In conclusion, we would observe that the object of taking the lenses as large as an inch in diameter is not that the margins of the lenses may be looked through, but simply because when they are too small their circumferences are seen by the eyes &id form two inter secting circles upon the solid picture. Were it not for this circum stance the lenses need not exceed half' an inch in diameter.
The stereosc,ope for exhibiting transparent pictures, (dia-positives,) is of the same general form as Knight's Cosmomma stereoscope, the the other points being the same as that of the Stereoscope described above.
It now only remains to explain the exact effect of the lenses upon the pictures. Turning to the figure at the commencement of this article ; if a lens of focus F were placed at L, and the eye pressed close to it, a pencil diverging from b would after refraction through the centre of the lens be converted into a pencil of parallel rays, without suffering deviation, so that the eye would see the point b in the same direction as if no lens were interposed, that is along the line LbB, but would have less difficulty in bringing to a focus upon the retina a parallel pencil than one diverging from a point so near as b. When therefore the eye is placed so close to the lens as to see things exactly through its centre there is no magnifi cation. If however the eye be placed at a little distance from the lens it does not look at the side objects exactly through the centre, and therefore the lateral pencils suffer a slight deviation, which in creases the apparent angle ALB, and produces magnification, which may however be counteracted by putting the pictures a little further from the lens than its focal length.
If then we leave out of consideration the unavoidable defects of all lenticular optical instruments, it appears that the form of stereo scope that has been described exhibits pictures, when properly taken, in such a way as to represent objects precisely as they would appear to unassisted vision, so that natural truth is perfectly realized. And the reader will particularly observe that in this form of stereoscope the images are not displaced or caused to overlap by any optical contrivance. How then, it may be asked, is the effect produced Let us examine carefully the pictures upon the card PQ.
In the first place the points a, a,—b, b,—c, c, are upon the same horizontal lines, which is simply because the cameras had their axes parallel, and not converging to a point ; (See remarks on page 65).
In the next place, if we measure the distances aa, bb, cc, we find aa the greatest because A is the most distant object ; bb the next because B is the -next object in point of distance ; and cc the least because C is the nearest object. But all these distances are less than L R. If, however, in addition to the three lamps, a fixed star D were introduced in any part of the picture, the visual rays LD, RD would be parallel, and the points d,d, where they cut the pictures would be at the same distance apart as L and R. If then we join L,R, with points c,c, nearer together than L and R, (and in the same plane with them) the lines Lc Rc meet at a finite distance C ; but if we join L,R with points d,d, at the same distance apart as L and R the lines L d, Rd are parallel, or only meet at an infinite distance. Hence it follows that in mounted stereoscopic pictures the furthest objects are the widest apart, and the nearest objects the nearest together; objects at an infinite distance only being as far apart as the distance between the eyes. These considerations will explain at once why it is that there is no necessity for displacing the images of the pictures by half lenses ; the objects being sufficiently displaced by the pers pective in the pictures themselves, and any further displacement being wrong in principle. The Brewsterian stereoscope has therefore been a step in the wrong direction, and the error has to be pointed out to the public, and the work begun over again.
We have now done with the Lenticular stereoscope. Its defects are those which are inseparable from all optical instruments in which lenses are used, and the pictures are so small that it is impossible to include in them the same amount of detail as in larger pictures. But at the same time for many purposes the smallness of the pictures, involving but a small expenditure of material and permitting the use of a light and portable form of apparatus in which both pictures may be taken simultaneously, is a real advantage.