COLOR-VISION AND COLOR-BLINDNESS. 691 



comes at once apparent. In birds, in some reptiles, and in fishes not 

 only are cones distributed over the retina much more abundantly and 

 more evenly than in man, but the cones are provided with colored 

 globules, droplets of colored oil, at their apices, through which the light 

 entering them must pass before it can excite sensation and which are 

 practically impervious to any color but their own. Each globule is so 

 placed as to intervene between what is regarded as the collecting por- 

 tion of the cone and what is regarded as its perceptive portion in such 

 a way that the latter can only receive color which is capable of passing 

 through the globule. The retinic of many birds, especially of the finch, 

 the pigeon, and the domestic fowl, have been carefully examined by 

 Dr. Waelchli, who finds that near the center, green is the predominant 

 color of the cones, while among the green cones, red and orange ones 

 are somewhat si)aringly interspersed and are nearly always arranged 

 alternately, a red cone between two orange ones, and nice versa. In a 

 surrounding portion, called by Dr. Waelchli the red zone, the red and 

 orange cones are arranged in chains and are larger and more numerous 

 than near the yellow spot. The green ones are of smaller size and fill 

 up the interspaces. ISTear the periphery the cones are scattered, the 

 three colors about equally numerous and of equal size, while a few 

 colorless cones are also seen. Dr. Waelchli examined the optical prop- 

 erties of the colored cones by means of the micro-spectroscope and found, 

 as the colors would lead us to suppose, that they transmitted only the 

 corresponding portions of the spectrum, and it would almost seem, ex- 

 cepting for the few colorless cones at the peripheral part of the retina, 

 that the birds examined must have been unable to see blue, the whole 

 of which would be absorbed by their color globules. It would be neces- 

 sary to be thoroughly acquainted with their food in order to understand 

 any advantage which the birds in question may derivo from the pre- 

 dominance of green, red, and orange globules over others, but it is im- 

 possible to consider the structure thus described without coming to the 

 conclusion that the birds in which it exists must have a very acute sense 

 of the colors corresponding to the globules with which they are so 

 abundantly provided and that this color sense, instead of being localized 

 in the center, as in the human eye, must be diffused over a very largo 

 portion of the retina. Dr. Waelchli points out that the coloration of 

 the yellow spot in man must, to a certain extent, exclude blue from the 

 central and most sensitive portion of his retina. 



It is hardly necessary to mention how completely the high differen- 

 tiation of the cones in the creatures referred to — tends to support the 

 hypothesis of Young, that a similar difierentiation, although not equally 

 manifest, exists also in man. If this be so, we must conclude that the 

 region of the yellow spot contains cones, some of which are capable of 

 being called into activity by red, others by green, and others by violet; 

 that a surrounding annnlus contains no cones sensitive to green, hut 

 such as are sensitive to red or to violet only ; and that, beyond and around 



