COLOR-VISION THEORY OF OIL-DROPLETS 193 



The ingenious color-vision theory of oil-droplet function falls to earth 

 under several blows: the number of oil-droplet colors does not in fact 

 correspond to the range of the bird's spectrum, which is now known to 

 be co-extensive or even a little wider than our own. Lizards have a com- 

 plete color-vision system, yet have only yellow oil-droplets. There are 

 vertebrates far below the birds — the fishes — that have color vision with- 

 out benefit of oil-droplets, which could then scarcely be considered a 

 primitive device for hue-discrimination. Most important of all, it has 

 been known (though almost forgotten) for decades that the cones of 

 the bird fovea contain only yellow droplets, the red ones stopping at 

 the margin of that all-important retinal pit. This demonstrates not only 

 how wholly illogical it is to suppose that the bird would be able to per- 

 ceive only yellow in the fovea, and all other colors only outside it, but 

 also that the different colors of droplets are of unequal importance and 

 have different uses, not one common function. The exclusively yellow 

 droplets in the avian fovea line up with the yellow filters, whether com- 

 posed of oil-droplets or not, of all other vertebrates. Yellow droplets 

 appeared first in evolution, in lower vertebrates; and where the oil-drop- 

 lets are decadent, as in nocturnal birds, some yellow ones may persist 

 but no red ones ever occur. The red filters of birds and turtles can be 

 temporarily ignored while we consider what the much more common 

 yellow filter may do for photopic vision. 



Yellow Filters and Chromatic Aberration — The image formed by 

 the natural dioptric system of the eye does not lie in a single plane or 

 spherical surface, even when the object is a plane or a curved surface 

 concentric with the eye. The image has thickness, owing to aberration 

 which is of two kinds, spherical and chromatic. Spherical aberration 

 results from the failure of the cornea and lens to bring parallel rays to 

 a single point, and since it is chiefly caused by the improperly curved 

 peripheral portions of the corneal and lens surfaces, it is effectively 

 combatted by the pupil which acts as a 'stop'. When the refractive power 

 of the lens is increased in accommodation, the pupillary aperture auto- 

 matically contracts to afford the smaller stop which is then demanded. 

 Chromatic aberration is due to the fact that the different wavelengths 

 of white light are not all bent to the same extent when they are refracted 

 at boundary surfaces. The refractive index of a substance is thus differ- 

 ent for each wavelength — it is this phenomenon of 'dispersion' which 

 makes it possible for a prism to form a spectrum by sorting the 'colors' 



