90 ELECTROMAGNETIC RADIATIONS AND MATTER 



In spite of the credence placed in the Young-Helmholtz three-pigment 

 theory of color vision, there is no direct evidence that three pigments exist 

 in the cones. There is direct experimental evidence for two, however; this 

 will now be recalled. Protanopes (color-blind people) cannot distinguish 

 green from red. By measurement of the intensity of the light reflected from 

 the retina as a function of incident wavelength on protanopes, it has been 

 shown that a definite absorption by a pigment, given the name "chlorolabe," 

 takes place with maximum at about 5400 A. 



Now the protanope can see green, but not red. This fact means that a sec- 

 ond pigment, given the name "erythrolabe," is missing in the protanope. 

 Difference spectra (unreliable) of two pigments in the normal fovea (collec- 

 tion of cone cells) show that the maximum absorption of the second, or miss- 

 ing, pigment is about 6000 A. Thus there is good knowledge of one pig- 

 ment, the chlorolabe, and knowledge of the existence of a second, erythro- 

 labe. There is no experimental knowledge of a third in cones. But, of course, 

 Land's new work indicates that only two are really necessary, one sensitive 

 above and one sensitive below 5800 A. The two pigments discussed have 

 these qualifications. Recall that the optical density maximum for rhodopsin 

 is at 5000 A. 



What the relation is between the excited pigment molecule and the color 

 perceived is poorly known. Experimental approaches include that of meas- 

 uring the electrical signals in the optic nerve (the electroretinogram, ERG) 

 during stimulation by light, the reflection densitometry experiments men- 

 tioned just above, studies of the rates of bleaching and recovery (adapta- 

 tion), visual acuity, color perception, and Land's new work. However, since 

 the excitation energy for electrons in large molecules is so dependent upon 

 structure, it would not be surprising if rhodopsin, chlorolabe, and erythro- 

 labe turn out to be very similar in composition. The answer will lie in 

 knowledge of the structure of these molecules. 



Incidentally, an important new fact, bearing upon acuity especially, is 

 that the eyeball is never still, but rather is in a state of small, almost im- 

 perceptible oscillations, such that the incoming light falls on a spot on the 

 retina for only a few microseconds before it is deflected away. If the eyeball 

 is fixed relative to the light source, color vision disappears. 



Physical Defects of the Eye 



If the lens is too thick or the eyeball elongated (myopia), the ciliary 

 muscles are not able to make sufficient adjustment of the focal length to 

 permit distant objects to be focused on the retina. The phenomenon is 

 known as nearsightedness, and can be corrected with the aid of glasses with a 

 concave lens of the proper focal length. If the length of the eyeball is too 



