COLOR VISION 361 



the presence of red, orange and yellow rays, and photosyn- 

 thesis in plants proceeds most rapidly in the red and orange. 

 The photochemical reaction in a given substance is therefore 

 primarily dependent upon the length of the light waves. 

 The reaction between quinine and chromic acid is affected 

 only by the light absorbed by the quinine, but the effect 

 is not proportional to the absorption. The maximum ab- 

 sorption takes place in the ultra-violet, but the maximum 

 efficiency is in the green. This shows that the location 

 of the maximum effect is dependent upon the power of 

 absorption and upon the distribution of energy as well as 

 upon wave length. Ozone in the presence of chlorine is 

 changed to oxygen by the visible light rays, while pure 

 ozone is not, showing that photochemical reaction in a 

 given substance may depend upon the presence of other 

 substances. 



(3) The distribution of brightness as judged by the human 

 eye is approximately the same for both normal and pris- 

 matic spectra of sunlight and gaslight. But the distri- 

 bution of energy in these spectra differs considerably. It 

 is therefore evident that brightness must be primarily a 

 function of the length of the waves and secondarily a func- 

 tion of the amplitude or energy contents. In lower in- 

 tensity the maximum is near the green, in higher it is in 

 the orange. In color-blind persons it is usually in the 

 green. 



(4) In the higher plants the maximum rate of curvature 

 in the spectrum takes place at the lower limit of the violet, 

 but there is a secondary maximum in the red. In some 

 fungi, however, the rate of curvature takes place under 

 potassium bichromate as rapidly as it does under an 

 ammoniacal solution of copper hydrate. That is, it takes 

 place as rapidly in the longer waves of the spectrum as it 

 does in the shorter. In the swarm-spore the region of 

 maximum stimulation in the solar prismatic spectrum is in 

 the indigo near the Fraunhofer line G. In Amoeba, Euglena 

 and Hydra it is in the blue, in Paramecium in the ultra- 



