SOME INTERACTIONS WITH LIVING MATTER 87 



important vital process, keeping pretty close to the facts, by-passing the 

 theories. 



Twilight Vision 



As mentioned above, cells of two general shapes are found on the retina, 

 rod and cone, the rod cells being responsible for the very sensitive detection 

 of light from dark when it is almost dark. These cells distinguish the shape of 

 the object, and although this is their primary role, they also permit us to dis- 

 tinguish colors. 



The pigment responsible for twilight vision is a molecule called rhodopsin, 

 the classical "visual purple." It is a condensation product of the carotenoid, 

 retinene, and a protein called opsin. Retinene is a 20-carbon, ringed com- 

 pound, the aldehyde of vitamin A, and its structure is well known. How- 

 ever, not very much is known about opsin. Another opsin has been identi- 

 fied, attached to retinene in the pigment todopsin. Further, an isomer of 

 retinene has been combined with the original opsin, and cyanopsin formed. 

 However, only rhodopsin is active in twilight vision. 



The extinction coefficient of rhodopsin, extracted in bile solution or in 

 digitonin, has a maximum value at 5000 A. It drops off rapidly at both 

 higher and lower wavelengths. Thus at 5500 A it is already down to about 

 25 per cent of the maximum, and at 5800 A is nearly zero; while at 4000 A 

 it is also 25 per cent of the maximum value, but then remains about the same 

 to wavelengths below those detected by the eye (smaller than 4000 A). The 

 Beer-Lambert law is obeyed exactly for weak solutions of rhodopsin. 

 Further, Figure 4-9 shows that the sensitivity of the human eye is deter- 

 mined directly by the absorption of light by rhodopsin. To man's eye 

 rhodopsin has a rose color; it absorbs strongly in the green (5000 to 5800 A) 

 and yellow (5800 to 6000 A) regions and to a lesser extent in the blue (4200 

 to 5000 A), and reflects all the rest; it is this reflected light which falls on 

 man's eye as he looks at the pigment, whether on the retina through an 

 ophthalmoscope, or in solution. This is why it is "colored" rose. 



It follows from the preceding paragraph that the fewest number of pho- 

 tons which will trigger the nerve will be those of wavelength 5000 A, for it is 

 here that the extinction coefficient is greatest. Incidentally, the unit of light 

 energy falling on the retina is the troland. At this wavelength it amounts to 

 about 100 quanta falling on a rod per second. However, the rhodopsin of a 

 rod is half-bleached by about 0.03 trolands, or 3 quanta per rod. It happens 

 that 1 troland is the retinal illumination when 0.1 millilambert (mL) is 

 viewed through a pupil 2 mm in diameter; and 0.1 mL is the brightness of a 

 white screen illuminated by 1 candle at a distance of 1 m. 



Rhodopsin is "bleached" by white light. Its color fades rapidly through 



