RESPONSE OF THE PUPIL WITH CHANGES IN BRIGHTNESS 115 



M the least mechanical equivalent of light; hence 



Br 2 AM 



E = 



& 



If M is taken as 151 X 10 -5 watt per lumen,* it can be shown that, 

 for a stimulus possessing an area of 2 sq mm viewed at a distance of 

 35 cm and possessing a brightness of 0.000362 millilambert, the energy 

 entering the eye is as low as 143 X 10 -10 erg per sec. The smallest num- 

 ber of photons per second to which the retina will respond can now be 

 calculated. 



Let us suppose that the test spot is emitting monochromatic light of 

 wavelength 5560 X 10 -8 cm, corresponding with the region of maxi- 

 mum retinal sensitivity. We can then calculate the energy content of 

 1 photon of this radiant energy incident on the retina. It is E = hv 

 = hc/\, where h is Planck's constant, of magnitude 6.62 X 10~ 27 erg sec; 

 c is the velocity of light and equal to 3 X 10 10 cm per sec; and X is the 

 indicated wavelength. Hence E = 0.036 X 10 -10 erg. 



If the above threshold power value is accepted as 143 X 10 -10 erg 

 per sec, then 4000 photons per second of this green radiation is used. 

 If the exposure to produce an experienced sensation occurs in 0.002 sec, 

 it follows that the retina responds to as few as 8 photons — an extraor- 

 dinary and remarkable sensitivity. Hevesy and Paneth [1938] find 

 that for a practiced eye about 30 photons of the above wavelength suffice 

 for the unaided visual perception of an alpha-particle scintillation 

 Barnes and Czerny [1932] find light flashes of 40 to 90 photons as the 

 minimum sensitivity of the dark-adapted eye. 



A solution of visual purple extracted from the retinas of frogs by 

 Dartnall et al. [1938] showed that the number of chromophoric group- 

 ings in visual purple destroyed in relation to the number of photons 

 absorbed is nearly equal to unity. 



Response of the Pupil with Changes in Brightness 



It has been found that the pupil has a different diameter for each 

 brightness level. This adjustment is due to the fact that the pupil is 

 automatically regulated so as to maintain constant light energy on the 

 retina. In this connection the evidence supplied by Reeves [1920] from 

 his study of the response of the pupil to changing intensities of light is 

 significant. The diameter of the pupil at any given brightness was 

 determined by means of a flashlight and motion-picture camera. The 

 apparent diameters viewed through the cornea and aqueous humor 



* H. T. Wensel, J. Research Natl. Bur. Standards, 22, 375, 1939. 



