will at length be seized with the eyes, which, having once fairly got, they will hold with considerable tenacity. The optical grasp of the stereoscopic picture, however, may, when once obtained, be greatly increased in power, and made more permanent, by placing a card, or the hand held edgewise, between the eyes and the slide, forming a division between the two photographs; not only is the natural tendency of the eye to return to its instincts thus lessened, but the two external pictures are obliterated, and the true binocular image only retained, every detail standing out in perfect and complete relief. The following diagram explains the rationale of the previous process at a glance:
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Here A A are the observer’s eyes, B the distant object, D B and E B lines of sight passing through the holes in the slide C. By this simple means each eye has been made to see only one picture, and that the one which the same eye would see in a natural inspection of the original objects represented. The conditions of binocular vision being thus complied with, the coalescence and consequent solidity of the photographs follow as a matter of course. The holes described will not be found requisite in more than one slide, for when once the artificial habit of converging the optic axes on a distant point is formed, the effect may be as readily obtained with an opaque picture as with one rendered transparent by the perforations. In any case of difficulty, however, such as combining subjects composed of many and minute, instead of few and prominent points, recourse must be had to the drilled slide, and the proper convergence having with its assistance been obtained, the refractory photograph may be slid over the prepared one, when success is pretty certain.
Before closing this paper, another and simpler mode of attaining the same results may be described. This, though easier of accomplishment, is less convenient for general application for reasons to be mentioned presently. It has its special merit, however, in being the only ocular stereoscope capable of combining larger pictures than the common photographs.
Bearing in mind what has previously been said in connection with axial convergence to a point beyond the picture, it will be clear on glancing at the sketch below that the selection of a point of convergence situated between the picture and the eyes will also be the means of presenting one image to each eye, .png)
but their relative positions will be reversed. Referring to the diagram, A A are the eyes, B the slide, E F and C D the lines of sight crossing each other in the point G. To carry out this arrangement, take a photograph, and holding it about twelve inches from the face, place a forefinger just half way between the eyes and slide; then look steadily at the finger tip, and, allowing the eyes as before to wander towards the picture, while they keep true to the convergent angle, the two views will be seen gradually to cover one another, and finally coalesce, presenting once again a central image; this time, however, not in relief. Convexity has changed to concavity, and things sink back where they should stand out. The reason is simple, but sufficient, as we just now hinted, to render this, the easiest method of producing ocular stereoscopy, practically useless. Although we have succeeded in combining the pictures, the change thus effected in their relative positions is of course fatal to the results. The right hand photograph (contrary to the natural state of things) has been presented to the left eye, and vice versâ. Now cut the slide in two, make its left side right, then combine as before, and again the sensation of complete relief will be produced; but since all ordinary stereoscopic slides are made for an instrument which insists on right being always right—and no one will care to divide all his pet pictures in this remorseless way—I think it will be best to substitute the perforated slide and patience for the scissors.
Method number two, however, can, as we said before, be used with larger photographs, properly arranged as regards reversed position, than number one. In the first, the point of convergence being beyond the picture, it is manifestly impossible to combine photographs whose centres are farther apart than two and a half inches, that being the average space between the eyes, inasmuch as when this dimension is exceeded convergence of the optic axes must of course cease and divergence begin. When this occurs the lines of sight recede from instead of approach each other, and might be extended indefinitely without meeting. Thus the point at which the pictures coincide being annihilated, their combination into a single image becomes impossible. In the second process, however, this is not the case, the lines of sight being capable of almost any amount of divergence when the point of intersection is brought between the picture and the eyes.
In the above remarks I have touched only on one or two out of many interesting points in connection with the peculiar phenomena incidental to binocular vision. Those who wish to go farther may readily find able guides to its deeper mysteries. This paper aims only at opening a few eyes to the sense of an interesting and not wholly useless power which they possess, and thus putting the ocular stereoscope—the oldest, cheapest, and best form of the instrument—in the hands of all who may think a little time and patience not too high a price to pay for its possession.
SIR JOSHUA’S PUPIL.
A young apprentice with very little heart in the study of his craft, after the manner of young apprentices, toiling in a watch and clock-maker’s shop in the town of Devonport, heard one day the fame of great Sir Joshua’s achievements in
