Sir David Brewster on New Stereoscopes. 17 



subject, I was led to construct the stereoscope in several new 

 forms, which, while they possess new and important properties, 

 have the additional advantages of cheapness and portability. The 

 first and the most generally useful of these forms is — 



1. The Lenticular Stereoscope. 



This instrument consists of two semilenses, placed at such a 

 distance that each eye views the picture or drawing opposite to 

 it through the margin of the semilens, or through parts of it 

 equidistant from the margin. The distance of the portions of 

 the lens through which we look must be equal to the distance of 

 the centres of the pupils, which is, at an average, 2~ inches. The 

 semilenses should be placed in a frame, so that their distance 

 may be adjusted to different eyes, as shown in Plate II. fig. 1. 



When we thus view two dissimilar drawings of a solid object, 

 as it is seen by each eye separately, we are actually looking 

 through two prisms, which produce a second image of each 

 drawing ; and when these second images unite, or coalesce, we 

 see the solid object which they represent. But in order that the 

 two images may coalesce, without any effort or strain on the 

 part of the eye, it is necessary that the distance of similar parts 

 of the two drawings be equal to twice the separation produced 

 by the prism. For this purpose, measure the distance at which 

 the semilenses give the most distinct view of the drawings ; and 

 having ascertained, by using one eye, the amount of the refrac- 

 tion produced at that distance, or the quantity by which the 

 image of one of the drawings is displaced, place the drawings at 

 a distance equal to twice that quantity, that is, place the draw- 

 ings so that the average distance of similar parts in each is equal 

 to twice that quantity. If this is not correctly done, the eye of 

 the observer will correct the error by making the images coalesce, 

 without being sensible that it is making any such effort. When 

 the dissimilar drawings are thus united, the solid will appear 

 standing, as it were, in relief, between the two plane representa- 

 tions of it. 



In looking through this stereoscope, the observer may pro- 

 bably be perplexed by the vision of only the two dissimilar draw- 

 ings. This effect is produced by the strong tendency of the 

 eyes to unite two similar, or even dissimilar drawings. No 

 sooner do the refracted images emerge from their respective draw- 

 ings, than the eyes, in virtue of this tendency, force them hack 

 into union ; and though this is done by the convergency of the 

 optic axes to a point nearer the eye than the drawings, yet the 

 observer is scarcely conscious of the muscular exertion by which 

 this is effected. This effect, when it does occur, may be coun- 

 teracted by drawing back the eyes from the lenses, and shutting 

 Phil. May. S. 4. Vol. 3. No. 15. Jan. 1852. C 



