VISION 399 



ment accommodation. The mind is extremely suspicious of 

 the veracity of its newsagents. Disparateness of images, con- 

 vergence of the eyeballs, shifting of accommodation for the 

 various levels of an object in space, should be indisputable 

 evidence of solidity or of hollowness. Conversely, the absence 

 of either factor should be conclusive proof of flatness. But 

 the mind does not trust to isolated sensations ; it looks for 

 associations of sensations. When the finger hints, " I could 

 touch that sharp point," it is useless for the eye to aver that 

 there is no point to be touched. 



If two exactly similar photographs are placed in a stereo- 

 scope, the fact that the eyes are not converged gives to the 

 common picture an appearance of depth, notwithstanding the 

 fact that corresponding points on the two retinae are stimu- 

 lated. If the two photographs have been taken, as they 

 should be taken for this purpose, with a double camera, the 

 disparity of the retinal images immensely enhances the impres- 

 sion of solidity. 



It is impossible to exaggerate the dependence of sensation 

 on judgment. At birth a child commences the long process 

 of education which enables it to associate the sensations 

 derived from its retinal images with the movements which 

 place it in contact with things. It discovers that, when it is 

 necessary to make the eyes converge, the object is near at 

 hand. It also associates the voluntary action of contracting 

 its ciliary muscle with nearness. Uncon verged and unaccom- 

 modated eyes come to mean distance. So, too, do indistinct- 

 ness due to absorption by the atmosphere, blueness due to the 

 same cause, a small image on the retina. But there are 

 obvious limits to its power of ascertaining the distance of an 

 object, and therefore, conversely, of its power of estimating 

 size. We have no idea of the size of the retinal image of the 

 sun. Very few people would be prepared to believe that the 

 angle which the sun subtends with the eye barely exceeds half 

 a degree. (The first finger, viewed in profile, at arm's length, 

 covers one degree of arc.) A disc of paper of the right size, 

 placed at the right distance, looks far too small to represent the 

 sun. The most brilliant of orbs bulks larger than this in our 

 minds. Everyone who for the first time looks at the sun 

 through well-smoked glass, or, better, through a flat-sided 



