Pictogram

“ It is more instructive if we experiment with pure colours, and so we must resort to our colour patch [Pg 128] apparatus described in Fig. 6. If a small circular aperture about quarter of an inch in diameter be cut in a card, and placed in front of the prism nearest the camera lens (Fig. 32) , the colour patch, instead of being an image of the face of the prism, will be an image of the circular hole, and when the slit is passed through the spectrum we shall have a coloured spot on the screen, on which we can superpose a patch of white light from the reflected beam. ”

“ But as we are able to form a distinct image of near objects, and as we notice when we turn our gaze from far to near objects there is a distinct feeling of muscular effort in the eyes, there must be some means whereby the eye can readily adapt itself for focussing near and distant objects. In a photographic camera the focus can be readily altered, either by changing the lenses, employing a lens of greater or less curvature, or by altering the distance of the screen from the lens. ”

“ The higher powers of the microscope are sometimes used to ascertain the character of inks with which the writing is done, and also to determine the character of the paper on which a signature is written, which at times becomes important. For all practical uses of the microscope in the examination of signatures the range of object enlargement occurring between a three-inch and an inch objective will be found to answer the purpose, as the various powers of the lenses become important in making the analysis. ”

“ Every micrographer has his own views as to the form of an installation to be adopted, and it will therefore be well to give an illustration of a definite apparatus which has been found to give satisfactory results. It consists of a microscope A with a firm base placed in a horizontal position. The microscope can be connected by a tube B with the expanded camera CC, at the end of which is the usual frame to receive the photographic plate. A practised observer can focus on a plate of clear glass by the aid of a subsidiary low-power microscope lens. ”

“ The first is that if the light upon the paper be stronger than that in the apparent field of the microscope, the image will not be well seen, or if the paper be too feebly lighted, it will be difficult to keep the point of the pencil in view. The light from the microscope is thrown into the eye, and the view of the image upon the paper is the effect of a mental act, the eye looking out in the direction from which the rays appear to come. ”

“ Each mesh, therefore, brings the image of the diaphragm to a focus on the negative, but the lens of the camera focusses the picture as a whole, thus the amount of light falling on the different pin-holes will vary in intensity, and hence the dots produced will vary in size, for it is assumed, with good reason, that each dot is built up from its centre and radially expands according to the amount of light acting upon it. ”

“ His camera lens is always used at a fixed height from the ground and forms its image on the plate at an exact focus. The print made from the negative is mounted on a card in a space of definite size, along the edges of which a metric scale is printed. In the way he has worked it out the distance between any two points in the picture can be determined. With a topographical plan and a metric photograph one can study a crime as a general studies the map of a strange country. ”

“ Different Positions of Object and Image.—The principal points are always near the surfaces limiting the lens, and consequently the lens divides the direct pencil containing the axis into two parts. The object can be either in front of or behind the lens as in fig. 10. If the object point be in front of the lens, and if it be realized by rays passing from it, it is called real. If, on the other hand, the object be behind the lens, it is called virtual; it does not actually exist, and can only be realized as an image. ”

“ If we take, for the sake of simplicity, not a picture, but a scale with four or five-tone gradations from light to deep black, the light tones will act more or less on the sensitive photographic plate according to their degree of brightness, but as the rays of light have to pass through the cross-lined screen interposed between the lens and sensitive plate, and as the rays can only pass through the transparent parts and not through the opaque, no homogeneous surface is obtained on the negative, but a tone produced by the cross-lines of the screen. ”

“ The camera, with the lens out of focus, should be pointed to a large sheet of white paper; insert the dark slide, draw the shutter, uncap the lens, and the light reflected from the white sheet through the lens, through the negative upon the sensitive plate, will result, after development, in a transparent positive of the negative.
The time of exposure will, of course, vary and can only be found by experiment. The operations of development, etc., are carried on as for a negative. ”

“ Strong systems produce in the proximity of their back focal plane an image of the scale, which can be inspected with a weak auxiliary microscope, and the length of the visible part of the graduation determined. The ratio of half the length of the visible piece of the scale to its distance from the diaphragm on the stage gives the tangent of half the angular aperture. The sine of this angle is the numerical aperture for dry lenses. With weak systems no auxiliary microscope is necessary, the eyepiece being removed and the scale viewed directly in the tube. ”

“ At the smaller aperture of the lens, caused by closing the diaphragm, the depth of focus would be considerably increased, at the same time much less light would be admitted to the camera. Looking upon our object, under the microscope, as comparable to a landscape, seen on the ground glass focussing screen of a camera, it is obvious that, unless our object has no thickness, and this is impossible, we cannot highly magnify its upper and lower surfaces at one and the same time. ”

“ Then the foci conjugate to the points A B are virtual, and their positions can be found by construction from the consideration that rays through A, B, parallel to the principal axis, will be refracted to F, the principal focus on the other side. The refracted rays, if produced backwards, must meet the secondary axis O A, O B in the required points. An eye placed on the other side of the lens will accordingly see a virtual image erect, magnified, and at a greater distance from the lens than the object. This is the principle of the simple microscope.
The Human Eye. ”

“ That circumstances may and do arise in which a better result is obtained in producing stereographs, when the chord of the angle of generation is more or less than 21⁄2 inches.
2nd. That the positions of the camera should not be parallel but radial.
I certainly thought that I had, as I intended, expressed the fact that I treat the cameras precisely as two eyes, and moreover I still contend that they should be so treated; my object being to present to each eye exactly such a picture and in such a direction as would be presented under certain circumstances. ”

“ In all microscopes the rays are limited, not in the eyepiece, but in the objective, or before the objective when using a condenser. If the pencils are limited in the objective, the restriction of the pencil proceeding from the object-point is effected by either the front lens itself, by the boundary of a lens lying behind, by a real diaphragm placed between or behind the objective, or by a diaphragm-image. ”

“ Now, in the same manner, a lens throws a picture of objects to which it is directed on the retina of the eye, and when that picture is larger than the image made in the eye by the object, without the aid of the lens, it is magnified, or the lens has served as a microscope, so called, from its use in seeing small objects, from mikros, small, and skopeo, to see. ”

“ Any ordinary cabinet card makes a neat mount of convenient size, upon which may be written the name of the object or specimen objective and magnification employed, and any other matter referring to print or negative which may be necessary to note.
A carefully kept note-book is a most important aid to the worker in photo-micrography, and it should contain all his failures and successes alike. Notes should be made of the specimen objective, magnification, exposure, plate and developer, which, carefully studied, will almost certainly enable him to secure a success with each exposure. ”

“ Soon, however, the eye will so adjust itself as to see the pencil and [442] the object perfectly well, and by a little practice the observer will be able to sketch every object as rapidly and firmly as if he were copying a drawing, by means of tracing-paper. The neutral glass is perhaps to be preferred to the camera-lucida, as it is learned more easily, and gives less trouble than that instrument. Its cost is five shillings.
After you have practised yourself well in the handling of the microscope, your ambition will take another step, and lead you to the preparation of permanent objects. ”

“ And in your camera you take a picture on a short strip of celluloid film, or on a glass plate, but in the moving picture machine the pictures are taken on a narrow strip of celluloid film perhaps a thousand feet long.
The camera consists of a narrow box. On one side is a handle, and there is a lens that can be adjusted or focused. Inside is varied machinery, but I will not tire you with a description of it. Sufficient to say that there are two wheels, or reels. On one—the upper—is wound the unexposed film. One end of this film is fastened to the empty, or lower, reel. ”

“ We may picture to ourselves a still more complete resemblance, by imagining that the lens of the camera is single, that we have fixed in front of it a watch-glass, with the convex side outwards, and that we have filled with water the whole of the interior of the camera, including the space between the watch-glass and the lens. The focussing-screen of the camera corresponds with the inner surface of the back of the eyeball, about which we shall presently have more to say. ”

“ As petroglyphs are much more permanent than pictographs on more perishable materials, they are more likely to be preserved from ancient times, but it is probable that the latter were actually of more frequent occurrence.
There is no single system of pictography. Everywhere a figure of a man means a man, and that of a tree stands for a tree, and to this extent pictographs can be deciphered by any one. ”

“ At the point of greatest clearness the image is said to be focused. Such being the principle of the camera, it is evident that in practice the instrument may assume many forms, provided always that it consists of a darkened box or chamber, having a hole at one end for the insertion of a lens or combination of lenses, and at the other a screen, generally made of ground glass, on which to receive the image. ”

“ In the course of a practical investigation (says Mr. Ross) with the view of constructing a combination of lenses for the object-glass of a compound microscope, which should be free from the effects of aberration, both for central and oblique pencils of great angle, I combined the condition of the greatest possible distance between the object and object-glass; for in object-glasses of short focal length their closeness to the object has been an obstacle in many cases to the use of high magnifying powers, and is a constant source of inconvenience. ”

“ When the magnifying power of a lens is considerable, or when its focal length is short, or it is wished to use it with greater precision and steadiness, it should be mounted on a short stand with a tubular stem, with rack-work focussing movement and mirror illumination. Fig. 52 represents a simple dissecting microscope, with a glass circular stage, 41⁄2 inches in diameter, supported on three legs—a handy and useful form of instrument for many purposes. ”

“ The commonest form of print is on paper, although the most satisfactory print from the photographic standpoint is the transparency on glass or celluloid film.
Transparencies.—Transparencies are made by the regular photographic processes of exposure and development, on glass plates or films placed in contact with the negative, or in the appropriate position in an enlarging camera. The sensitometry and the terms used to describe the qualities of plate or film for this purpose are those already given in connection with the general discussion of plates and films. ”

“ He must now learn that so large a pencil of light passing through a single lens would be so distorted by the spherical figure of the lens, and by the chromatic dispersion of the glass, as to produce a very confused and imperfect image. ”

“ The construction of this eye-piece is shown in Fig. 101, a divided scale being cut on a strip of glass in 1⁄100ths of an inch, every fifth of which is cut longer than the rest to facilitate the reading of the markings, and at the same time that of the image of the object, both being distinctly seen together, as in the accompanying reduced micro-photograph of blood corpuscles, Fig. 102. 143
The value of such measurements in reference to the real object, when once obtained; is constant for the same objective. ”

“ At this point, termed the focus, the sensitive photographic material is placed for the purpose of obtaining the required picture.
The great desideratum in a photographic camera is perfect lenses. They should be achromatic, and the utmost transparency should be obtained; and under the closest inspection of the glass not the slightest wavy appearance, or dark spot should be detected; and a curvature which as much as possible prevents spherical aberration should be secured. ”