The book of the lantern

Front matter

F. A. GRAHAM, \ I

I Family and Dispensing! I

(Ehemtst, Jj

Contents

Chapter

I. — The Construction of the Lantern ....

II. — The Optical System of the Lantern

Preface

it is customary for an author to say a few words to his readers by way of introduction, before they become better acquainted with one another, let me briefly state the reason why this book is written, and why I felt some confidence in undertaking the work. The magic lantern has always been one of the most popular instruments ever made. So popular has it been, that children by the thousand recognise its charms, while many of more mature years have a secret hankering after it, which they would fain leave unacknowledged : “ For it is but a toy,” think they, “and we have left toy land behind us since we reached man's estate.” Let me sympathise with these feelings, and own for my part a weakness for pantomimes and fireworks, which weakness I have occasionally the opportunity of indulging, on the plea of taking my children out for a treat. But let me say at once that the magic lantern

Till

PREFACE.

is now no toy, but is recognised as a valuable aid to education far and wide. The reason for this is not far to seek. First, we have to look at the vast improvement in the instrument itself. So long as the greasy, evil-smelling oil lamp was almost the sole illuminant available, and roughly executed daubs in varnish colours on glass the only works of art (?) which could be purchased for the lantern, it did not much signify that the lenses were also of a faulty character, and no better in quality than the “ bulks eye 33 of the nocturnal policeman. But when the brilliant limelight came to be adapted to the lantern, it was at once seen that the capabilities of the instrument were not only much increased, but almost without limit. It would be difficult to say offhand how many persons visited the Royal Polytechnic Institution in the forty years during which it was open to the public ; but it is within my own experience that at one period, at the time of the “ ghost 33 illusion, they came at the rate of two thousand per diem. There is no doubt whatever that the Polytechnic caused this form of amusement to become popular, for the lecturers affiliated to the Institution travelled the country round, and gave similar entertainments in all parts.

There are few branches of science in which the optical lantern cannot be made useful for purposes of demonstration, and as this fact becomes better known, every schoolroom in the kingdom will be provided with one. In every lecture theatre worthy of the

PREFACE.

IX name the instrument is already constantly called upon to illustrate various subjects,, and I venture to state that its use will be greatly increased now that so much attention is being paid to the art of photo - micrography, by which enlarged pictures of microscopic objects can be easily rendered available for projection by means of the lantern. A large class of students can thus at the same moment study the structure of an organism which may be in reality invisible to unaided sight by reason of its minute size.

But during the past few years, the number of those who interest themselves in the lantern and its capabilities has been vastly increased by the sudden popularity of the art of photography. Amateur photographers are now to be found in every town in the kingdom, and they are beginning to find out that there is no better method of showing their friends the pictures which they have taken than by means of the optical lantern. The same instrument, too, as it offers a means of making permanent enlarged copies of small photographs, serves with them a double purpose.

The introduction as illuminants of the hydrocarbons, under the name of petroleum, paraffin, kerosene, &c., has also had its share in the recent development of the optical lantern, for now a few pounds will purchase a better instrument than was procurable at any price twenty years ago.

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X

PREFACE.

With these facts in view, I have long thought that a thorough guide to the working of the optical lantern, the preparation of its diagrams and pictures, the colouring of the same, the production of photo-micrographs, and everything pertaining to lantern work, would fill a vacant place in technical literature. Beyond the two or three shilling handbooks which have appeared, there is nothing of the kind procurable; and in the space available in such books, it is obvious that the various matters to which I have referred cannot be sufficiently dealt with.

Some of the matter here printed has already appeared in The Amateur Photographer , and in The Camera. The author is indebted to the proprietors of both those periodicals for their courtesy in allowing both text and cuts to be reproduced in this work.

T. C. HEPWORTH.

45, St. Augustine’s Road, Camden Square,

London, N.W.

November , 1888.

Chapter I. The Construction Of The Lantern

N that wonderful autobiography of Benvenuto Cellini, which Horace Walpole described as being *' ‘ more amusing than any novel,” we find the account of a weird incantation scene which took place in the Colosseum at Rome. Cellini tells us that he had made the acquaintance of a Sicilian priest who volunteered to initiate him into some of the secrets of necromancy. A meeting was appointed at the Colosseum, where “ the priest, having arrayed himself in necromancer’s robes, began to describe circles on the earth with the finest ceremonies that can be imagined. I must say that he made us bring precious perfumes and fire, and also drugs of fetid odour. When the preliminaries were completed, he made the entrance into the circle ; and taking us by the hand, introduced us one by one inside it. Then he assigned our several functions : to the necromancer, his comrade, he gave the pentacle to hold ; the other two of us had to look after the fire and the perfumes ; and then he began his incantations. This lasted more than an

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hoar and a half, when several legions appeared, and the Colosseum was all full of devils.” It has been suggested that these effects were produced by some form of lantern casting images on the smoke from the burning drugs. Should this surmise be correct, it would refer the use of the instrument back to the early half of the sixteenth century — Cellini having been born at Florence in the year 1500 — and the event spoken of having occurred in his early manhood.

But on careful perusal of the entire account of these supernatural wonders, I feel convinced that no kind of optical instrument can have been used. To produce any remarkable effect in such a large space as that covered by Vespasian’s Amphitheatre would certainly tax the powers of the best modern lantern. Besides which, Cellini was a remarkably clever and observant man, and would probably have detected the employment of any such apparatus. It is far more probable that the priest was aided by a number of confederates, and that these were in

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a reality the legions of devils which so impressed the superstitions mind of the Florentine goldsmith and sculptor. With far more reason might we suspect the use of the' lantern in those manifestations which are said to take place among the so-called spiritualists and their mediums of to-day.

We are certainly on much firmer ground when we ascribe the first conception of the instrument to Athanasius Kircher, the learned Jesuit of the seventeenth century,, who has left so many volumes to testify to the great gifts which he possessed. For in one of these books, Ars Magna Lucis et Umbra , we not only find descriptions and diagrams of numerous optical contrivances (I may note in passing that many of these drawings, redressed and elaborated, appear in modern text-books as new ideas), but several which show that Kircher quite understood the main principle upon which the optical lantern depends. A tracing of one of these rude cuts is given at fig. 1, from which it will be seen that the design to be projected by the lens is illuminated by three candles — the brightest form of artificial light then known — and an inverted image is thrown upon a screen at a distance.

Here w~e have practically the germ of the aphengescope, or opaque form of lantern. But modern writers on the subject, in referring to Kircher, have curiously overlooked this most suggestive drawing, and have given another one from his book, which they erroneously describe as the first form of magic lantern. This I also reproduce (see fig. 2). The description appended to the cut certainly does not bear out that view, but points rather to a means of in-

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creasing tlie light from any lantern by using a parabolic reflector behind the lamp or candle flame. The passage (translated) runs as follows : —

u To construct an ingenious lantern which may show things written at a great distance so that they can be read —

“ Let a lantern be made of the same cylindrical figure

C as you see here represented, in whose base let a concave mirror be placed, having as parabolic a shape as is possible. Within the focus of this mirror let F, the flame of a candle, be fixed, and you will have what is required, for it will shine with such unwonted splendour as to show by night, without any trouble, even the smallest letters when examined by the aid of a telescope. But persons looking at the flame from a distance will think that it is a great fire. If the inner sides of the cylinder are fashioned of polished tin in the form of an ellipse, they will increase the light. But the figure here given will sufficiently show

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the invention. E marks the handle and the opening (or window) ; C the chimney or funnel.”

It is worthy of note that Sir David Brewster, in his “ Natural Magic,” quotes the incantation scene from Cellini at length, and states his conviction that the appearances were brought about by optical apparatus, although he admits that little was known of1 the action of mirrors and lenses until the time of Kircher.

It is obvious, however, that such primitive instruments were of the crudest kind, and can only be regarded as interesting curiosities. Up to within quite reeent times lanterns for projection held about the same relation to the modern instrument as does the bone needle of the cave men to the sewing machine. Like most instruments of precision, the optical lantern is the outcome of many years of patient thought and labour, and is the result of the working of many minds. No individual can be credited with its invention or discovery. The crude idea is, as we have seen, to be found in Kircher’s book, and one improvement has been suggested here, and another there, until we have before us a very perfect optical appliance.

The gradual advance in the instrument very naturally follows the introduction of improved illuminants for more general purposes. The oil lamp was superseded by the argand gas-burner, and this was in its turn supplanted by the whiter and better light afforded by mineral oil, while before this, for the better kind of lanterns, Lieutenant Drummond’s brilliant limelight was quickly adopted as the best for the purpose. - This, too, may possibly, in the near future, give way to the still more brilliant electric arc

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light. But the introduction of mineral oil, in conjunction with the adaptation of photography to lantern pictures, have been the main factors in giving the instrument its present popularity.

The first lantern burning mineral oil, and called the Sciopticon, came to us from America. It was constructed on scientific principles, and was far in advance of anything of the kind before produced. It possessed good lenses and a powerful lamp, the two broad wicks of which were placed edgeways towards the condenser. The lamp was so closed in that it formed a combustion chamber, and burnt the oil under the best conditions. The lantern, however,

Fig. 3.

had its faults. The front glass of the lamp was apt to break, and a dark vertical line was always seen upon the sheet — a line which was in reality the image of the dark space between the two wicks. By adding a central wick, and by making certain alterations in the ventilation of the lamp chamber, Messrs. Newton conquered both these difficulties, and a far more perfect form of lantern has

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been the result. The same makers have, too, made the lamp distinct and separate from the lantern, so that, if required, it can readily be removed, and a lime- jet used in its stead. The form of lamp referred to is shown in fig. 3, both open for trimming and closed as in nse, while a complete mineral oil lantern, of the kind now adopted by most makers, is seen at fig. 4. The

Fig. 4.

management of oil lanterns is so simple, really resolving itself into the necessity for keeping the burning wicks at a correct height, and putting the slides or pictures on the stage provided for them, that no more space need be devoted to this portion of my subject.

The great advantage of using a pair of lanterns is that whilst a picture is being shown by one, another picture is being made ready in the other, and there is no pause or blank screen when the change is made. The so-called dissolving views, which are produced by making this change slowly, made a great sensation when they were first intro-

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duced, perhaps because few knew exactly how they were managed. But they are now so common that many persons consider them rather tiresome than otherwise. Still, they give the operator a ready means of varying his work, if the dissolving apparatus be used with judgment. Thus most beautiful effects can be obtained in landscapes, more especially in seascapes, by using photographic cloud pictures, the gradual blending of one clouded sky into another giving fine aerial and very natural results. In one set of pictures which I prepared to demonstrate the beauties of cloudland, in connexion with a lecture on

Fig. 5.

ballooning, a sunrise picture was made to melt into a sunset picture, and in due time this latter gave place to a moonlight effect. I am convinced that much can be done in this direction if time can be given to the preparation of the pictures.

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Originally for such effects two separate lanterns were used side by side, but now a biunial lantern, with one optical system above the other, is employed (see fig. 5). The modern arrangement is far more convenient for the operator, for the apparatus is compact and every adjustment is within easy reach of his hand. A lantern so constructed generally consists of a strong wooden body, lined with metal, with cells for the reception of the condensing lenses. Openings at the back shaped thus JL allow for the necessary to and fro motion of the lime jets and their trays. Between these openings, on the outside of the lantern body, is fixed the dissolving key or tap, connected by indiarubber tubes with the two jets (see fig. 6). In front of the

Fig. 6.

instrument are the stages for the reception of the slides, and the metal plates to which are affixed the telescopic tubes for holding the objective lenses. These plates are hinged, and their inclination upward or downward to make the two discs concentric on the sheet or screen is governed by milled headed screws. The objectives can by this means be made to slightly approach one another, while the condensers remain fixed. It is obvious that it would be better

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if the condensing iens also moved with the objective so that the optical axis of one shonld agree with the other. This could easily be done by making the upper lantern move on a central pivot, and clamping it with a fixed screw ; but the more faulty and elaborate plan has been adopted by manufacturers, and will probably hold its own for a long time yet.

It is certainly too much the fashion to adorn lanterns with a mass of heavy brass-work. Like any other adornment, the brightly-lacquered brass looks well enough, but represents, to my thinking, a waste both of material and of workmanship, which adds greatly to the cost of an instrument, without adding one jot to its efficiency. Indeed, this brass-work is a positive disadvantage when a lantern has to be carried from place to place by a busy lecturer, and constitutes, not only an inconvenience, but a tax, in the shape of “excess luggage.” This superfluous metal must, I suppose, be looked upon as a custom of the trade, which it is very difficult to break down. It is the same case with the microscope, the delicate brass-work of which often costs more than the lenses, expensive though the latter are. In each case the metal-work represents a convenience in operating the instrument, but much of it could be dispensed with, without in any way detracting from its performance. We may, I think, gain a lesson in the construction of an ideal lantern by examining a modern photographic camera for tourists’ use, where the greatest rigidity is combined with extreme lightness, and metal is used but sparingly. Looking at such an instrument, we find that it must be extended for focus-

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sing purposes, and that its chief expanding part is made of folded leather (concertina bellows fashion). Now, why should not the same arrangement be adopted for the optical lantern ? We here require a similar extension of the front of the instrument, in order to suit the foci of the different lenses employed, and according to the distance and size of the picture which we wish to project on the screen before us. Surely some arrangement of the kind could be adapted to the lantern. If such a change of construction were brought about, and with analogous alterations in other parts, the weight of a double or triple lantern would certainly be reduced to about one-third of of what it is at present. It may, perhaps, take some time for opticians to appreciate this view of the case, and there may be good trade reasons for not making a change of such a radical nature. I am not behind the scenes, and so cannot tell. It might certainly be urged with some truth that there is no need to be in a hurry to alter the present type of lantern, seeing that lightness of construction may soon be brought about by the substitution of aluminium for brass. The production of the former metal is daily becoming cheaper, and as its weight, bulk for bulk, is about one-third that of brass, and as it is strong, not easily tarnished, and in other respects is suitable for the purpose of lantern construction, we may look forward to its adoption in this service.

Where a lantern is used for educational purposes it requires certain additions which are quite unnecessary in exhibition instruments. On the other hand, several ornamental adjuncts which are desirable in the latter

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form of lantern may be dispensed with in tbe instrument designed for tbe lecture or school room. Indeed, in another chapter I point out how a very simple arrangement

Fig. 7.

of lamp and lenses can be made serviceable for educational work. One of the most important additions to an instrument used for teaching is #what is known as the vertical attachment. The object of this arrangement

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is to show certain preparations and other objects which must be kept in a horizontal position during exhibition. “ Horizontal Attachment ” would, therefore, perhaps be a more sensible name for the apparatus — the construction of which can be easily understood by reference to the annexed cut (fig. 7). The round opening in front, four inches in diameter, is the place where the apparatus fits on to the lantern, the lenses, &c., of the latter having been removed for its accommodation. So that the light from the lantern is received by the sloping mirror, and is reflected upwards through a condensing lens, which is placed horizontally. This lens forms a table or stage upon which different slides or preparations can be laid for exhibition. The image is formed by the lens above, and by the prism above that is redirected and cast upon the sheet or screen. It is obvious that some loss of light must result from filtering the rays through so many media, but this cannot be helped. In the chapter dealing with experiments possible with the lantern the use of this vertical attachment will be further alluded to, when its value will be better appreciated.

Double lanterns to burn oil are usually placed side by side, but fig. 8 shows a convenient form of lantern which has been recently introduced, and which is so constructed that one lantern can be detached from its fellow. Thus, when the lime-light is used they are adjusted one above the other, and when the oil lamp is employed they are placed side by side. Mr. Tyler has still more simplified the matter by inventing a biunial lantern which burns oil, although the position of the two lamps is one above the other. This he achieves by the use of a bent chimney,

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which proceeds from the lower lantern and carries away the hot air, so that it cannot influence the lamp above.

Fig. 8.

In the triple lantern (fig. 9) we have three optical systems, and of course three lime-lights. This is the exhibition instrument par excellence , and is commonly used by those exhibitors who may he described rather as entertainers than lecturers. The third lantern is in reality not often used. It is kept in reserve for producing occasional effects while the other two lanterns are at work.

In some respects the optical lantern resembles the king of musical instruments, for the effects which can be produced by it are dependent upon and limited by the number of its parts. If an organ has but one row of keys, the instrument can do little beyond furnishing an accompaniment. With

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two rows its capabilities are much increased, for the organist can use them alternately in contrast with one another, or can combine them. With three claviers, his powers are once more amplified, and he can introduce those changes of musical colour for which our language provides no descriptive word. So it is with the lantern. If it have

Fig. 9.

but one optical system the operator can only use it as a means of showing simple pictures or diagrams. If it have two systems he can produce the popular dissolving views ; and if it be provided with a third set of lenses, he can mingle with the views shown those “ effects,” as they are called, which are so welcome to an audience of young folk, and very often to children of a larger growth.

Chapter II. The Optical System Of The Lantekn

! HE three great essentials of a good optical lantern are the light, the condenser, and the objective, the two latter forming the optical system of the instrument. Theoretically, the light to give the best results should be a mere point of radiance, but unfortunately this is at present almost unattainable. I say almost, because I believe it to be quite within the bounds of possibility to construct an electric arc light which shall fulfil all the conditions as to steadiness, uniformity of action, and maintenance of a fixed position, which are required in lantern work. Such a light has not yet been found, but when electricity becomes more general as a source of illumination in our cities and towns, — as it surely will, — an arc light or regulator of the required description will, I feel confident, soon be forthcoming. The invention of such a contrivance, when the means of obtaining the requisite current to feed it is so limited, would at present have the disadvantage of coming before its time. The electric arc light has been used more than once experimentally in the lantern, with the most promising results.

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The best source of illumination for the lantern is, — when we exclude electricity for the reasons just indicated, — the lime-light. It has the good qualities of intense whiteness, steadiness, ease of management, portability, and although not a point, its area of radiance is not much greater than the space covered by a pea. I shall describe in detail the method of its production and management later on.

Having then a convenient form of intense light, we have next to consider the means of using it to the best advantage. We must start with the acknowledgment that in enlarging the image of a picture we must sacrifice a large amount of light. But by using properly-constructed lenses, we can make this loss as little as possible. As already pointed out, the optical system of a lantern consists of two distinct parts, — the condenser and the objective lenses.

As many of my readers may be quite unacquainted with the matter under consideration, I will point out why this double system is necessary, and describe the work performed by each set of lenses. In the annexed diagram, fig. 10, L represents the lime cylinder, with rays of light emanating from it and illuminating the picture P, which

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we require to show in an enlarged form. O is the objective lens by which this enlargement is to be brought about ; the sheet or screen upon which the picture is projected being supposed to be far away to the right. With such an arrangement of parts what should we see on that screen ? In the first place we should have but a very feeble light, for as will be seen by reference to the diagram, most of the luminous rays are wasted altogether, only the central ones proceeding through the lens 0. It will be seen also that these rays go through the central portion of the picture only, and that therefore only this part can be projected on to the distant screen. So that as a result of our first efforts at lantern projection, we get an indistinct and badly-lighted portion of a picture presented to us. How can we remedy this state of things ? Obviously, the thing to be done is to cause more of the rays from our light source to be utilised, and this can be brought about by placing between that light and the picture a lens which shall condense the light upon that picture, and which is therefore known as the condenser.

In fig. 11 we see a repetition of the diagram, fig. 10, with p the addition of a condenser, shown in this case, for the sake of simplicity, as a single lens. Referring once more

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to this imaginary picture upon our screen, we now see that it is complete. It is no longer the central fragment of a design, but covers the sheet, and is equally illuminated. We can at once see the reason for this welcome change by looking once more at our revised diagram, fig. 11. The rays of light instead of being wasted in illuminating the inside of the lantern box, are refracted by the lens which we have introduced, and are bent towards the objective.

In my diagrams, for the sake of simplicity, I have represented each lens as consisting of a single piece of glass of plano-convex form. Such lenses are found in toy lanterns of the cheapest kind, but are, as might be expected, extremely faulty in performance. In the enlarged picture, they give, owing to their total want of correction, badly-defined margins, curved lines, and fringes of colour.

Having now seen the purpose fulfilled by the condensing lens of the optical lantern, let us further consider its best form, and let me at once correct an error into which a purchaser is likely to fall. I have sometimes heard the possessor of a lantern speak somewhat boastingly of his instrument as one with 5, or perhaps 6 -inch condensers, the more ordinary size being 4 inches, and often only 3 \ inches. For lantern projection, any size over 4 inches is a positive disadvantage, and instead of representing a gain, means really a great loss of light. The reason for this is readily seen. What may be called the standard size for a lantern picture is 3 inches in diameter (the entire slide with its margin measuring 3J inches). If the picture be framed in a circular 3-inch mount, a 3^-inch condenser will amply illuminate it. If, however, the orifice of the

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mount be square or cushion-shaped, such a small condenser would infallibly cut off the corners of the projected image, and for such pictures, therefore, a 4 -inch condenser is necessary. But the smaller condenser transmits more light, for the reason that being of shorter focus, the lime cylinder is brought nearer to it. Some well-known exhibitors, seeing the great importance of getting all the light upon the screen that they possibly can, use nothing but 3J-inch condensers, but this obliges them to confine themselves to round pictures. On the whole, I prefer myself the 4 -inch condenser, for although I lose some light, I can make use of any shaped pictures or diagrams which may be required. (If the lantern be used for photographic enlarging purposes, then a large-sized condenser is the great thing needful, at least, we must have one of a size large enough to cover the negative which has to be enlarged ; a quarter-plate size necessitating a 5-inch condenser and so on. But in that case brilliancy of image is quite a secondary matter, and is compensated for by extension of time occupied in the operation.) It must also be borne in mind that, quite apart from the question of focal length of the condensing lens, there is a limit to the near approach of the incandescent lime cylinder towards it, for the intense light is naturally accompanied by a fervent heat, which will surely crack a lens if it be too near to it. In the chapter on the working of the lime-light, precautions against this accident are fully dealt with.