The Electrical Response of the Eye to Stimulation by Light 888 



According to Blondel ^ the light intensity of 1 mm.' of the crater surface 

 amounts to 158 bougies, according to PetaveP to 147 candle-power. To 

 make the two values comparable one to the other it is necessary to express 

 them in the same units. We choose for this purpose the Hefner 

 candle (HK). Regarding 1 bougie as equal to 1075 (HK) and 1 candle- 

 power equal to 1095 (HK)^, and if the mean of the figures found by 

 Blondel and Petavel be taken, a value of 165 (HK) per mm.^ crater 

 surface is obtained. 



If we assume the intrinsic brilliancy of the crater light to be i(HK) per 

 mm.^, then the intensity of radiation of our square spot in a direction at 

 right angles to its surface equals that of a light source of 



I = Sin(HK) . . . . (5) 



The total flux of light passing out from this spot is the light flux 

 which is directed upon the inner surface of a hemisphere. The centre of 

 this hemisphere coincides with the centre of the diaphragm, while the plane 

 upon which the hemisphere rests is the plane of the diaphragm. 



Taking Lambert's law into account we calculate the total flux of light 

 as amounting to 



^ = 7rl Lumen'* .... (6) 



The flux (f>^ which forms the image in the pupil is but a part of <p, and 

 indeed is 



^j=tsin2^0j Lumen . . . (7) 



where — is that part of the light which remains after the loss by absorp- 

 tion in the refracting media and reflection from the refracting surfaces. 



From (f>■^^ the strength of illumination of the image in the pupil can 

 easily be calculated. If the flux of light were distributed equally on an 

 area of 1 M^, the illumination intensity of the surface would equal (f>^ metre- 

 candles or Lux. But the flux here being concentrated on the area of the 

 image =s^ square millimetres, the illumination intensity amounts to 



E=10«x^Lux . . . (8) 



To calculate the value of E numerically we remember that i = 165 HK, 

 Sj=l'9 mm., S3 = 033 mm., sin |Oi = 0118. p alone remains to be deter- 

 mined. Referring to Section IV., regarding the energy of stimulation in 

 absolute measurement, we put p = 2,and then find for E the value 120 x 10* 

 metre-candles. 



The illumination of a plane whereon the direct rays of the sun in 

 zenith fall vertically through a clear atmosphere is given as 288000 



' Cf. Liebenthal, Praktische Photometrie, Braunschweig, 1907, p. 139. 



2 Loc. cit., p. 475. = Cf. Liebenthal, loc. ciL, p. 434. 



•* One Lumen is the flux of light that radiates from a puni'.Liform light source of 1 

 (HK) to an area =1 of the surface of a sphere described round the light point with & 

 radius =1. 



