738 



LIGHT AND LIFE 



this leaves time for only the earliest steps in bleaching — the formation 

 of the lumi- or at most the meta-pigment. One must attempt to under- 

 stand what in these processes could result in a nervous impulse. 



This would jnesent a stubborn problem under any circumstances, 

 for we do not yet understand the mechanism of any biological ex- 

 citation, whether of nerve, muscle, sensory receptors, or eggs. In the 

 eye, however, this problem takes a peculiarly stringent form, unique 

 among all these objects, for experiments with the human eye have 

 shown that a completely dark-adapted rod can be excited by the ab- 

 sorption of a single quantum of light (19; 4, 35, 5) . 



Perhaps the simplest way to arrive at this conclusion is through 

 such a computation as that in Table 2. This shoAvs the numbers of 

 quanta per second incident upon, and absorbed by the rhodopsin of 

 a completely dark-adapted human rod at various levels of brightness, 

 at 507 m^, close to the wavelength of maximum sensitivity for rod 

 vision. No alloAvance has been made in the last column for bleaching 

 of the rhodopsin by the incident light; this would of course result 

 in less absorption. 



It should be understood that the threshold of the dark-adapted 



TABLE 2 



Absorption of I.ir.in in Rhodopsin in Himan Rods at Various Liminances 



The first two columns show the flux density at the retina, corrected for the open- 

 ing of the natural pupil (2), and allowing for an ocular transmission of 0.5. Con- 

 verted to quanta per sec. on the basis that for scotopic vision at .W? mfj., 1 lumen = 

 5.72 X 10-' watt = 1.37 X 10-" cal/sec = 1.47 X lO^'^ quanta/sec. Cross-sectional area 

 of himian rod outer segment, 2.54 X 10-" mm-. The outer segment contains about 

 4 X 10^ molecules of rhodopsin when dark adapted, on the assumption that it 

 absorbs about .SO per cent of incident light of wa\elength 507 ni/u (36). In the 

 last column, no allowance is made for the bleaching of rhodopsin. which at higher 

 luminances would lower the numbers of quanta absorbed. 



Flux Density at Retina 



Field 



luminance 



(millilamberts) 



Lumens per 

 sq. mm 



Quanta per 

 sec. per .sq. mm 



Quanta per sec. per rod 



Incident 



.-Absorbed 



10-" 



10-=' 



10-' 



10-= 



0.1 



1.0 



10 



100 



1000 



0.0013 

 0.0127 

 1.17 

 10.6 



87 

 605 

 3430 

 I 7.7.')0 

 103,000 



0.00043 



0.0043 



0.39 



3.6 



29 



202 



114 1 



.5917 



34,334 



