SPECTRAL COLOR AND STIMULATION 489 



The relative distribution of energy in the spectrum used in the 

 following experiments was ascertained by Dr. W. E. Forsythe 

 and Mr. Francis E. Cady, members of the staff of the Nela Re- 

 search Laboratory. Mr. Cady also measured the light used. 

 I am greatly indebted to these gentlemen for their generous 

 assistance. 



At the end of the experiments the lamp used was matched in 

 color with one of the laboratory standard lamps whose distribu- 

 tion was known in terms of a standard radiator. A new lamp 

 precisely like the one used in the experiment was then similarly 

 matched and the results agreed so closely that it was evident that 

 there was no appreciable change in the lamp during the time it 

 was used in my experiments and that the distribution of energy 

 in the spectrum remained practically constant. The results of 

 the above mentioned tests are plotted in figure 3. 



By referring to the accompanying curve it will be seen that 

 beginning at the violet end of the spectrum and proceeding 

 toward the red end the energy first increased rather gradually 

 but later very rapidly, so that while there was, in the green at 560 

 MM, 30f times as much energy as in the violet at 400 mm, there 

 was, in the red at 700 mm, more than 138 times as much. This 

 great difference in energy in different -regions of the spectrum 

 shows clearly that if one should obtain a response in the longer 

 waves and none in the shorter, it would not necessarily prove 

 that the stimulating efficiency of the former is greater than 

 that of the latter, for the difference in the response might be due 

 solely to the difference in the amount of energy involved. 



To ascertain the relation between wave-length and stimulating 

 efficiency, it is consequently necessary to make corrections for the 

 unequal distribution of energy in the spectrum. Such correc- 

 tions were made in all of the experiments described in the follow- 

 ing pages. 



Most of the organisms investigated were collected in temporary 

 clay pools formed in the immediate vicinity of the laboratory at 

 Nela Park, owing to the imprecedented abundance of rainfall 

 during the season. In these pools, many of which were so small 

 that they contained only a few gallons of water, unicellular and 



