172 THE HORSE OF MR. VON OSTEN 



assertion that it made no difference whether we regarded 

 by means o£ -the speculum the seeming movement of a 

 fixed retinal point, or whether the image of an external 

 moving object is passing over the horse's retina. As a 

 matter of fact, however, these two processes are very 

 different from one another. In moving the mirror, with 

 its small opening we are looking through ever changing 

 portions of the horse's lens, — testing it out, as it were. 

 The horse, on the other hand, sees with all parts of the 

 lens simultaneously, in so far as the lens is not covered 

 by the iris. The arcuate deflection, which is nothing but 

 a registration of the difference in the indices of refraction 

 of the different parts of the lens used consecutively, 

 might thus be formed for the observer using the mirror, 

 but never for the horse. For these reasons we cannot 

 conclude that the kind of astigmatism described can 

 really increase the horse's acuity in the perception of 

 movements. 



Since the light-refracting apparatus of the horse's eye 

 does not offer a satisfactory explanation for the extraor- 

 dinary keenness of visual perception possessed by the 

 Osten horse, we must go a step further and ask whether 

 it may not perhaps be found in the part immediately 

 sensitive to light, the retina. That portion really would 

 seem to be adapted to the perception of movements of 

 minimal extent, and for this reason : it is more than three 

 times as great in extent as the human retina, and the 

 horse's retinal images are likewise larger owing to the 

 position of the nodal point. The cells of the retina that 

 are sensitive to light, the rods and cones, might therefore 

 be correspondingly larger than those of the human eye, 

 without thereby making the whole organ less efficient 

 than the human eye. But the most recent measure- 



