PHYLOGENY, CHEMISTRY OF FILTERS 205 



we would see without our filters, by determining how much more sharply 

 we see with additional yellow filters placed outside the eye. The fact that 

 we can gain appreciably in visual acuity by that means — as any expert 

 rifleman knows — shows, by analogy with the squirrel species of various 

 brightness-preferences, that the human eye is not purposed for use in 

 the very brightest of light. The prairie-dog, which prefers such light, 

 has his intra-ocular filter already so deeply colored that any extra-ocular 

 supplement to it would probably take more away from his vision than 

 it conferred. We are also led to consider man as not inherently strictly 

 diurnal by the fact that the ground-squirrel, the bird, or the diurnal 

 reptile unblinkingly tolerates intensities which force us to screw up our 

 eyelids or run for a pair of dark goggles. 



