252 S. B. VINCENT 



the ophthalmoscope but determined the refraction by retinoscopy 

 and measured the angle of divergence of the optic axes with a 

 specially devised goniometer. Chievitz determined ♦the sensitive 

 area microscopically for many animals, including 20 mammals. 

 Johnson with the ophthalmoscope for 182 mammals and Slonaker 

 for 93 vertebrates including 18 mammals. Harris bases his 

 discussion of binocular and stereoscopic vision upon evidence 

 of complete or incomplete decussation of the optic nerves. He 

 studied the ocular movements and the pupil light reflexes and 

 gathered his evidence from operative experimentation and from 

 examination of normal animals. 



The tabulation includes facts from other mammals for pur- 

 poses of comparison, and those animals have been chosen which 

 have been most used in comparative psychology. A discussion 

 of the data may be useful since there are so many different 

 factors which help to make vision good or bad. 



The color of the fundus is the first thing noticed in an ophthal- 

 moscopic inspection. As will be seen it varies greatly in different 

 orders and species. The significance of this color is as yet but 

 imperfectly understood, although it is thought that it may 

 prove to be in some way connected with the color vision of 

 j the animal. 



Another thing which the ophthalmoscope reveals at once is 

 the shape and size of the optic disc. Large discs and opaque 

 nerve fibres mean blind areas on the portion of the visual field 

 affected and hence must be noted among the facts which con- 

 tribute to poor vision. 



None of the mammals below the primates possess foveas, but 

 there is often a definite sensitive area whose size and position 

 is important for vision. This has been determined by Johnson 

 for a number of forms with the ophthalmoscope and microscop- 

 ically for many forms by Chievitz, Slonaker and others. It 

 seems to be larger in those animals where the divergence of the 

 axes is great and when combined with nearly spherical lenses 

 and large corneas it gives the animal a wide range of vision which 

 may possibly be binocular. Probably considering the mode of 

 life such wide range is of more importance than the sharp focus 

 made possible by a fovea; but the fovea, when it does appear, 

 is accompanied by only slight divergence of the optic axes — a 



