The Electrical Response of the Eye to Stimulation by Light 411 



practically with an absolutely black body. If the temperature of the body 

 is known, the radiation energy of any part of the spectrum may be 

 calculated with the aid of Wien-Planck's^ formula. If the energy of 

 the rays whose wave-lengths lie between X and d\ is expressed hy HdX, the 

 factor H according to that formula is 



In the circumstances of our investigation the error we make is 

 negligible if we employ, instead of Wien-Planck'^s formula, the original 

 formula of Wien, 



_« 

 H = CX-5e~*^ . . . . (9) 



Here X signifies the wave-length expressed in centimetres, e the base of 

 the natural logarithms, T the absolute temperature, c a constant = 146, 

 and C another constant, which for the radiation from an area of 1 cm.^ has 

 the value of 0-896 x IQ-^^ g gal. cm.Vsec. 



According to Lummer and Pringsheim^ the temperature of the 

 crater must lie between 4200° and 3750° abs., while Waidner and 

 Burgess,^ after a detailed critical and experimental investigation, think it 

 most probable that the temperature of the hottest part of the positive 

 carbon lies between 3900° ana 4000° abs. It is permissible, therefore, for 

 us to assume that the temperature is 4000° abs. 



Further, H denotes the entire radiation which is emitted by a flat area 

 of 1 cm.2 This radiation would be entirely received by an imaginary lens 

 with an angle of aperture of 180°. 



We denote by Z cm.^ the area of the slit used in our experiments ; that 

 is to say, the magnitude of the radiating area, while 0^ is the angle of 

 aperture of our collimator lens. 



Further, we must take into account that all the light rays falling upon 

 the collimator lens do not enter the frog's eye. A part of the rays, as we 

 have already mentioned, is lost by reflection from the refractive surfaces 

 and absorption in the refractive media of the spectroscopic apparatus. 



If we denote by - that part of the light which passes, then we find for 



Hj — the radiation actually entering the pupil — the formula 



H,=H— sin2i0i .... (10) 

 P 



In our experiments Z = 0-0209 cm.^, sin i0i = 0118, while the value of 



p is taken as 2. 



> The formulae here used may be found in Kohlrausch's text-book, Lehrbuch der 

 praktischen Physik, 10 Aufl., 1905. 



» Verhandl. d. Deutsch. Physikal. Gesellsch., i., p. £3. 1899 ; ibid., p. SIV 

 3 Bulletin of the Bureau of Standards, Wa.shington, vol. i. p. 123. 1904. 



