TABLE 61.— THE SENSIBILITY OF THE EYE* 

 Part 1. — Contrast or photometric sensibility 



89 



For the following table the eye was adapted to a field of o.i millilambert and the sensi- 

 tizing field flashed off. A neutral gray test spot (angular size at eye, 5 X2.5°) the two 

 halves of which had the contrast indicated (i transparent, i covered with neutral screen 

 of transparency = contrast indicated) was then observed and the brightness of the trans- 

 parent part measured necessary to just perceive the contrast after the lapse of the various 

 times. One eye only used, natural pupil. Values are log brightness of brighter field in 

 millilamberts. 



Part 2. — Glare Sensibility 



When an eye is adapted to a certain brightness and is then exposed suddenly to a much 

 greater brightness, the latter may be called glaring if uncomfortable and instinctively 

 avoided. Observers naturally differ widely. The data are the means of three observers, 

 and are log brightnesses in millilamberts. The glare intensity may be taken as roughly 

 1700 times the cube root of the field intensity in millilamberts. Angle of glare spot, 4°. 



Log. field . 

 Log. glare 



-6.0 

 1.35 



-4.0 

 1.90 



-2.0 

 2.60 



-1.0 



2.90 



.0 +1.0 

 3.28 3.60 



2.0 



3.90 



3.0 



4.18 



4.0 



4.48 



Part 3. — Rate of adaptation of sensibility 



This table furnishes a measure of the rate of increase of sensibility after going from 

 light into darkness, and the values were obtained immediately from the instant of turning 

 off the sensitizing field. Both eyes were used, natural pupil, angular size of test spot, 4.9°, 

 viewed at 35 cm. Retinal light persists only 10 to 20 minutes when one has been recently in 

 darkness, then in a dimly lighted room ; it persists fully an hour when a subject has been 

 in bright sunlight for some time. A person who has worked much in the dark "gets his 

 eyes" quicker than one who has not, but his final sensitiveness may be no greater. 



For reference, see footnote 25, p. 87. 



TABLE 62.— MINIMUM ENERGY NECESSARY TO PRODUCE THE SENSATION 



OF LIGHT 



Ives 38 X 10" 10 erg sec 



Russell .... 7.7 X 10" 10 " " 



Reeves .... 19.5 X 10" 10 " " 



Buisson ....12.6X lO" 10 " " 



Taylor ....Minimum threshold for dark-adapted 

 eye, a surface, at a brightness of 

 1.8X10" 7 millilamberts, source 

 color temperature 2850 °K. 



Hecht 2.2-5.7 X 10" 10 ergs at cornea, con- 

 sidering losses the amount of 

 energy that reaches the retina is 

 such that i quanta is absorbed by 

 from 5-14 retinal rods. 



Astrophys. Journ.,vol. 44, p. 124, 1916. 

 Astrophys. Journ., vol. 45, p. 60, 1917. 

 Astrophys. Journ., vol. 46, p. 167, 1917. 

 Journ. de phys., vol. 7, p. 68, 1917. 

 Journ. Opt. Soc. Amer., vol. 32, p. 506, 

 1942. 



Journ. Opt. Soc. Amer., vol. 32, p. 42, 

 1942. 



SMITHSONIAN PHYSICAL TABLES 



