VISUAL JUDGMENTS 



593 



FIG. 306. 



but as single, and a very correct impression may be obtained of its size 

 and position. In this judgment or interpretation a number of separate 

 processes must be involved. The fact that the eyes are not accommodated 

 for the thread evokes at once the associated movements which would be 

 necessary, if time allowed, to bring the non-corresponding images on to 

 corresponding points of the retina. We do not therefore assume a double- 

 ness of an object, even when its images fall on non-corresponding points, 

 unless our gaze is voluntarily directed on the object. 



THE J UDGMENT OF SOLIDITY. The fusion of non- corresponding images 

 to a single visual conception is responsible for our appreciation of solidity. 

 If we look at any object which is not too far 

 from the eyes, first with the right and then with 

 the left eye, we shall see that the images in 

 the two eyes are not identical. If we look, for 

 instance, at a truncated pyramid, we shall see 

 with the right eye rather more of the right side 

 of the pyramid and with the left eye rather more 



of the left side (Fig. 306). If these two images a 

 and b be so arranged that they fall on corre- 

 sponding points of the two retinae, there is no 

 confusion of sensation, but the resulting impres- 

 sion is that of a solid object. This is the principle 

 involved in the stereoscope, which allows us to 

 combine two images of this character so that they 

 fall on corresponding points of the two retinae. 



I 



W \ 



In Brewster's stereoscope (Fig. 307), which is almost 



invariably used at the present time, the combination of the 

 two pictures is effected by means of two ha If -lenses with 

 convex surfaces, and their thinner margins directed in- 

 wards 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 photo- 

 graphs 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 exaggeration of the solidity of the combined images. 



When only one eye is used the external world has a much flatter appear- 

 ance. Some idea of solidity is still gained from the fact that the accommo- 

 dation 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. 



B' 



A 



FIG. 307. Brewster's 

 stereoscope. 



