THE FIELD OF VISION 27 



Horse . 6 to 100 



Ox 7 to 100 



Calf . .... 5. 5 to 100 



Sheep . 7 . 6 to 100 



Pig 8 to 100 



Dog . .. 7. 7 to 100 



Cat . 10 to 100 



Rabbit 8 to 100 



Man 4. 2 to 100 



As pointed out by Lindsay Johnson; 3 with a large spherical 

 lens, rays falling obliquely on the cornea will be brought to 

 a focus with comparatively little distortion, the image being 

 received upon a surface which is nearly equidistant from the 

 nodal points. This is a matter of considerable importance 

 in securing a large monocular field for moving objects, in 

 animals with a retina having a widely-extended sensitive 

 area. In the Primates, who require acute vision for small 

 fixed objects, and who have developed a highly sensitive, 

 small, central spot for this purpose, it is essential that the 

 image of the object looked at should be accurately focussed 

 on that spot only. It is advantageous, in the interests of 

 binocular vision, that the images of objects formed on parts 

 of the retina peripheral to the central spot should be but 

 dimlv seen. Hence we find in the Primates the cornea 



*/ 



and lens so constructed as to sacrifice the interests of the 

 monocular field in favor of those of acute central fixation. 



The alteration in the shape of the lens in Primates, so 

 that instead of being nearly spherical it is considerably 

 flattened antero-posteriorly, has considerably diminished 

 its refractive power. To compensate for this to some extent 

 we find that the refractive power of the cornea has been 

 considerably increased. The following interesting table 

 has been constructed by Kalt and Dufour; 4 it shows that 

 in the eyes of man and birds, which have flattened lenses, 

 the refraction of the cornea relative to that of the lens is 



