ENERGY, PHOTOCHEMICALS AND BRIGHTNESS 307 



with those Haycraft obtained by measuring the size of 

 the pupil in different parts of the spectrum and by deter- 

 mining the distance at which small areas differing in color 

 become invisible. It will be seen by referring to Fig. 34 

 that the maximum brightness obtained in these experiments 

 in light of high intensity is very near the Fraunhofer line D, 

 that is, between the yellow and the orange, which agrees 

 with the maximum in the solar spectrum obtained by 

 Fraunhofer and Vierordt. It is interesting to note that 

 in low light intensity the maximum is in the green, and 

 that this corresponds fairly well with the maximum for 

 color-blind individuals. 



In case of color-blind individuals the difficulty of com- 

 paring intensity of different colors is of course obviated. 

 The brightness distribution in the spectrum has been 

 ascertained in several cases, all of which are in approxi- 

 mate agreement, the maximum being in the green near 

 the Fraunhofer line E (Fig 34), in fairly close agreement 

 with the maximum for the normal eye in the spectrum of 

 low light intensity. 



By comparing the curves in Fig. 34 it is at once evident 

 that the brightness distribution in the spectrum is not 

 proportional to the energy. The distribution of brightness 

 in the normal solar spectrum, the prismatic solar spectrum 

 and the normal gas spectrum agrees fairly well in certain 

 respects, while the distribution of energy in these spectra 

 is very different. Brightness sensation is therefore asso- 

 ciated with some specific effect of the length of light waves, 

 as well as with the amplitude of the wave. Color sensa- 

 tion, on the other hand, is associated with the specific effect 

 of the length of the waves and with the effect of combina- 

 tion of waves of different lengths. The specific effect of 

 waves of a given length and amplitude is no doubt due to 

 chemical changes in the retina. Visual purple is most 

 rapidly bleached by the rays in the spectrum between the 

 lines D and E, the region containing the rays which are 

 absorbed most readily, and the region which contains the 



