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In addressing you on the subject of photography, there are then two wholly distinct points of view from which I may look at the subject. Either I may consider the scientific principles on which photography rests, and how these truths are the rules of our art; or I may attention to the application only of our rules to the production of photographs, paying little heed to their existence or general truth as principles. As, for instance, in regarding the steam-engine, we might, on the one hand, examine the mechanical principles and physical laws on which the machine works; or, on the other hand, we might confine our attention to the details of construction, or the stoking and driving.

I have preferred, and I hope I am justified in so doing, the former course; I believe it will be more interesting generally. Both are equally important; indeed the latter, the examination of details, is perhaps the most important, as we cannot produce a photograph, with a perfect knowledge of principles, without a thorough knowledge of details; but at the same time it would be impossible in one lecture to treat of both. The details are most fully given in many hand-books on photography, such as Hardwich's, and given from the result of experience ten times greater than I can boast of, and far better than I can give. But at the same time from the aim of these works—to enable the reader to become a practical photographer-it is difficult to obtain at first a general knowledge of the principles from them, as the statement of the principles is .constantly interrupted by a description of manipulative details. I have, therefore, disregarded these details: it would be very wearying and quite useless, to give a number of photographic formulæ without at the same time actually manipulating a photograph ; while I hope a slight general knowledge of the principles may be interesting and useful to many officers, if at any time they wish to take up the subject practically.

In conclusion, I propose to mention some applications of photography to military purposes: these are at present not very numerous; but these instances will, I trust, protect me from the appearance of assumption in lecturing on a subject which many non-military photographers who have paid it far greater attention would be better able to handle. I have undertaken the subject with great hesitation, and only because I believe that it may often be applied with great advantage to the service. " Photography includes all processes in which

pictures are taken by the action of light.

These processes are very numerous: in some different substances are used for the sensitive film, in others the same substances are employed under different conditions. As it would be very confusing to revert from one to the other, I propose taking the ordinary or most generally employed process as a type, and explaining it.

We will therefore take the wet collodion process, and its accompanying printing process on paper.

It is not my intention to assume that you know anything of photography, and as there may be some present who have never watched a photograph being taken, I may be permitted to define, as it were, what I am going to explain, and roughly to show the various stages of a photograph: the effect of each step I hope afterwards to explain.

I have here à camera. It must not be fancied that there is any peculiar photographic action or virtue in a camera. I cannot now explain

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the optical principles of a lens, but I think the action of a camera may be readily understood from that of a telescope or opera glass.

When we look through a telescope we have a distinct image, slightly magnified, of the objects on which the telescope is directed; that is, the eye, instead of looking at the objects directly, is examining the picture of the objects formed by the telescope. Now, the telescope consists of two distinct portions; the object or front glass forms the image or picture, and the eye-piece magnifies the image in order that we may examine it the better. In the camera we do not employ the magnifying or examining portion. If we place a screen in its place, that is, where the rays come to a focus we have still a picture of the objects formed. Now this, the telescope with the eye-piece removed, is the counterpart of the camera; instead of the tube of the telescope we have a box, with the object-glass fixed in it; the picture is then formed on this ground-glass screen. Having formed our picture, that is, focussed the camera, and so on, in place of the ground-glass screen we place the sensitive film; the picture is formed on it, and by the action of the light impressed on the sensitive substance. (By means of diagrams, figs. 1 and 2, and an explanation which it is unnecessary to enter into fully here, as it may be found in any elementary treatise on optics, it was shown how the image is formed in a camera.

How, for instance, in a darkened room, if a small hole be made in the shutter, an image of the illuminated objects in the street will be formed on the opposite wall, but reversed in consequence of the rays of light crossing. How, by diminishing the opening, the image becomes better defined at the expense of its illumination, till perfect definition could be obtained by the admission of only one ray.

This simple form of camera not being adapted to our purpose where strong light is required, how the difficulty is overcome by the lens, which catching, as it were, a large number of diverging rays from each point of the object, bends them together to one point-the focus, except in so far as spherical aberration comes into play, by which means we obtain a welldefined and at the same time well-illuminated image.

And further, that rays coming from points at different distances are not brought to a focus at the same distance from the lens, that is, in the same plane; and hence distortion of hands and objects in the foreground.)

I have here a piece of white paper with the word "object" printed on it in black.

Suppose I photograph it.

From all the white portions rays of white light are transmitted on to the lens, and brought to a focus in the camera on the ground glass.

From the black portions, speaking roughly, there are no rays: black being the absence of light. So that, if we place a sensitive substance in the place of the screen, we have light acting over some parts—those that correspond to the white parts of that paper, and no light on those portions which correspond to the black letters.

The light having rapidly produced a certain change in the portions on which it has acted, I remove the plate to the dark room, continue the action by applying certain chemicals, and having thoroughly changed or reduced these portions that have been acted on, I apply what is called a fixing agent, that is, I remove all the unchanged portionsthose portions which correspond to the black letters.

The action of the light and chemicals has been to make the portions which were semi-transparent before wholly untransparent. I have then a glass with a coating all over except where we have the word "object" (reversed), with the portions of the glass that answer to the black letters transparent, the other portions opaque and dark.

If we hold this up and view it by means of transmitted light, what was dark in the original is here shown transparent. This then is called a “ negative." If we place this on sensitive paper and expose it to the light, the light will pass through these transparent portions, and, acting on the sensitive paper, blacken them—this will be a positive; and from this one negative we can produce any number of positives.

The action is the same if, instead of the printed word, we had a landscape or a portrait to take; for, from the whitish skin, rays of light would be transmitted, and from the hair and eyes, if these were black there would be no light, except in so far that there are bright spots of light reflected from the moisture of the eye, and patches of light from the hair, especially if this is artificially glossy; however, to this I shall again have occasion to refer-I mean to the reflected light, not the glossy hair.

If, now, instead of using this glass negative as a matrix from which to obtain a number of copies, we only care to have one picture, all that is necessary is to blacken up the back. The black shows through the transparent portions, and the other portion, which is metallic to a great extent, appears white by reflected light. This is a glass positive. They are often met with as cheap photographs. The small portraits now so largely executed for sixpence and a shilling, are done in this manner. There is a slight but not a great difference in the details of manipulation of the two processes.

In the ormer case, when we wished to produce a negative, the deposition of silver is made as opaque as possible by the aid of organic matter; whereas in the latter case it is required to be as metallic and bright as possible, as you will easily see; but this metallic deposit would not be opaque enough to produce a good positive by printing through; the light would pass through the dark parts, and what should be the light parts would become more or less dark. In fact, a good positive is a bad negative, and vice versa. I may mention that a daguerreotype is of the same class as the positive on glass: one picture of the object is produced by an exposure of the metal plate in the camera, and it cannot be multiplied. Hence the disadvantage of this process.

There are other processes, such as the Calotype, or Talbotype, so called from the inventor, Mr. Talbot. These are of the same class as negatives on glass; but the negative is taken on paper and the paper is then made transparent by waxing, and then printed through. To Mr. Talbot is, I believe, due the idea of taking negatives, that is, taking a photograph which may act as a matrix from which to obtain any number of impressions.

In taking a photograph, then, there are two actions: one, the action of light, the other a chemical action. We take advantage of natural laws and use them for our purpose; and as Bacon, in one of his aphorisms, says: “ The knowledge and power of man are coincident; for, while ignorant of causes, he can produce no effects."

On this principle we will examine these two causes and effects; and first the laws of light, though necessarily only in a very cursory manner, and only those points with which we are more particularly concerned.

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