As a set-off against these disadvantages, it has been averred that in the reflecting stereoscope we can use larger pictures, but this, as we shall shew in a future chapter, is altogether an erroneous assertion.
Description of the Lenticular Stereoscope.
Having found that the reflecting stereoscope, when intended to produce accurate results, possessed the defects which I have described, and was ill fitted for general use, both from its size and its price, it occurred to me that the union of the dissimilar pictures could be better effected by means of lenses, and that a considerable magnifying power would be thus obtained, without any addition to the instrument.
Fig. 11.
If we suppose a, b, [Fig. 11], to be two portraits,—a a portrait of a gentleman, as seen by the left eye of a person viewing him at the proper distance and in the best position, and b his portrait as seen by the right eye, the purpose of the stereoscope is to place these two pictures, or rather their images, one above the other. The method of doing this by lenses may be explained, to persons not acquainted with optics, in the following manner:—
If we look at a with one eye through the centre of a convex glass, with which we can see it distinctly at the distance of 6 inches, which is called its focal distance, it will be seen in its place at a. If we now move the lens from right to left, the image of a will move towards b; and when it is seen through the right-hand edge of the lens, the image of a will have reached the position c, half-way between a and b. If we repeat this experiment with the portrait b, and move the lens from left to right, the image of b will move towards a; and when it is seen through the left-hand edge of the lens, the image of b will have reached the position c. Now, it is obviously by the right-hand half of the lens that we have transferred the image of a to c, and by the left-hand half that we have transferred the image of b to c. If we cut the lens in two, and place the halves—one in front of each picture at the distance of 2½ inches—in the same position in which they were when a was transferred to c and b to c, they will stand as in [Fig. 12], and we shall see the portraits a and b united into one at c, and standing out in beautiful relief,—a result which will be explained in a subsequent chapter.
Fig. 12.
The same effect will be produced by quarter lenses, such as those shewn in [Fig. 13]. These lenses are cut into a round or square form, and placed in tubes, as represented at r, l, in [Fig. 14], which is a drawing of the Lenticular Stereoscope.