OP ARTS AND SCIENCES. 411 



globules anrl lentiform bodies observed in the cones of birds and rep- 

 tiles ; and by the bright appearance of the points of the cones, as seen 

 from the back of an illuminated retina. This would be necessary, if 

 rays of each color wei-e to be focused on their appropriate disks. The 

 distance between the focus of red and that of violet, as formed by the 

 lens and humors, would then have to be reduced to, let us say, one fif- 

 teenth its estimated extent. This, again, would require such a disper- 

 sive surface as would bring rays, coming to different foci from different 

 directions, into approximate parallelism ; and, finally, a very strongly 

 conyerging, and at the same time dispersive, power. Both these prob- 

 lems are theoretically solvable by the calculus of geometrical optics ; 

 but there still remains some doubt whether such a surface as is required 

 in the first lens could really exist, and whether any known substance 

 combines the high degree of refractive and dispersive power requisite 

 for the second. It is, of course, conceivable that a substance might 

 exist which should refract rays of one end of the spectrum, and trans- 

 mit those of the other unaltered; and it is, again, very probable that 

 each disk has a refractive power of its own. But, fortunately, none 

 of the above suppositions are necessary. We may suppose that each 

 disk responds to its own ray, from whatever direction it comes. In 

 that case, the tardy action of red rays in producing their maximum 

 sensation might be explained, in part, by the smallness of the red 

 disks. 



It is by no means necessary, as was assumed by the critics of Zen- 

 ker's theory, tliat the thickness of the disks should in any way corre- 

 spond to the length of the waves of light, or even that the phenomena 

 of interference should be invoked. The difference may lie solely in 

 size ; it may depend on the angle of incidence ; or, again, there may 

 be such a special sensibility among the disks, and such minute accuracy 

 in the refractive apparatus, that every red ray is thrown directly 

 upon the sensitive point of its own disk, and that even another ray 

 thrown there would seem to have the hue corresponding to the disk. 

 All we want is a ponderable ether ; while the fact that millions of 

 vibrations are necessary before the faintest trace of color can be pei-- 

 ceived sufficiently indicates its tenuity as compared with the medium 

 of sound-waves. 



The number of disks must be, at least, several hundred. Aubert 

 was able to distinguish one thousand hues in the spectrum. Gnblet 

 and Rood believe we can learn to distinguish many million hxxes and 

 shades. So it is probable that we shall here have to resort to the same 

 explanation which Helmholtz gives of the fact that musicians can per- 



