3o8 LIGHT AND THE BEHAVIOR OF ORGANISMS 



maximum brightness. There is probably some interrela- 

 tion between these phenomena. It is however not our 

 purpose to discuss theories of vision. We wish merely to 

 emphasize that experimental results appear to show that 

 brightness is not proportional to the energy in light, that 

 it is a function of wave length as well as of amplitude. 



3. Distribution of Actinic Effect in the Spectrum 



That light causes profound changes in chemical com- 

 pounds is a matter of common information to all familiar 

 with the process of photography. The fact that the 

 shorter waves of the spectrum, the ultra-violet, violet and 

 blue are chiefly active in causing changes in the halogen 

 salts of silver and various other metals used in this pro- 

 cess, is at least in part responsible for the idea that photo- 

 chemical changes in general are largely if not entirely 

 brought about by the action of the shorter waves, which 

 are usually referred to as the actinic rays. 



Photochemical reactions are far more numerous in both 

 the inorganic and the organic realms than is generally 

 supposed. Davenport (1897, pp. 1 61-165) brought to- 

 gether many instances under the following heads: syn- 

 thetic, analytic, substitutional, isomeric, polymerismic, 

 fermentative effects of light. Recent investigations have 

 made known others which are of especial interest to us. 

 Most important among these are numerous reversible 

 reactions, reactions which take place in one direction in 

 daylight or in light of a given wave length, and in the 

 opposite direction in darkness or in light of a different 

 wave length. 



The following reversible equations are referred to in a 

 recent paper by Stobbe (1908) on photochemical reactions. 

 The first five are quoted by Stobbe, the rest were dis- 

 covered by him. In these equations the arrows indicate 

 the direction in which the reaction takes place in the 

 different conditions of light with which they are labeled. 



