416 



PHYSIOLOGY 



CHAP. 



our eyes were at the distance of the two outer mirrors, and thus 

 a perceptible stereoscopic effect is obtained. In consequence, a 

 landscape viewed with the telestereoscope seems to be nearer than 

 it is in reality, because under ordinary conditions we have an 

 impression of relief in objects only at a comparatively small 

 distance. In this case the relief gives rise to an erroneous 

 judgment of the distance of the objects. 



The same principle of the telestereoscope underlies the 

 binocular telescope of Zeiss, in which the mirrors are replaced 

 by prisms ; the distance between these is considerably greater 



Kic.. 203. Diagram of Helmholtz' telestereoseopr. 



than that between the two eyes, and the stereoscopic effect is 

 proportionately greater. 



To Dove we owe a useful application of the stereoscope for 

 testing the genuineness of bank-notes. We saw that on looking 

 at two identical images in the stereoscope, they fuse into one 

 simple, flat, two-dimensional image. But if they differ in any 

 detail, the observer is at once aware of the smallest discrepancy, 

 either because the two images do not fuse, at the points of 

 incongruence, or because they appear out of the plane, or in front 

 of or behind it. 



IX. Visual perceptions and spatial representations of the 

 outer world, i.e. the recognition of objects contained in the visual 

 field, and judgment of their size, distance, and characters depend 

 on the sum total of the many and varied visual impressions 



