334 LIGHT AND THE BEHAVIOR OF ORGANISMS 



TABLE XI. (After Wilson, i8gi, p. 424-) 



Yellow decrease [in number of Hydras] .... 56 per cent. 



Red " " " " " 55 " " 



Green " " " " " 70 " " 



Blue increase " " " " 92 " " 



TABLE XII. (After Wilson, p. 427.) 



Total increase [in number of individuals during period of 

 observation] 421 to 674, i.e., 60 per cent. 



Blue, increase 327 per cent. 



Yellow, decrease 30 



Dark screen, decrease 37 



Daylight, increase 30 



a (I 



Wilson says that hydras are positive in blue, that they 

 go fairly directly toward the source of light, and that the 

 other colors are inactive, but he does not show definitely 

 how the aggregations in the blue are formed. We shall 

 discuss the movement in different colors more in detail 

 later. 



The colors produced by the plates of glass used were 

 not monochromatic. Thorough spectroscopic examina- 

 tion showed that the red transmitted a little orange, the 

 yellow some green, orange and red, the green some yellow 

 and a trace of red, and the blue some indigo and violet 

 and a trace of green and red. It is not at all likely that 

 such slight transmission of foreign colors as represented 

 above modifies the reactions of organisms that have not 

 well-developed eyes and are no more sensitive to light 

 than Hydra, although much has been said regarding this 

 and many results have been branded worthless owing to 

 the use of slightly impure colors. Wilson fortunately con- 

 firmed the results obtained with colored glasses by critical 

 tests in a spectrum produced by focusing light from an 

 Argand gas burner on a narrow slit in an opaque screen 

 in front of a large carbon bisulphide prism. '' The appa- 

 ratus was placed in a perfectly dark underground room and 

 every pains was taken, by the use of suitable screens, etc.. 



