93.—Sun-glass.—Sextants and theodolites are supplied with a very dark glass or a combination of dark glasses fixed in a rim to form an eye-piece front, which screws or fits on in front of any eye-piece, to take observations of the sun for longitude or bearing, Fig. 19, SG. It needs no description, but is necessary to be mentioned to complete the optical arrangements of a telescope, as it is sometimes used for surveying purposes.

94.—The Body of a Telescope that forms part of a surveying instrument is constructed of a pair of triblet drawn tubes, Fig. 21, TT′ T′. These tubes should be truly cylindrical and straight, so as to fit smoothly together, the one within the other, and slide in and out quite freely but without any play. The inner tube should be as long as practicable, so as to remain steady when racked out to the full extent required to focus near objects. The object-end R is generally enlarged so as to take the cell in which the objective O is placed, without cutting off any part of the light, or entailing the weight of larger tubes than is necessary to make use of the full field of the objective. The objective is generally held in its cell by an internally fitting screwed ring with milled edge, so that the glasses may be taken out and separated to be cleaned, and be easily replaced. Two notches or grooves are commonly made in the edges of the glasses, each of which is deep enough to take a small brass pin which is soldered to the inside of the cell. The second notch indicates relative position, so as to secure the glasses being replaced properly. In all cases the double convex crown glass is placed outwards from the telescope. A glass of large size should have a loose ring within the cell to act as a spring to save distortion of the objective from expansion or contraction of the metal; but this is not necessary in small surveying instruments. In some common telescopes the object-glass is burnished into its cell, in which case the glasses of the objective cannot be separated for cleaning.

95.—Stops.—Within the inner tube two or more thin metal rings, termed technically stops SS and S′S′, are placed to cut off any extraneous light that may enter the telescope obliquely, and which, if not stopped off, would produce a fogginess over the whole field of view. It is important that these stops should not cut out any part of the full aperture of the object-glass if it be a good one. In the manufacture of the telescope this is easily seen by looking in at the eye-piece of the unglazed telescope to see if the stops clear the objective cell. In the finished glazed telescope another method will be discussed further on.

96.—The inner or the outer tube of the body of the telescope slides towards or from the objective for focussing by means of a rack R″ and pinion P. The rack is soldered to the inner tube, and the pinion fitted in a cock-piece, as shown Fig. 22 C, on the outer tube. The pinion is moved by a large milled head M. This fitting should be made with care. The pinion should be very free, so that it does not lift the body at any tooth, and at the same time there should be no shake on the gearing. It needs considerable practice to rack a telescope properly.

97.—The outward part of the object end of the telescope is generally turned to fit the interior of a separate short tube, shown at R, which is placed over it. The outer end is closed by a ring to the size of the aperture of the objective. This is termed a ray-shade or sometimes a dew-cap. The ray-shade is extended when the telescope is directed to such an angle that the sun's rays would fall upon any part of the objective, and thereby cause internal reflections. A swivel shutter, Fig. 21, R′, is placed upon the outward end of the ray-shade, which, when closed, as shown in the cut, forms a cap to the telescope. The eye-piece EP before described, art. 82, Fig. 16, is placed in a tube constructed upon the end of the telescope, in which it slides freely, to focus upon the diaphragm to be presently described. The telescope is mounted sometimes solidly upon a transverse axis, or it is mounted in turned bearings, or it has two collars placed round it which are turned quite equal and true, and are mounted on Y's to be hereafter described.

98.—Mechanical Adjustment of the Eye-piece.—In some large instruments the eye-pieces are racked for adjustment in the same manner as the object-glass already described. A better plan is to have an inner tube to the socket tube cut with a screw into this, and provided with a milled edge, so that the eye-piece may be screwed gently to focus upon the webs of the diaphragm.

99.—The Diaphragm of the Telescope is so constructed as to permit the displacement of spiders' webs or other fine objects in any direction at right angles to the axis of the telescope, or in the vertical only in the dumpy level, to be described, the object in all cases being to adjust the crossing of the webs, lines, or points to the axis of the telescope. It will be convenient here to discuss a general form of diaphragm applicable to theodolites, mining-dials, and plane-tables only, which gives movement in two directions at right angles to each other.

100.—The diaphragm, Fig. 23, is formed of a stout disc of brass having a centre hole of about ·30 inch diameter. Upon the side which is placed next the eye-piece the hole is brought to a thin edge by an internal bevel or countersink, which leaves the hole much larger at its off surface, Fig. 24 P. The disc is held in its place and adjusted by four capstan-headed screws, termed collimating screws, two of which are shown in section as CC′, the screws being tapped into the rim of the diaphragm frame P. The screws are placed through a stout collar. The theodolite diaphragm has generally three spiders' webs or lines crossed in the manner shown in the centre of Fig. 23. The eye-piece is screwed into the thick plate, Fig. 24, TT′, and adjusts to the focus of the webs.

101.—Webs.—It is a somewhat delicate process to web a diaphragm, but it is necessary that every surveyor abroad, out of the reach of an optician, should understand the method if his instrument were originally webbed. The webs are taken from a small or young garden spider. The best are taken when the spider has first commenced spinning. To wind off the web a fork is bent up out of a piece of thin brass wire. A long hairpin will answer for this purpose very well, or even a fork formed of a thin branching twig of a shrub; but if this last be used it should be thoroughly dry, or the webs will be broken or be baggy by its warping in drying.

102.—The web in connection with the spider is first attached to one prong of the fork by looping or by any sticky matter, if the web be not sufficiently sticky naturally. The spider is then suspended from the fork and jerked down a foot or so, and the web is wound off as shown in Fig. 25. The last length of web being attached by gum. A dozen or so of the forks may be taken from the same spider before she is exhausted. The webs are then gummed or varnished to the sides of the fork, and are ready for use at any future time. They are best preserved if placed in an air-tight box, which may have slots in an internal fitting to hold them. The small amount of spring given by the fork keeps the webs always taut. Where a living spider cannot be found, the open ties of an old web may be taken; but in this case, after the web is wound on the fork, it should be carefully washed by immersing it in clean water, and, if necessary, brushing it gently under water with a light camel-hair brush, examining it occasionally with a magnifier to see that it is sufficiently clean and free from knots for its purpose.

103.—To Fix the Webs, lines are drawn on the diaphragm, into which the webs are to fall. It is then varnished over the divided side with Canada balsam, laudanum, or other quick-drying, sticky varnish—at a pinch, sealing-wax dissolved in strong whisky will answer. The outer, or the unused web upon the fork, is lowered carefully over one of the most nearly vertical lines, and lightly pressed down to assure its perfect adhesion to the varnish. It is then either broken off or cut loose. The second nearly vertical line is then webbed in the same manner, and the horizontal line finally, being sure that this last cuts the intersection of the others. The diaphragm should then be put in a warm place to be allowed to thoroughly set without disturbance before it is fitted in the telescope.

104.—Platinum Wires are sometimes used in place of webs. These wires are made by drawing a piece of fine platinum wire, which has been previously soldered into a silver tube, to the greatest fineness possible with the draw-plate, and afterwards dissolving the silver off the platinum by nitric acid. The platinum wire is thus produced of less than ·001 inch diameter. For a time these wires were very popular, and it was thought that they would supersede the use of webs, but they do not appear entirely to answer expectation. The platinum drawn in this manner appears to lose some part of its elasticity. It is not easily attached, that is, it is liable to shift from its fixing, possibly from its contraction and expansion with change of temperature, not being of the same metal as the diaphragm. It also oxidises a little or becomes in some way corroded in use out of doors. It appears to answer better for astronomical telescopes, but the finest platinum wire obtainable is not so fine as a spider's web.

105.—Lines Ruled upon Glass.—A glass diaphragm is frequently used in a surveying instrument to replace the webs. Lines are ruled upon the glass in similar positions to the webs already described. They appear quite sharp in the eye-piece, and are more permanent than webs. Glass is also convenient for permitting space lines to be ruled for subtense measurements, a subject to be considered further on. The objections that have been found to glass are that it obstructs a little light, and is subject to dewing. The dewing is particularly annoying when temperature is lowering quickly, as a diaphragm may become bedewed many times in a few hours. In all cases where a glass diaphragm is used it should be placed in a ground metal fitting, so that it may be taken out in a minute to clean and be replaced with perfect certainty of its adjustment. It is a very convenient practice where webs are used to have a spare glass diaphragm to replace them should they become broken. This may be constructed by means of a ground metal fitting to be put in a webbed instrument in perfect adjustment in cases where it might be impossible to find a new web.

106.—Points.—The author for a large number of instruments employs very fine points in place of webs, which he highly recommends. These are fixed for support upon the margin of the diaphragm, and projected therefrom into the field of view. The points are formed of a special alloy, 75 platinum, 25 iridium, which has the hardness of steel, and is perfectly non-corrosive in air or moisture. They are made sufficiently stiff to be dusted with a camel-hair brush, supplied in the instrument case, without the slightest fear of disturbance of position in the instrument. They form a perfectly permanent index of sufficient stability to last in perfect adjustment as long as the instrument lasts in wear. One objection is that a point gives less field of observation for levelling than a line, but this does not hold if there is tangent adjustment to the instrument to bring the point up to its reading position. The value of the reading from these points will be discussed further on.

107.—Position of the Diaphragm in the Telescope.—If the objective be accurately centred, and its mounting true, the intersections of the webs, lines, or points should come exactly in the axis of the telescope; but it would never do to accept this without critical examination. Therefore the webs may be placed approximately in the centre, and adjusted true to the axis of the objective and the telescope by what is technically termed collimation. The first point, however, to be studied in this adjustment is to get the eye-piece and the objective accurately in focus with the webs. The same description of focussing which answers for collimation will answer also for ordinary use of the telescope.

108.—Adjustment of the Eye-piece to the Webs is effected by pushing in or drawing out the eye-piece in its tube with a slight screwing motion until the webs, lines, or points appear quite distinctly. To prevent confusion from the sighting of objects, it is better to take off the ray-shade, to point the telescope to the distance in opposition to the direction of the sun, and to keep the telescope rack fully extended, so that it is quite out of focus. When the light is not very bright a sheet of notepaper or an envelope may be placed obliquely in front of the object-glass to obtain a soft reflection from the sky. This method is always employed by some observers.

109.—Adjustment to Focus of the Objective.—Parallax.—The eye-piece remaining in focus, the telescope is racked out until the object desired to be brought into view, either for the collimation or for ordinary reading, is sighted. After this the milled head is moved as slowly as possible until what is thought to be the exact focus is obtained. The certainty of exact focus is not easily obtained by direct observation, but it may be obtained by what is termed observation for parallax, which must be taken in all cases when adjustment is required for collimation. Thus, having obtained the nearest possible adjustment by sighting a small object or a division upon the staff, bring the object to read exactly in a line above the horizontal web in the centre of the stop or the corner against a vertical web. If now the eye be moved up and down as far as the range of the eye-piece will permit vision of the centre of the webs, and the object sighted appears fixed at the same position to the webs, the focus is perfect. If, in moving the eye, the object sighted appears to follow its motion about the intersection of webs, the focus of the telescope lies beyond the webs; the objective must therefore be moved slightly nearer the webs by turning the milled head very gently. If, on the other hand, the object sighted moves in the opposite direction to the eye about the intersection of the webs, the focus of the telescope is towards the eye-piece, and the telescope requires slightly racking outwards by moving the milled head in the reverse direction. After a few trials the object and webs appear stationary, however obliquely observed.

110.—Collimation is the adjustment of the crossing of the webs of the diaphragm to the axis of the telescope and its object-glass. This is effected by adjustment of the opposite collimating screws, Fig. 24, CC′, in two directions at right angles to each other. Where the telescope is placed in Y's or collars, this adjustment is made by placing the webs or lines in focus of the eye-piece and the object-glass of the telescope in focus upon a small distant object. Then if the telescope is rotated in all directions, and the small distant object cuts the crossing of the webs in all positions, it is said to be truly collimated. It is necessary to discuss the structure of various instruments to show the methods of collimating in special cases; therefore this subject will be again brought forward.

111.—The Qualities of a Telescope of a surveying instrument are best ascertained by its performance. The general method is to place a staff at the full range, 10 to 15 chains, and to see if the ·01 foot in fine bright weather is read clearly and sharply. This outdoor observation is not always possible, particularly in large towns, but it may very well be supplanted by reading at a short distance. The author made for the late Colonel Strange, F.R.S., whose knowledge of scientific instruments was of the highest order, a test-card for the Lambeth Observatory, to be placed at 25 feet from the instrument. This card had on one part fine lines ruled ·01 inch apart. A 14-inch telescope was considered sufficiently good if these lines could be clearly separated at this distance by the telescope when it was in correct focus. The dial of a watch, or an ivory scale, answers very well as a test object, as sharpness of outline is the point to be ascertained.

112.—A more refined technical method than that described above, which also tests the general accuracy of the optical arrangement of the telescope, is to fix a small disc of white writing-paper, say 1/8 inch diameter, cut out with the point of a pair of compasses with sharp outline, on a black surface of a board, paper, or cloth. If this be placed as before, 30 feet or more distant in a good light, and be correctly focussed in the telescope, a sharp image of it should be obtained. This focal position of the telescope may be temporarily marked upon the inner tube with a fine soft black-lead pencil. If now the object-glass be racked outwards or inwards from this line, say for about a twelfth of an inch, and the image appears to be surrounded with a uniform haze, the objective may be considered to be correctly formed, or to be free from spherical aberration, as it is termed, and the combination to be correctly centred. If the haze appears more on one side than the other the centring is defective. If the object remains fairly sharp when out of exact focus the curves of the lens are defective, as the shorter the range of focus the more perfect is the correction from spherical aberration.

113.—If the curves are not sufficiently correct to bring the image from all parts of the objective to a focus, such incorrect parts are useless, and a good glass of smaller size would be better. The fault is generally found in the marginal portion of the objective, which requires the greatest skill of the glass-worker. Therefore, a very good test to find whether the whole of the aperture of the objective is in effective use is to cut out a piece of card of the size of this aperture and to cut a second piece out of the centre of this, of half the diameter, so as to form a disc and a ring. If the objective be now covered by the ring and accurately focussed upon a test object, and this be then removed and replaced by the disc fixed over the centre of the objective, and the focus remains equally sharp, the curves may be said to be, practically, correctly worked.

114.—As the central part of an objective is more easily brought to correct curvature than the marginal parts it is not uncommon in inferior instruments to make the aperture of the central stop of the telescope cut off the margin of the objective. This renders it only equal to a smaller glass.

115.—Whether the full aperture of a telescope is used may be discovered by employing a second eye-piece—outside the regular eye-piece that is placed in the telescope—to pick up the image of the object glass formed through the eye-piece which is placed against the telescope in the manner of using a dynameter, art. 87. With the ordinary surveyor's level, two eye-pieces are commonly sold; one of these may be placed in the telescope and the other used to pick up the image of the object-glass. With a theodolite one eye-piece may be placed in the telescope, and one of the readers used to magnify the divisions of the limb may be used to pick up the image. The best manner of proceeding is to fix with water or thin gum two or three small pieces of paper, say 1/20, 1/10, and 1/7 inch square, close against the edge of the cell upon the face of the objective. Then focus the telescope on an object at some distance, say a chain or two. Now use the second eye-piece in front of the one in the telescope, and an image of the object-glass will be seen; and if the aperture is fully open all the pieces of paper in their places will be clearly distinguishable. If one or other piece is invisible, the margin of the glass is cut off to this extent. If the objects in front of the telescope tend to confuse, a piece of white paper may be placed obliquely to reflect the light of the sky into the telescope, which will at the same time fully illuminate the objective.

The discussion of the principle of the anallatic telescope, used only with the tacheometer, is deferred to another chapter, wherein subtense instruments are described.