No sooner had Ross constructed ¼-inch achromatic objectives on Lister’s formula than he discovered an error which had hitherto escaped attention, viz., that the thinnest cover-glass of an object produced a considerable amount of refractive disturbance. A marked difference was observed in the image when viewed with or without a cover-glass. This difficulty was first met by the addition of a draw-tube to the microscope body. But as this also impaired the image, Lister overcame the difficulty by mounting the front lens of the objective in a separate tube made to fit over a second tube carrying the two pairs of lenses. This arrangement led up to his invention of the screw-collar adjustment, the mechanism for applying which is shown in Fig. 114. The anterior lens a at the end of the tube is enclosed in a brass-piece b containing the combination; the tube a, holding the lens nearest the object, is then made to move up or down the cylinder b, thus varying the distance, according to the thickness of the glass covering the object, by turning the screw ring c, thus causing the one tube to slide over the other, and clamping them together when properly adjusted. An aperture is made in the tube a, within which is seen a mark engraved on the cylinder, on the edge of which are two marks, a longer and a shorter, engraved upon the tube. When the mark on the cylinder coincides with the longer mark on the tube, the adjustment is made for an uncovered object; and when the coincidence is with the shorter mark, the proper distance is obtained to balance the aberrations produced by a cover-glass the hundredth of an inch thick; such glass covers are now supplied. The adjustment should be tested experimentally by moving the milled edge which separates or closes the combinations, and at the same time using the fine adjusting screw of the microscope. The difficulty associated with the cover-glass of old has, by the introduction of the homogeneous immersion system, been very nearly eliminated. There still remains, however, a disturbing amount of residual colour aberration in the achromatic dry objective, and for the correction of which Zeiss proposed mounting the several lenses on a method somewhat different to that so long in use in this country. Fig. 115 shows an objective in which the screw-collar ring b b is made to adjust the exact distance between the two back lenses placed at a a. The value of the screw-collar is not questioned. It is difficult to obtain at all times cover-glasses of a perfectly uniform thickness; they will vary, and therefore perfect definition must be obtained, as heretofore, by adjusting for each separate preparation while the object is under examination.

As early as 1842 the excellence of Andrew Ross’s achromatic objectives were acknowledged, and his formula for their construction was generally followed. No doubt many of these early objectives of his manufacture are still regarded as treasures. I possess a ½-inch and a ¼-inch, which I believe to be comparable with any achromatic objectives of the same apertures of the present day. These I have always found most serviceable for histological work.

In 1850 Mr. Wenham produced an achromatic objective of considerable achromatic value. This consisted of a single hemispherical front combination, shown in the accompanying enlarged diagram, Fig. 116. Wenham’s formula seems to have been generally adopted by Continental opticians, who sold these lenses at a reduction of price. In Paris, Prazmowski and Hartnack—I have had one of Hartnack’s earliest immersions in use for many years—brought this form of objective to greater perfection, and in 1867 Powell and Lealand adopted the single front combination system in their early water-immersion objective, whereby the focal distance was said to be “practically a constant quantity, while reduction of aperture by making the front lens thinner ensures a much greater working distance without affecting the aberrations, since the first refraction takes place at the posterior or curved surface of the front lens, the removal of any portion of thickness at the anterior or plane surface simply cuts off zones of peripheral rays without altering the distance—any space being filled by the homogeneous immersion fluid, or by an extra thickness of cover-glass.”21

Great improvements were brought about by R. B. Tolles, of Boston, 1874, in the objective, as well as in the optical and mechanical parts of the microscope, most of which, however, must be ascribed to the criticisms and suggestions of amateur workers skilled in the exhibition of test-objects—the late Dr. Woodward of Washington, for example, whose series of photographs of the more difficult frustules of diatoms have rarely been surpassed. Such results were due to improvements made in the optical part of the microscope at his suggestion. He came to the conclusion, arrived at about the same time by mathematical scientists, that increase of power in the microscope was only possible in two directions, the qualitative and the quantitative.

It was now that microscopists turned to the late Professor Abbe for assistance in perfecting the objective in the dioptric direction. This, he pointed out, must be looked for in further improvements in the art of glass-making.

A series of experiments ultimately brought to light a mineral substance, Fluorite, which, when combined in the proper proportion, one part to two of German crown and flint glass, was found to have the qualities looked for, and to possess different relations of a dispersive and refractive power. From Professor Abbe’s researches, begun in 1876, we have had the aperture of the objective greatly enlarged, and the homogeneous system brought into general use.

Previous to this date the best made objective merely approximated to colour correction. Undoubtedly the chief object to be obtained was the removal or diminution of the secondary colour aberration. This, together with other residual errors Abbe pointed out in 1880, led to the improvement of the optical quality of the glass used in the manufacture of all optical instruments, the chief difficulties being surmounted in the Jena glass factory, whereby a complete revolution was effected in the microscopic objective. The apochromatic glasses of Zeiss, Powell, Beck, Ross, Watson, Swift, and other makers, in which the secondary spectrum has been totally eliminated, or only a negligible tertiary spectrum remains—that is to say, the objectives of these makers—are now corrected for three spectrum rays, and not two, as in the older objectives; and only those who look forward for making further discoveries in the intimate structure of bacilli or for resolving the finest diatom markings can be said to fully appreciate the importance and value of the investigations of the late Professor Abbe, and which have, so to speak, entirely changed old empirical views as to the value of high aperture, and demonstrated that high amplification, unless associated by proportionally high aperture, necessarily produces untrue images of minute structures. It was he also who introduced a practically perfect system of estimating apertures, known as the “numerical aperture notation,” by which not only can an accurate comparison be made of the relative apertures of any series of objectives, whether dry or immersion, but their resolving power under the various conditions of the kind of light employed. Their penetrating power and their illuminating power can now be estimated with mathematical exactness.

The practical advantages, then, secured by the adoption of the homogeneous system were, on the whole, greater than any before made or believed to be possible, and when taken into account in connection with the improvement of the eye-piece (also due to Abbe), almost perfect achromatism and homogeneity between objective, object, and eye-piece is secured, together with a sharp definition of the image over the whole visual field. These, with an increase of working distance between the object and the objective, and other important results, have been placed within the reach of the microscopist by men of science, and the outcome is the general adoption of the homogeneous system, termed by Carl Zeiss, a fellow-worker with Abbe, the22 apochromatic system of constructing objectives.

Relative Merits of the English and German Objectives.

As to the relative merits of German-made objectives, no superiority can be claimed for them over those made by English opticians.

The Continental form of the ¹⁄₁₂-inch oil-immersion objective, shown in Fig. 118, on the scale of 6 to 1, consists of four systems of lenses, namely, the front, a deep hemispherical crown lens of high refractive index; the second front of the system, an achromatic lens of such a form that it gathers the light from the hemispherical front; the middle lens, a single meniscus; and the back an achromatised lens, the second front of the back being connected in such a way as to compensate for the spherical and chromatic aberrations of the front lens.

The first homogeneous immersion objective which came under my observation was manufactured in the well-known Jena workshop of Carl Zeiss, December, 1877. This had a very considerable increase of numerical aperture, upwards of 50 per cent.; a clear gain, as an oil angle of even 110° proved to be of greater value than an angle of 180° in air, while the resolving power of the objective was increased in like proportion. There does not at present appear to be a bar to the construction of objectives of yet higher power, with increase of aperture. The available course open in this direction is the further discovery of another vitreous material and a suitable immersion fluid with an index of 1·8 or 1·9, and glass with a corresponding index, so as to ensure homogeneity of the combination. Zeiss asserts that in the more difficult departments of microscopical research the apochromatic lenses will supplant the older objectives, yet there are many problems in microscopy awaiting solution which do not demand the highest attainable degree of perfection in the objective, and in the majority of cases the older achromatic objective is all that is needful, provided it is good of its kind. The achromatic objectives and eye-pieces of the older type have still an advantage, as, owing to their simpler construction, really good lenses of the class required can be purchased at considerably lower prices than the objectives of the new series. These, from being more complicated in construction, involve a greater amount of skilled manual labour.

The German glasses of to-day afford satisfactory evidence both of skill and workmanship displayed in their production. Their cost is greater, then, for the reason given, as will be seen on reference to Continental catalogues. The dry series of objectives cost somewhat less, a ½-inch (numerical aperture 0·30) can be had for £1 10s., and a ¹⁄₆-inch (numerical aperture 0·65) for £2. On the other hand, the apochromatic series rapidly increase in price as the numerical aperture approaches the limit of numerical aperture 0·40. The best of Zeiss’s series are the 12 mm. (½-inch) and the 3 mm. (¹⁄₈-inch), numerical aperture 1·4, both of which possess the optical capacity assigned to them. These objectives are undoubtedly the finest to be met with in the workshop of any optician. Achromatic objectives of Continental manufacture have been as much improved as those of English make by the introduction of the newer varieties of glass, as already explained, while a new nomenclature has sprung up in consequence. We now have semi-apochromatic and parachromatic. The German opticians have followed Zeiss’s lead, since almost the same series of objectives are given in the catalogues of Leitz, Reichert, and Seibert, while the quality of both dry and immersion objectives is found to be much the same. The low price of Reichert’s immersion objectives should be noted, as their performance is quite perfect. A ¹⁄₁₂-inch (numerical aperture 1·30) of Leitz’s, with which I have worked at bacteria, has given me much satisfaction; supplied by Watson and Baker at £5. A ¹⁄₁₂-inch dry objective by the same maker (numerical aperture 0·87) costs £3, and a water immersion ¹⁄₁₂-inch (numerical aperture 1·10) £3 5s. Leitz reminds me that it requires a good lens of from six to seven hundred magnifying power for the examination of bacteria. For this reason he has constructed a new form of lens, a ¹⁄₁₀-inch oil-immersion of 2·5 mm. focus, for the purpose of adding to the resources of bacteriology. This lens necessarily has a lower magnification than his former ¹⁄₁₂-inch oil-lens, but as it is less costly to manufacture it is sold at a smaller price. The before-mentioned ¹⁄₁₂-inch, with a No. 3 compensating eye-piece, gives a magnification of over seven hundred or eight hundred diameters. To secure the best results in using the higher powers of Leitz’s, from No. 5 upwards, a cover-glass of 0·17 mm. in thickness should be used, and care taken to make the length of the draw-tube equal to 170 mm. This length of tube should be adhered to in the use of this optician’s oil-immersion lenses. If the microscope be provided with a nose-piece, the draw-tube should be drawn out to 160 mm.; in its absence it should be set at 170 mm., a deviation of 10 mm. or more from the correct tube-length deteriorates from the value of Leitz’s oil-immersion objectives as of other opticians. It is suggested that the German apochromatic combination of three cemented lenses is that adopted by Steinheil long before, in the construction of his well-known hand-magnifier (see page 77, Fig. 51). Zeiss’s 3 mm. objective has a triple front, balanced by two triple backs—in all nine lenses—a somewhat amplified diagram of which is represented in Fig. 118. The formula for this combination was furnished by Tolles, of Boston, America, and it at once secured increase of aperture (the value of this optician’s many contributions to microscopy has since his death been generally acknowledged). The metrical equivalent focus assigned by Zeiss to his series of dry achromatic objectives is given in somewhat ambiguous terms, which tend to confuse rather than classify them; for instance, two lenses of the same aperture—24 mm. and 16 mm.—corresponding to the English 1-inch and ²⁄₃-inch, each have assigned to them an aperture of 0·30; a 12 mm. and 8 mm., corresponding to the English ½-inch and ¹⁄₃-inch, have an aperture of 0·65; while a 6 mm. = ¼-inch, and a 4 mm. = ¼-inch and ¹⁄₆-inch, have each an aperture of 0·95.

Nachet exhibited at the Antwerp Exhibition a fine ¹⁄₁₀-inch oil-immersion, which was highly praised by the jurors.

It is necessary, to make the fact perfectly clear, that dry and immersion lenses having the same angular aperture have also a similar defining power. The pencil of rays, however, differs in intensity and density as the rays emerging from the cover-glass of the object into air are very considerably deflected, and the cone suffers a corresponding loss of brightness. On this important point, then, I believe it will prove of value to interpolate a clear and full exposition of the change brought about by the cover-glass.

It is not difficult, then, to perceive the importance of Amici’s discovery as to the value of a drop of water inserted between the object and the objective, and it now seems somewhat surprising it should have been so long neglected by opticians, since it is at once seen to diminish the reflection which takes place in the incidence of oblique light. The film of water not only gives increased aperture, but also greater cleanness and sharpness to the image. The film, then, as already shown, collects the straying away of peripheral rays of light, and sends them on to the eye-piece, and greatly assists in rendering the image more perfect, and materially aids in the removal of residuary secondary aberrations; while with air, or dry objectives, a certain amount of aberration takes place, sufficient to affect the pencils on their passage from the radiant to the medium of the front lens, adding a considerable ratio to the total spherical aberration with the objective, which, in the case of wide angles, increases disproportionately from the axis outwards. This can only be corrected by a rough method of balancing; that is, by introducing an excess of opposite aberration in the posterior lens. An uncorrected residuum, rapidly increasing with larger apertures, is then left, and this appears in the image amplified by the total power of the objective, so that with a non-homogeneous medium there is a maximum angular aperture which cannot be surpassed without undergoing a perceptible loss of definition, provided working distance is required. If we abolish the anterior aberration for all colours, by an immersion fluid which is equal to cover-glass in refractive and dispersive power, the difficulty is at once overcome. If, for instance, we have an objective of 140° in glass (= 1·25 N.A.) and water as the immersion fluid, the aberration in front would affect a pencil of 140°. Substituting a homogeneous medium, the same pencil, contracted to the equivalent angle in that medium of 112°, will be admitted to the front lens without any aberration, and may be made to emerge from the curved surface also without any disturbing aberration, but contracted to an angle varying from 70° to 90°. The first considerable spherical aberration of the pencil then occurs at the anterior surface of the second lens, where the maximum obliquity of the rays is already considerably diminished.

Figs. 119 and 119a will doubtless make this clearer. If the objective of 140° works with water (Fig. 119), there would be a cone of rays extending up to 70° on both sides of the axis, and this large cone would be submitted to spherical aberration at the front surface a. But with homogeneous immersion Fig. 119a) the whole cone of 112° is admitted to the front lens without any aberration, there being no refraction at the plane surface; and as the spherical surface of the front lens is without notable spherical aberration, the incident pencil is brought from the focus F to the conjugate focus F′, and contracted to an angle of divergence of 70°-90° without having undergone any spherical aberration at all.

The problem of correcting a very wide-angled objective has thus been reduced by the homogeneous oil-immersion system, both in theory and practice.23

Abbe’s Test-plate.

Abbe designed the test-plate (Fig. 120) for testing the spherical and chromatic aberrations of objectives, and estimating the thickness of cover-glasses corresponding to the most perfect correction: six glasses, having the exact thickness marked on each, 0·09 to 0·24 mm., cemented in succession on a slip, their lower surface silvered and engraved with parallel lines, the contours of which form the test. These being coarsely ruled are easily resolved by the lowest powers; yet, from the extreme thinness of the silver, they form also a delicate test for objectives of the highest power and widest aperture. The test-plate in its original size is seen in Fig. 120, with one of the circles enlarged.

To examine an objective of large aperture, the discs must be focussed in succession, observing in each case the quality of the image in the centre of the field, and the variation produced by using, alternately, central and very oblique illumination.

When the objective is perfectly corrected for spherical aberration, the outlines of the lines in the centre of the field will be perfectly sharp by oblique illumination, and without any nebulous doubling or indistinctness of the edges. If, after exactly adjusting the objective for oblique light, central illumination is used, no alteration of the focus should be necessary to show the outlines with equal sharpness.

If an objective fulfils these conditions with any one of the discs, it is free from spherical aberration when used with cover-glasses of that thickness. On the other hand, if every disc shows nebulous doubling, or an indistinct appearance of the edges of the line with oblique illumination, or, if the objective requires a different focal adjustment to get equal sharpness with central as with oblique light, the spherical correction of the objective is more or less imperfect.

Nebulous doubling with oblique illumination indicates over-correction of the marginal zone; indistinctness of the edges without marked nebulosity indicates under-correction of the zone; an alteration of the focus for oblique and central illumination points to an absence of concurrent action of the separate zones, which may be due to either an average under or over correction, or to irregularity in the convergence of the rays.

COVER-GLASS GAUGE.

Zeiss has gone a step further to lay the microscopist’s ghost of the cover-glass. He invented a measurer (Fig. 121) whereby the precise determination of thickness of glass-covers can be obtained. This measurement is effected by a clip projecting from a circular box; the reading is given by an indicator moving over a divided circle on the lid of the box. The divisions seen cut round the circumference show ¹⁄₁₀₀ths of a millimeter. This ingenious gauge measures upwards of 5 mm.

This necessary and important digression has led me away from the consideration of the achromatic objective, and to which I shall now return.

English Immersion and Dry Objectives.

The homogeneous immersion system met with its earliest as well as its staunchest advocates among English opticians. Among its more energetic supporters were Messrs. Powell and Lealand, who were the first to construct a ¹⁄₈-inch immersion objective on a formula of their own, and which was found to resolve test-objects not before capable of resolution by their dry objectives. This encouraged them to make a ¹⁄₁₆-inch, acquired by Dr. Woodward for the Army Medical Department, Washington, and subsequently a ¹⁄₂₅-inch; neither of which surpassed their ¹⁄₈-inch in aperture, and a new formula was tried in the construction of their first oil-immersion objective. This had a duplex front, and two double backs; but even this did not quite accomplish what was expected of it, and another change was subsequently made; the anterior front combination became greater than a hemisphere—a balloon-lens. This at once gave an increase of aperture to a ¹⁄₁₂-inch objective of 1·43 numerical aperture. After some few more trials a more important change of the formula took place. The front lens was made of flint-glass, and the combination took the form represented in diagram (Fig. 122). This, on an enlarged scale, represents Powell’s ¹⁄₁₂-inch numerical aperture 1·50. It is a homogeneous apochromatic immersion of high quality and very flat field. It will be noticed that in this combination the four curves of the lenses are very deep compared with those of other opticians.

Messrs. Ross have made many important improvements and changes in the construction of their several series of achromatic objectives; the calculations and formulæ for which were made exclusively for them by Dr. Schrœder. The list is too long to quote, but most of these lenses are of a high-class character, and work with admirable precision. Among the best of their objectives, I can commend a 1-inch of 30° and two oil-immersions, a ¹⁄₈-inch of 1·20 and a ¹⁄₁₂-inch of 1·25 numerical aperture, each of which bear the highest oculars equally well; a good test, as I have always maintained, of excellence. Their ¹⁄₁₀-inch has a somewhat larger aperture, and therefore shows a fine image of the podura scale. The finish of Ross’s several series of objectives fully maintains the high character and reputation of this old-established firm of opticians.

Messrs. R. and J. Beck have bestowed great attention upon the improvement of their dry-objective series, much in demand for histological work, especially among the students of city hospitals, who usually commence their pathological work with the cheaper forms of objectives. In that case an inch objective of about 25° air angle, a ½-inch of not less than 40°, and a ¼-inch or ¹⁄₅-inch magnifying from 50 to 250 diameters, is quite sufficient for most of their work. For bacteriological research, Messrs. Beck supply a ¹⁄₆-inch immersion taken from a series, having a high aperture and a better finish at a moderate price. Their ¹⁄₁₀-inch immersion has in my hands proved a serviceable power for bacteriological research; it requires a good sub-stage illuminating achromatic condenser to obtain the best results.

Messrs. Watson and Sons have much enhanced their reputation by the marked improvement lately brought about in the manufacture of their whole series of objectives. This probably is chiefly due to the introduction of the Jena glass into their manufacture, and which has enabled them to give increase of aperture to one series in particular, that of the para-chromatic, all of which in consequence are of very high quality. It is difficult to particularise their several objectives, the whole having special features in proportion to their magnifying powers, while much care seems to have been bestowed on them for the elimination of residual colour. A ¹⁄₈-inch with correction collar is comprised of a single deep and rather thick front lens, plano-concave flint, and double convex-crown for the middle and triple combination for the back, the latter consisting of two crown lenses cemented to a dense flint (Fig. 124) drawn to scale of 5-1, with lined portions intended to represent the flint, and white the crown glass lenses of the combination. The initial magnification of this objective is 83 diameters, and the numerical aperture ·94. This superior objective can be had for the small sum of £2. Another remarkably useful and cheap objective, their 1-inch numerical aperture 0·21, consists of two achromatic systems forming the front and back with the separation between them of about half an inch, and may also be especially recommended for students’ work.

In the accompanying diagram the lenses are drawn on too large a scale, and therefore the distance between the two combinations should be much greater.

Among the more useful of Watson’s series, the 1-inch, the ½-inch, and the ¹⁄₆-inch, together with the ¹⁄₈-inch dry-objective, and a ¹⁄₉-inch, will be found the most serviceable.

Messrs. Baker have their own series of objectives, most of which are so very nearly allied to those of the continental opticians; and what has been said of Zeiss’s and Leitz’s objectives may be taken to apply also to Baker’s, who have an established reputation for their histological series, all of which are well suited for students’ and class-room work.

Messrs. Swift and Son have a new series of objectives, semi-apochromatic and pan-aplanatic, most of which are excellent in quality and show increased flatness of field together with that of achromatism; the index of refraction in each series having been correctly determined together with exact radial focal distance, thus affording more available aperture. I may select for special commendation their ¹⁄₁₂-inch £5 5s. homogeneous immersion objective, which is in every way suitable for bacteriological work; its definition is very good, as is seen in a micro-photograph of podura scale, given further on. Their dry ¹⁄₆-inch can be had for £1 16s.—a marvel of cheapness. Of their general series the most useful for histological work are the ½-inch, the ¹⁄₃-inch at £1 12s., and their ¹⁄₅-inch of numerical aperture 0·87 at £3.

Mr. Pillischer, of Bond Street, has manufactured many excellent objectives. A fine homogeneous oil-immersion ¹⁄₁₂-inch numerical aperture 1·25 is worthy of special notice; it will be found suitable for bacteriological work; it has fine definition with a considerable amount of penetration.

A more intelligent idea of the magnifying power of the objective combined with the eye-piece will be gained by consulting the table given below; precision in this respect has long been a desideratum with microscopists.

Magnifying Powers of Eye-Pieces and Objectives.

A TYPICAL AND INITIAL SELECTION OF POWERS OF EYE-PIECES CALCULATED FOR THE 10-INCH TUBE-LENGTH.

Compensating Eye-pieces for use with Apochromatic Objectives.

This may be taken as a typical set, further treated of among Eye-pieces.

Initial Powers of Objectives calculated for the 10-inch Tube-length.

This is ascertained by dividing the distance of distinct vision 10 inches by the focus of the objective, thus—

A reference to the above table will at once show that the nomenclature of objectives expresses at once the initial magnifying powers, but as makers have great difficulty in so calculating their formulæ so as to obtain the exact power, these figures must be taken as approximate. Thus a ¼-inch, which should magnify 40 diameters if true to its description, might actually magnify a little more or less.

The magnifying powers of Zeiss’s and other apochromatic objectives can be ascertained by dividing the focal length of the objective in millimeters into 250 mm. (the distance of distinct vision), thus

The total magnification, when any eye-piece is working in conjunction with an objective, is ascertained by multiplying the initial power of the objective by that of the eye-piece.

The above calculations are all for a 10-inch tube-length. Should, however, a shorter or longer length of body be employed, the magnification can at once be ascertained by a proportion sum. If the magnification be 180 with 10-inch tube-length, what would it be with a 6-inch body—10 : 6 :: 180 = 108 diameters.

Abbe designed three different forms of eye-pieces: 1, the searcher eye-piece; 2, the working eye-piece; and 3, the projecting eye-piece. The Searcher is a negative form of low power. The working is both negative and positive, the positive form of which is constructed on a newer principle; while the projection is chiefly intended for microphotography, its field being small and its definition superlatively sharp. These are severally explained among eye-pieces.

High-Power Objectives.

Points of Importance for securing the best results with High-power Objectives.—Always give to the body-tube of the microscope the length for which the objective is corrected, 0·160 mm. for the short continental tube, and 0·250 mm. for the English tube (10-inch). Employ both dry and immersion objectives mounted for correction, commencing with a numerical aperture of 0·75 (that is about 100° in air). If the graduation is not given in thickness of cover-glass apply to the maker to correct this omission.

With the homogeneous oil-immersion objective it is highly necessary to utilise all marginal pencils of light, to optically unite the upper lens of the condenser with the preparation as well as the front lens of the objective by means of a liquid having the same index of refraction or at least equal to that of the immersion. Cedar Oil has been generally adopted for the purpose mentioned, the better way of using which is as follows: place a drop on the centre of the front objective, or on the top of the cover-glass, and then lower the objective by means of the coarse adjustment until it comes in contact with the oil, and carefully bring into focus by the fine adjustment. If the slide is held between the finger and thumb of one hand and moved from side to side, while the other hand is working the fine adjustment, there can be no danger of injuring either the objective or the specimen. Before putting the microscope away, take a fine camel-hair brush dipped in ether, alcohol, or methylated spirit, and carefully remove the oil from the objective and the glass cover of the object; a soft chamois leather or cambric pocket handkerchief will dry it off, or a piece of fine white blotting paper answers equally well. Should the lens come accidentally into contact with the Canada balsam, it must be very carefully removed either by ether or alcohol. The former is by far the safest, as alcohol, if not very carefully used, quickly dissolves out the balsam and loosens the cover-glass of the object.

Achromatic Condensers.

The Achromatic Condenser can no longer be classed among the accessories of the microscope, since it is an absolutely indispensable part of its optical arrangements. Its value, then, cannot be overrated, and the corrections of the lenses which enter into the construction of the condenser should be made as perfect as they can be made—in fact, as nearly approaching that of the objective as it is possible to make them. It may therefore be of interest to know something of the rise and progress of the achromatic condenser. In my first chapter I have noticed the earlier attempts made by Dr. Wollaston, whose experiments led him to fit to the underside of the stage of his microscope a short tube, in which a plano-convex lens of about three-quarters of an inch focal length was made to slide up and down (afterwards moved up and down by two knobs); to improve definition he placed a stop between the mirror and the lens. The stop was found to act better when placed between the lens and the object. From this improvement Dr. Wollaston enunciated that “the intensity of illumination will depend upon the diameter of the illuminating lens and the proportion of the image to the perforation, and may be regulated according to the wish of the observer.” Dujardin in France and Tully in England were at work in the same direction. The former a year or two later on contrived an instrument, which he termed an eclairage, to remedy the defects of Wollaston’s, and for illuminating objects with achromatic light. This was submitted for approval to Sir David Brewster, who, when the use of the achromatic condenser was first broached, used these encouraging words:—“I have no hesitation in saying that the apparatus for illumination requires to be as perfect as the apparatus for vision, and on this account I would recommend that the illuminating lens should be perfectly free from chromatic and spherical aberration, and that the greatest care be taken to exclude all extraneous light both from the object and eye of the observer.” This far-seeing observer in optical science has borne good fruit, and the outcome of his views is seen in the great development and improvement of the achromatic condenser. In 1839 Andrew Ross made his first useful form of condenser, and gave rules for the illumination of objects in an article written for the “Penny Cyclopædia.” These, epitomised, read as follows: 1. That the illuminating cone should equal the aperture of the objective, and no more. 2. With daylight, a white cloud being in focus, the object has to be placed nearly at the apex of the cone. The object is seen better sometimes above and sometimes below the apex of the cone. 3. With lamplight a bull’s-eye lens is to be used, to parallelise the rays, so that they may be similar to those coming from the white cloud. It has been seen that Mr. Lister foreshadowed the sub-stage condenser.

The early form of Ross’s condenser consists of two small brass tubes made to slide one in the other. To the outer one is attached a flat brass plate which slides underneath the stage of the microscope, and by means of a screw the adjustment of the axis of the illuminator is effected. The upper portion of the apparatus carries the achromatic combination, which by a rack and pinion movement is brought nearer to, or removed further from the object on the stage. The several parts of the illuminator unscrew, so that the lenses may be used either combined for high powers, or separated for low powers.

Messrs. Smith & Beck greatly improved upon Ross’s condenser by adding another achromatic lens to the combination, three being employed when used with high-power objectives and two or even one with the lower, the adjustment and focussing being made by rack and pinion arrangement beneath the stage. Some further changes for the better were made in the condenser by Powell, and in 1850 an amateur microscopist, Mr. Gillett, fully grasping the value of controlling the cone of rays passing into the microscope, devised a new form of condenser, in connection with which a revolving series of diaphragms of different values were made to pass between the achromatic lenses and the source of light.

Andrew Ross constructed the first condenser on Gillett’s principle, and this proved to be one of the most successful pieces of apparatus contrived. Gillett’s Condenser consists of an achromatic lens c, about equal to an object-glass of one quarter of an inch focal length, with an aperture of 80°. This lens is screwed into the top of a brass tube, and intersecting which, at an angle of about 25°, is a circular rotating brass plate a b, provided with a conical diaphragm, having a series of circular apertures of different sizes h g, each of which in succession, as the diaphragm is rotated, proportionally limits the light transmitted through the illuminating lens. The circular plate in which the conical diaphragm is fixed is provided with a spring and catch e f, the latter indicating when an aperture is central with the illuminating lens, also the number of the aperture as marked on the graduated circular plate. Three of these apertures have central discs for circularly oblique illumination, allowing only the passage of a hollow cone of light to illuminate the object. The illuminator above described is placed in the secondary stage i i, which is situated below the general stage of the microscope, and consists of a cylindrical tube having a rotatory motion, also a rectangular adjustment, which is effected by means of two screws l m, one in front, and the other on the left side of its frame. This tube receives and supports all the various illuminating and polarising apparatus, and other auxiliaries.

Directions for using Gillett’s Condenser.—In the adjustment of the compound body of the microscope for using with Gillett’s illuminator, one or two important points should be observed—first, centricity; and secondly, the fittest compensation of the light to be employed. With regard to the first, place the illuminator in the cylindrical tube, and press upwards the sliding bar k in its place, until checked by the stop; move the microscope body either vertically or inclined for convenient use; and, with the rack and pinion which regulates the sliding bar, bring the illuminating lens to a level with the upper surface of the object-stage; then move the arm which holds the microscope body to the right, until it meets the stop, whereby its central position is attained; adjust the reflecting mirror so as to throw light up the illuminator, and place upon the mirror a piece of clean white paper to obtain a uniform disc of light. Then put on the low eye-piece, and a low power (the half-inch), as more convenient for the mere adjustment of the instrument; place a transparent object on the stage, adjust the microscope-tube, until vision is obtained of the object; then remove the object, and take off the cap of the eye-piece, and in its place fix on the eye-glass called the “centring eye-glass,”26 which will be found greatly to facilitate the adjustment now under consideration, namely, the centring of the compound body of the microscope with the illuminating apparatus of whatever description. The centring-glass, being thus affixed to the top of the eye-piece, is adjusted by its sliding-tube (without disturbing the microscope-tube) until the images of the diaphragms in the object-glass and centring lens are distinctly seen. The illuminator should now be moved by means of the left-hand screw on the secondary stage while looking through the microscope, to enable the observer to recognize the diaphragm belonging to the illuminator, and by means of the two adjusting screws to place this diaphragm central with the others: thus the first condition, that of centricity, will be accomplished. Remove the white paper from the mirror, and also the centring-glass, and replace the cap on the eye-piece, also the object on the stage, of which distinct vision should then be obtained by the rack and pinion, or fine screw adjustment, should it have become deranged.