EXPLANATION OF OBSERVATIONS 167 



were not astigmatic. We cannot, however, immediately 

 conclude from the fact that an objective movement is 

 imaged as being greater in extent on the retina, that it 

 will therefore be more readily perceived by much less 

 that it will appear greater to, the horse, than would 

 be the case if the lens were normally constructed. The 

 visual percept is not immediately dependent upon the 

 retinal processes, for between the two are interpolated 

 complex, inaccessible nervous processes. Still, Berlin 

 believes that he is justified in drawing this conclusion 

 from a number of relevant considerations. Accepting 

 it, he believes that it would be possible for the horse to 

 perceive movements, that for the human eye. which is 

 not subject to this form of astigmatism, would lie below 

 the threshold. 



This theory, the simplicity of which certainly must 

 make a strong appeal, has been adopted by a number of 

 well-known investigators (Schleich*", Konigshofer *^). 

 If we also could accept it, then Hans's phenomenal power 

 of perceiving the movements of objects would be ex- 

 plained. But doubts arise which restrain us. Even if 

 we were to accept Berlin's view in general, we should 

 still come upon the following difficulties. \ In the first 

 place, it is questionable whether the peculiar form of 

 astigmatism mentioned is indeed as common as he sup- 

 poses.* The references in the literature are exceedingly 



* Since no opportunity was given us to examine Hans's eyes we do 

 not know wliat their condition is in this respect. Though it would have 

 been interesting to know, it would hardly make any difference in the 

 views presented. If Hans should prove to be either far or near-sighted, 

 then, if we are to make any supposition at all, it would be that the defect 

 could not be very great, since near sightedness exceeding 2 or 3 diopters 

 and far-sightedness exceeding one diopter is seldom found in the case of 

 the horse. According to Mr. von Osten, Hans at one time manifested 



