592 



PHYSIOLOGY 



& 



FIG. 301. To show the 

 difference in the images of a 

 truncated pyramid as given 

 by the right and left eyes. 



FIG. 300. The eyes are directed to the point &. A thread hung obliquely at a 

 under these circumstances gives rise to the images shown in the upper figures 

 i.e. two images which do not lie on corresponding points. Nevertheless the 

 thread is seen as single. 



which, are discrepant. Two questions therefore 

 arise : (1) do we see such objects doubled ? 

 (2) if we see a single image only, is it because 

 one of the images is displaced from conscious 

 ness by the antagonism of the other ? An 

 answer is given by the following experiment : 

 a Brewster's stereoscope is taken, the optical 

 arrangement for which is shown in Fig. 302. At 

 B and B' two similar lantern slides are placed which show a view of any 

 distant objects. On looking through the instrument at the point S the direc- 

 tions of the rays are changed so that the images of 



the slides are seen to overlap one another. By shift- | 



ing one of the slides the images may be made to fall / \ 



on corresponding points of the retinae, and they then / \ 



form a single combined picture. In front of these 

 slides are now placed another pair of slides which 

 show the photograph of an index mark. If the 

 indices are adjusted so that they occupy correspond- 

 ing positions in relationship with the objects on the / w \ 

 slides below them, on looking into the instrument 

 it will be seen that these marks appear to lie in the 

 same plane as the distant objects placed on the 

 slides below them. If one of the index marks 

 be moved towards the axis of the instrument, it 

 will be seen on looking into the eyepieces, that the 

 indices now appear to lie in a plane considerably in 



B' 



*c a 



I 



l',n-\\ slrr's 

 Stereoscope. 



