690 COLOK-VISION AND COLOR-BLINDNESS. 



visible as distinct and separate objects at 8 metres, I can not fairly and 

 distinctly see all three colors until they come within 10° of the center. 

 Beyond 40°, albeit with slight differences between individuals and on 

 different meridians for the same individual, colors are only seen by the 

 degree of their luminosity; that is, they appear as light spots if upon 

 a dark ground and as dark spots if upon a light ground. Speaking 

 generally therefore, it may be said that human vision is only tri-chro- 

 matic, or complete for the three fundamental colors of the solar spec- 

 trum, over a small central area, which certainly does not cover more 

 than 30° of the field ; that it is bi-chromatic, or limited to red and vio- 

 let, over an annulus outside this central area ; and that it is limited to 

 light and shade from thence to the outermost limits of the field. 



The nature and imitations of the color sense in man long ago sug- 

 gested to Thomas Young that the retina might contain three sets of 

 fibers, each set capable of responding to only one of the fundamental 

 colors 5 or in other words, that there are special nerve fibers for red, 

 special nerve fibers for green, and special nerve fibers for violet. It has 

 also been assumed that the differences between these fibers might essen- 

 tially consist in the ability of each set to respond only to light vibra- 

 tions of a certain wave length, much as a tuned string will only respond 

 to a note with which it is in unison. In the human subject, so far as 

 has yet been ascertained, no optical differences between the cones are 

 discoverable ; but the analogy of the ear and the facts which have been 

 supplied by comparative anatomy combine to render Young's hypothesis 

 exceedingly probable, and it is generally accepted, at least provision- 

 ally, as the only one which furnishes an explanation of the facts. It 

 implies that elements of all three varieties are present in the central 

 portion of the retina ; that elements sensitive to green are absent from 

 an annulus around the center ; and that the peripheral portions are 

 destitute of any elements by which color sense can be called into ac 

 tivifcyo 



According to the observation already made, that the highest degree 

 of acuteness of vision is necessarily attended by a corresponding acute- 

 ness of color sense, we should naturally expect to find such a highly 

 developed color sense in birds, many of which appear, as regards visual 

 power, to surpass all other creatures. I need not dwell upon the often- 

 described acuteness of vision of vultures or upon the vision of fishing 

 birds, but may pass on to remark that the acuteness of their vision ap- 

 l)ears not only to be unquestionable, but also to be much more widely 

 diffused over the retina than is the case with man. If we watch domes- 

 tic poultry or pigeons feeding we shall frequently see a bird, when 

 busily picking up food immediately in front of its beak, suddenly make 

 a lateral dart to some grain lying sidewise to its line of sight, which 

 would have been practically invisible to a human eye looking in the 

 same direction as that of the fowl. When we examine the retina the 

 explanation both of the acuteness of vision and of its distribution be- 



