Fig. 154.—Image on the Retina.
We have in the previous chapter mentioned the effect of light upon the eye and its direction, and when an object is placed very near the eye we know it cannot be distinctly seen; a magnified image is thrown upon the retina, and the divergency of the rays prevents a clear image being perceived. But if a small lens of a short “focal length” be placed in front of the eye, having PQ for its focus, the rays of light will be parallel, or very nearly so, and will as such produce “distinct vision,” and the image will be magnified at pq. In the Compound Refracting Microscope, BAB is the convex lens, near which an object, PQ, is placed a little beyond its focal length. An inverted image, pq, will then be formed. This image is produced in the convex lens, bab′, and when the rays are reflected out they are parallel, and are distinctly seen. So the eye of the observer at the point E will see a magnified image of the object at PQ brought up to pq (fig. 155).
Fig. 155.—The Microscope lenses.
Sir Isaac Newton suggested the Reflecting Microscope, and Dr. Wollaston and Sir David Brewster improved the instrument called the “Periscopic Microscope,” in which two hemispherical lenses were cemented together by the plane surfaces, and having a “stop” between them to limit the aperture. Then the “Achromatic” instrument came into use, and since then the Microscope has gradually attained perfection.
Fig. 156.—Concave lens.
We have so frequently mentioned lenses that it may be as well to say something about them. Lenses may be spherical, double-convex, plane-convex, plane-concave, double-concave, and concave-convex. Convex lenses bring the parallel lines which strike them to a focus, as we see in the “burning-glass.” The concave or hollow lens appears as in fig. 156. The rays that follow it parallel to its axis are refracted, and as if they came from a point F in the diagram. But converging rays falling on it emerge in a parallel direction as above, or diverge as in fig. 158.
Fig. 157.
1. Focus of parallel rays. 2. Focus of divergent rays. 3. Focus of divergent rays brought forward by more convex lens.
The use of spectacles to long or short-sighted people is a necessity, and the lenses used vary. The eye has usually the capacity of suiting itself to viewing objects—its accommodation, as it is termed—near or far. But when the forepart of the eye is curved, and cannot adapt itself to distant objects, the person is said to be short-sighted. In long sight the axis of the eyeball is too short, and the focus falls beyond the retina; in short sight it is too long. In the diagrams herewith fig. 159 shows by the dotted lines the position of the retina in long sight, and fig. 160 in short sight, the clear lines showing in each case the perfectly-formed eye. For long sight and old sight the double-convex glass is used, for short sight the double-concave (fig. 162). We know the burning-glass gives us a small image of the sun as it converges the rays to its focus. But lenses will do more than this, and in the Photographic Camera we find great interest and amusement.
Fig. 158.—Diverging rays.
Photography (or writing by light) depends upon the property which certain preparations possess of being blackened by exposure to light while in contact with matter. By an achromatic arrangement of lenses the camera gives us a representation of the desired object Fig. 163 shows the image on the plate, and figs. 164 and 165 the arrangement of lenses.
Fig. 159.—Hypermetropia (long sight).. Fig. 160.—Myopia (short sight).
Fig. 161.—Concave and convex lenses.
Fig. 162.—Lenses for long and short sight.
To Porta, the Neapolitan physician, whose name we have already mentioned more than once, is due the first idea of the Photographic Camera. He found that if light was admitted through a small aperture, objects from which rays reached the hole would be reflected on the wall like a picture. To this fact we are indebted for the Camera Obscura, which receives the picture upon a plane surface by an arrangement of lenses. In fact, Porta nearly arrived at the Daguerreotype process. He thought he could teach people to draw by following the focussed picture with a crayon, but he could not conquer the aërial perspective.
So the camera languished till 1820, when Wedgwood and Sir Humphrey Davy attempted to obtain some views with nitrate of silver, but they became obliterated when exposed to the daylight.
Fig. 163.—The Camera.
As early as 1814, however, M. Niepce had made a series of experiments in photography, and subsequently having heard that M. Daguerre was turning his attention to the same subject, he communicated with him. In 1827 a paper was read before the Royal Society, and in 1829 a partnership deed was drawn up between Daguerre and Niepce for “copying engravings by photography.” Daguerre worked hard, and at length succeeded in obtaining a picture by a long process, to which, perhaps, some of our readers are indebted for their likenesses forty years ago. By means of iodine evaporated on a metal plate covered with “gold-yellow,” and exposing the plate then in a second box to mercurial vapour, he marked the image in the camera, and then he immersed the plate in hyposulphate of soda, and was able to expose the image obtained to daylight.
Fig. 164. Arrangement of lenses Fig. 165.
But the mode now in use is the “collodion” process. We have all seen the photographer pouring the iodized collodion on the plate, and letting the superfluous liquid drain from a corner of the glass. When it is dry the glass-plate is dipped into a solution of nitrate of silver, and then in a few minutes the glass is ready. The focus is then arranged, and the prepared plate conveyed in a special slide—to keep it from the light—to the camera. When the “patient” is ready, the covering of the lens is removed, and the light works the image into the sensitive plate. The impression is then “brought up,” and when developed is washed in water, and after by a solution which dissolves all the silver from the parts not darkened by the light. Thus the negative is obtained and printed from in the usual manner.
Instantaneous photography is now practised with great success. An express train, or the movements of a horse at full speed, can thus be taken in a second or less. These results are obtained by using prepared plates, and the “emulsion process,” as it is called, succeeds admirably. The mode of preparation is given in a late work upon the subject, and the photographic plates may also be obtained ready for use. Gelatine and water, mixed with bromide of ammonium, nitrate of silver, and carbonate of ammonium, mixed with certain proportions of water, form the “emulsion.” We need not go into all the details here. Information can easily be obtained from published works, and as the plates can be purchased by amateurs, they will find that the best way.
Aside from the art interest in the new plates is quite another, that springs from the fact that it is now possible to take pictures of men, animals, and machinery in rapid motion, thus enabling us to view