VISUAL JUDGMENTS 



663 



S 



In Brewster's stereoscope (Fig. 305), which is almost invariably used at the 

 present time, the combination of the two pictures is effected by means of two 

 half-lenses with convex surfaces, and their thinner 

 margins directed inwards so that they act as prisms. 

 A vertical black screen is placed between the two 

 lenses. On looking through the lenses towards the 

 point s, the direction of the rays is changed by the 

 prisms so that the image of the pictures B and B' 

 fall on corresponding points of the retinae B into 

 the left eye and B' into the right eye. The ordinary 

 photographs for these stereoscopes are taken by 

 means of a double camera with lenses at a distance 

 apart considerably greater than that between the two 

 eyes. On this account there is actually an exaggera- 

 tion of the solidity of the combined images. 



B' 



W 



When only one eye is used the external 

 world has a much flatter appearance. Some 

 idea of solidity is still gained from the fact 

 that the accommodation has to be altered in 

 order to bring different parts of the solid objects 

 into focus. The judgment is also aided by the effects of light and 

 shade. The inadequacy of such means in default of the stereoscopic 

 images in binocular vision is at once appreciated when we try to 

 dissect or to carry out any fine manipulation using only one eye. 



FIG 305, Brewster'a 

 stereoscope. 



