The magic lantern and its management

AND ITS MANAGEMENT.

There is no optical instrument so well known or so highly held in popular estimation as the Magic Lantern. It is somewhat unfortunate that its old appellation, given to it when it was nothing more than a mere toy, should have stuck to it so long. And more than one attempt has been made by those conversant with its capabilities as a scientific instrument, and as an important aid to education, to give it a name more consistent with its real value. The late Mr. Dalmeyer was, I believe, the first to re-christen it the ‘Optical Lantern;’ and there is evidence that others are adopting the term, and that in time the old name will sink into oblivion.

With regard to the early history of this deservedly favourite contrivance, but little is known. Its invention has been ascribed to Friar Bacon, but the evidence on this point is of the most legendary character. It seems, however, certain that the first to give an intelligent description of it was Kircher—a Jesuit who lived two and a half centuries ago, and who published a work entitled ‘Ars Magna Lucis et Umbræ,’ or ‘The Great Art of Light and Shadow.’ The body of the lantern, as described by Kircher, consisted of a room several feet across, furnished with an opening in which a lens was placed. The source of light was an oil-lamp, and the pictures for exhibition were of the roughest kind. Indeed, we may presume that the effects produced fell far short of those attained by the cheapest toy lantern of our schoolboy days. Even if we search the columns of an Encyclopædia or any similar book of reference of forty years ago, we shall not find any great improvement on the description afforded by Kircher. The magic lantern will there be found described as ‘an optical toy, by which glass pictures executed with coloured varnishes can be thrown upon a wall or screen.’

But from this time a gradual improvement began. First, the old oil-lamp was replaced by the argand burner; then came gas; finally the brilliant lime-light—and some time in the near future, perhaps, will come electricity. The improved means of obtaining light have naturally led to larger pictures being thrown on the screen; for the size of such pictures is governed only by the amount of light available. Thus, supposing we own a modern lantern constructed with one of the improved three or four-wick lamps, and that it will give us a good picture of about six feet diameter, by using the same lantern, and without any change whatever in its optical arrangements, save fitting it with a lime-light jet, the disc can be increased to fifteen feet diameter. A picture of the same size could be produced with the original lamp, provided that the operator retired to the necessary distance from the screen, but the light would be so attenuated that the picture would be but a ghost of what it ought to be. It will therefore be understood that the enlarged image formed by the lantern lens can be made to fall at any point in front of that lens, and may be of any size; but to render it practically available its size, and therefore its distance from the lens, must be regulated by the amount of light at disposal.

During the past few years two circumstances have combined to render the optical lantern more popular than ever: one being the introduction of photographic transparencies representing scenes from the remotest parts of the earth, and the other the construction of metal-bodied lanterns of improved character, and burning mineral oil. As a good paraffine lamp is compared with a farthing rushlight, so is one of these lanterns to the toy of our boyhood. The light given, although it falls far short of the brilliance of the lime-light, is much more intense than could have been hoped for some years ago from oil. But its brilliance is not all due to the paraffine, but quite as much to the careful ventilation and general construction of the flame-chamber. There is, too, no danger in the use of this form of lantern, for the reservoir containing the paraffine is far below the combustion-chamber, and therefore it cannot become unduly heated.

The lenses, too, in this new form of lantern, although placed in the same position as those in the old-fashioned magic lantern, are of a very different stamp. And this leads me to a brief consideration of the optical system comprised in these instruments.

There are two sets of lenses: one being known as the condenser, and the other as the objective. The condenser is made up of two or more glasses, the usual form consisting of two plano-convex lenses placed face to face. The duty of this condenser is to take up as much as possible of the light afforded by the lamp, and to change the direction of the rays so that they are cast through the picture or slide placed in front of it. A glance at Fig. 1. will show how this is accomplished, and how the rays indicated by the dotted lines would be lost if not turned to account by the condenser. The use of the objective, on the other hand, is to magnify the image of the picture, and to present it in as perfect a form as possible on the screen placed for its reception. Its distance from the picture is governed by the distance of the lantern from the screen, and can be regulated to a nicety by the focusing screw attached to it. It is generally acknowledged by all acquainted with the requirements of the lantern that the photographic camera lens (known as the quarter-plate portrait lens) fulfils admirably this duty; and this particular form of lens, which is by no means expensive, is generally fitted to modern lanterns of the mineral-oil type. The condensers vary from four to three inches in diameter; perhaps the best size is three and a half inches. The relative positions of condenser, light, slide, and objective are indicated in the diagram to which attention has just been called, Fig. 1.

The optical lantern always inverts the image of any picture projected by it, an inconvenience readily obviated by placing the slide on the stage upside down. At the same time the picture must have its front turned towards the condenser, not away from it, otherwise every part of the slide suffers reversal on the screen. In some cases this does not matter, but when any wording, such as that on a signboard, for instance, is contained in the picture, it reads backwards. Such accidents can be prevented by a white label, easily seen in semi-darkness, placed in a certain position on the picture. If such a precaution had been observed at a certain exhibition at which I was present, General Garfield would not have been shown standing on his head—not a dignified position for the President of the American Republic.

The first lanterns, constructed to burn mineral oils, were very different to those of modern make, and took their pattern from the cumbrous instruments which before them were provided with colza-oil lamps. The reservoir for the oil was carefully kept in a cistern at the back of the lantern, and bubbled through a pipe to the lamp on the same principle that water is supplied to an earthenware poultry fountain. It was doubtless thought necessary in those days to maintain the paraffine in the cistern at a height equal to or above the wick, as in the case of colza and other heavy oils. But paraffine is more of the nature of a spirit—for it volatilizes very readily—and in modern lamps for lanterns the reservoir is kept some inches below the point of combustion. By the time that the liquid reaches this point, it has become so heated in the course of its passage along the metal wick-holders that it issues as a gas. This can be readily proved by turning down the wicks after the lantern has been burning some time, when the flames will remain above the wick-holders, although the wicks themselves have been turned down quite out of reach. In the older lanterns, too, the glass chimney was thought to be indispensable, but now this has been done away with, and with it the cylindrical wick has also gone. The modern combustion-chamber is made of charcoal iron, and is closed at one end with a thin pane of glass, just outside which the condensing lenses are placed, and at the other end by a silver-plated reflector. The necessary air to promote combustion is admitted by suitable orifices below, and a draught is established by a long iron chimney above.

There is no great difficulty in managing one of these lanterns. The loosely plaited cotton wicks should in the first instance be carefully cut parallel with the wick-holders. After this they do not require much attention. It is a far better plan to remove the charred end of the wicks by scraping them along with the back of a knife, than to be always cutting them afresh. However, any loose strands that may project from the cotton should be certainly snipped off with scissors.

After lighting the lantern—which is best done with a long wax taper—each wick should be turned down quite low, and the combustion-chamber closed. In about two minutes, by which time the front glass and the lantern generally will have become warmed, the wicks should be turned up to their highest; that is to say, to a pitch just short of smoking-point. If the wicks are turned low for any length of time the lamp will be sure to smell. It will also smell if, in the process of charging with oil, some of the liquid has been smeared against any part of the lantern which may afterwards become heated. The best oil only should be used.

I have already hinted at the educational value of the optical lantern, but those who have not experimented with it have but a faint idea of its capabilities in this respect. I am of the opinion that every school and college should possess one, and that both pupils and teachers would soon discover its many advantages. Now that it is manufactured in such a portable and efficient form, and can be so easily managed, there is no possible hindrance to its adoption, unless it be its old name of ‘magic lantern,’ which is associated with things too childish for consideration. But this objection cannot hold good when it is pointed out how many different branches of knowledge can be illustrated by its use.

The schoolmaster of a London Board School once told me of some of his difficulties in teaching such a subject as geography. Most of the children had never seen the sea, and many of them had never even travelled so far as the Thames. What notion could such waifs have of a mountain, a valley, a cliff, or even of a rock? The value of the optical lantern at once becomes apparent here. But better informed children in high-class schools, although they see many places beyond the London streets, can yet reap much advantage from the mode of instruction which I am advocating; and as the range of knowledge increases, so will some such means of illustration be forced upon teachers. The labour and expense of tracing a dozen diagrams on glass, in the way hereafter explained, is considerably less than that involved in drawing a single large diagram for the schoolroom wall. By this easy method the illustrations in any book can be roughly copied and rendered available for an entire class.

In the higher branches of knowledge the lantern can also give its help. Botany, zoology, and natural history generally can be illustrated by diagrams, photographs, and in many cases by natural preparations. The revelations of the microscope can be transferred to the lantern, and the most complex organisms can be seen clearly defined and magnified many hundred times their natural size. It is possible, too, to show by the lantern many interesting experiments in chemistry and electricity which cannot be shown to a large number of persons in any other way. Let me cite for instance the formation of crystals, which can actually be watched in progress, highly magnified, by the simple expedient of smearing a piece of glass with a solution of sal ammoniac and placing it in the lantern. There are several adjuncts which may be fitted to a first-class lantern which I have not space to notice in detail. With the lantern-microscope ordinary microscopic slides can be utilized, and tanks for containing the living denizens of our ponds and ditches may be employed. The marvels of polarised light may be demonstrated with the lantern-polariscope. By another special device called the ‘aphengescope’ and which is made to fit on the lantern, opaque objects can be thrown on the screen. It is possible, for instance, to utilize ordinary photographs or diagrams on card, to show the moving works of a watch, sections of fruit, and many other objects as explained in a subsequent chapter.

The lantern, at the time of exhibition, should stand firmly. It can, if it be a small one burning oil, be placed on a box standing upon a table. I myself prefer to place it on a photographic tripod-stand, and this I effect by screwing on to the top of the stand a base-board upon which the lantern firmly fits. For large lanterns, such as the lime-light biunial, a far more solid arrangement is requisite. My own method is this. The lantern screws on to the top of its travelling-box, at the lower corners of which are sockets to admit four wrought-iron legs. These legs are bent outwards, and what I may call their toes are turned out, and have a hole into which a screw may be inserted and driven into the floor. The lantern is fastened to the box by two hinged pieces at the back, so that the nozzles can be raised to any extent desired.