SOME INTERACTIONS WITH LIVING MATTER 89 



slow. The reader is invited to contemplate the expression of the Weber- 

 Fechner law in this organ: 



5" oc log r/M, 



It says that the sensitivity, S, increases as the difference between the 

 threshold intensity and that of the background decreases. 



This photochemical description of twilight vision, although satisfactory in 

 general, apparently needs revision, for serious troubles arise when quantita- 

 tive description is attempted. It now seems likely that individual pigment 

 molecules are attached to individual nerve endings, and the excitation of just 

 one pigment molecule by incoming radiation is sufficient to trigger the nerve. 

 Thus, although it takes upwards of half an hour for dark adaptation to oc- 

 cur—that is, for the bulk rhodopsin to be regenerated in man after a bleach- 

 ing — the minimum time during which the eye can recover enough from a 

 flash to see another flash is about 0.01 sec. 



Color Vision 



The cone cells somehow distinguish between wavelengths, and thus dis- 

 tinguish colors. The Young-Helmholtz theory, usually accepted, and now 

 nearly 100 years old, suggested that three color-sensitive pigments exist, 

 each one sensitive to one of the basic colors: red (6200 to 7800 A), green 

 (5000 to 5800 A) and blue (4200 to 5000 A); and that various intensities mix 

 to give the colors and qualities commonly referred to as hue, brightness, etc. 



The Young-Helmholtz theory is based on the experimental fact that by a 

 proper mixture of red, blue, and green light in an object, any shade of color 

 can be matched. The theory is that the three pigments absorb definite frac- 

 tions of the visible spectrum and overlap one another, and that the optic 

 nerve can receive and transmit signals which correspond to any and all 

 wavelengths of the spectrum. Apparently this theory now requires major 

 modification as a result of the very recent (1959) work of E. H. Land. 7 In 

 some remarkable experiments he has shown in effect that the full range of 

 colors can be recorded by the brain provided only that the proper mixtures of 

 intensities of two wavelengths (one greater than, and one less than, 5880 A 

 (yellow)), fall on the retina! It seems that the information about colors other 

 than the two incoming wavelengths is developed in the retina. The possibility 

 that the pigment molecules are in intimate contact in the cone cells, and dis- 

 tribute the excitation energy among themselves in a manner controlled by 

 the intensity pattern of the incoming light, immediately suggests itself. But 

 more work is clearly needed following this surprising turn. Another recent 

 surprise is that some evidence has been turned up that other molecules in the 

 neurones, in the nerve pathway itself, contribute to the color perceived in 

 human vision. 



