ENERGY, PHOTOCHEMICALS AND BRIGHTNESS 305 



At 450^^^ (with a bat's wing burner) the energy is repre- 

 sented by o.ii; at 500^'' by 0.52; at 700'''' by 12.70. In 

 the red, then, the energy is 100 times as great as in the 

 violet and about 20 times as great as in the green. In 

 case of direct sunUght the maximum energy in the normal 

 spectrum is between the yellow and the orange. From 

 this point there is a gradual decrease toward the red end 

 and a somewhat more rapid decrease toward the violet. The 

 difference in energy in different parts of the normal sun- 

 light spectrum is not nearly so great as in the normal gas 

 spectrum. In the prismatic spectrum of both gas and 

 sunlight it is, however, much greater than in the normal 

 spectrum, owing to the relatively greater condensation of 

 the rays toward the red end. Whereas the maximum in 

 the normal sunlight spectrum is between the yellow and the 

 orange at about 600''", in the prismatic spectrum it is, 

 according to Langley (1884), at the end of the red or per- 

 haps even in the infra-red. (Fig. 34.) 



2. Brightness Distribution in the Spectrum 



The determination of relative brightness of different 

 parts of the spectrum is a matter of considerable com- 

 plexity, owing largely to the difficulty of comparing differ- 

 ent colors with reference to brightness and to the fact that 

 the relative brightness of the different colors varies with 

 the absolute intensity, a characteristic known as Pur- 

 kinje's phenomenon. In spite of these difficulties the 

 results obtained by eight or more different methods, all 

 refined in every detail, are in close agreement, indicating 

 that the conclusions stated below are in all probability 

 reliable. 



The method most widely employed Is the direct com- 

 parison of the different colors with white light of known 

 intensity. By means of this method Fraunhofer in 18 17 

 located the maximum brightness in the prismatic solar spec- 

 trum in the neighborhood of the line D. In 187 1 Vierordt 



