26 BOTANICAL GAZETTE [july 



of the spectrum, the presence of which can be detected only by a 

 spectroscopic analysis. For example, certain results are frequently 

 ascribed to blue light, with no record of just what range of the 

 spectrum is used, nor what wave lengths other than the predomi- 

 nating ones are acting. 



Another source of inaccuracy has been the neglect or oversight 

 of the great variation in the intensity or quantity of radiant energy 

 transmitted by the color screens. Biological experimenters for 

 the most part have failed to take into consideration the fact that 

 the quantity as well as the quality of the light stimulus varies 

 with the different colors, and that the former variable must be 

 eliminated before results can be attributed to differences in wave 

 length alone. In some cases the importance of differences in the 

 intensity factor has been recognized, but no method was known 

 to the writer whereby the different colored lights could be com- 

 pared as to their amounts of radiant energy (10). 



There have been, of course, several methods devised by means 

 of which the relative intensity of monochromatic lights can be 

 measured. The first investigation in which the attempt was 

 made to get monochromatic light of known wave length and 

 equal energy was that of Blaauw (i). He used glass color screens, 

 and states that the lights transmitted were equal in intensity when 

 measured with a Weber photometer. The accuracy of this method 

 can be seriously questioned. 



The next exact work of this nature was done by Kniep and 

 Minder (ii). They used a blue and a red color screen, and a 

 green solution, with sunlight as the source of light. The wave 

 lengths to which each was transparent were known; and the 

 energy behind each was determined by means of a thermopile and 

 d'Arsonval galvanometer. The interference of the long heat rays 

 was prevented by inserting a water layer in a parallel-sided con- 

 tainer between the thermopile and the source of light. 



Day (5) obtained light of known wave length by means of a 

 spectrum from Nernst glowers, formed by a carbon bisulphide 

 prism and cut down by a diaphragm with narrow vertical slits 

 which could be adjusted so as to allow any desired region of the 

 spectrum to be transmitted. In this adjustment a spectroscope 



