140 



ESSENTIALS OF PHYSIOLOGY. 



are more familiar, and the relation in space of which to the unfamiliar 

 object can be determined. 



The judgment of solidity is dependent mainly on binocular vision. 

 A solid object not too far away from the eyes gives rise to a slightly 

 different image in each eye, and the fusion of these images in conscious- 

 ness results in the idea of solidity. The same fact is made use of in 

 connection with the stereoscope. Two pictures taken from slightly 

 different points of view are fused by means of prisms, and in this way 

 give the impression of depth which cannot be obtained from a single 

 flat picture. 



Binocular vision is thus of the greatest importance in assisting the 

 formation of judgments of the solidity of objects, and still more in 



estimating accurately the position 

 of those which are close at hand. 

 It follows, therefore, that whereas 

 the permanent loss of an eye 

 involves a certain diminution of 

 the field of vision, it also involves 

 the much greater disadvantage 

 of increasing the difficulty of 

 making the visual judgments on 



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iii 



44. Zijllner'ajlines (Staging 

 frincibles ofvPhysivlogy. )! 



1 



which depend the performance of 

 accurate mechanical work. 



OPTICAL ILLUSIONS. 



It follows from what has 

 been said as to the interpreta- 

 tion of visual sensations that the 

 judgments based upon these are exceedingly fallible, and that this 

 is so is a matter of everyday knowledge. Judgments based upon 

 experience are biassed by that experience, as is well shown by the 

 accompanying illustration (fig. 44), in which parallel lines appear 

 to be alternately convergent and divergent because of the short 

 oblique cross lines. Another simple illustration may be made by taking 

 two straight lines of equal length and drawing divergent lines away 

 from the ends of the one and centralwards from the ends of the other, 

 when the former will appear the longer of the two. 



