344 LIGHT AND THE BEHAVIOR OF ORGANISMS 



positive orientation to the light conditions in which the 

 animals collected, or to negative orientation to the other 

 condition; or the organisms may have wandered into the 

 light in which they collected by random movements and 

 have remained because of a definite reaction when the 

 border of this area was reached, somewhat similar to the 

 reactions of Paramecium on reaching the limit of an area 

 containing carbon dioxide; or they may have remained 

 because they came to rest somewhat as planarians come 

 to rest in a given light condition under some circum- 

 stances. It is therefore impossible to be certain as to the 

 interpretation of many of the results. Moreover only 

 a limited number of different colors was used, so that 

 the maximum effect in the spectrum cannot be located. 

 We shall consequently consider only a few of Graber's 

 observations. 



Colored glass and solutions were used almost exclusively 

 to differentiate the rays. These were all, however, thor- 

 oughly examined spectroscopically, and the relative inten- 

 sity of the light transmitted was also fairly accurately 

 ascertained. In this work Graber had the assistance of 

 the physicist Professor Mach. The lower forms only are of 

 interest to us here. 



Lumbricus is well known to be negative in its light reac- 

 tions. Graber's results are in harmony with this. In a 

 trough one half of which was shaded he found over five 

 times as many in the shaded region as in the illuminated 

 region. When one half of the trough was red and the other 

 blue there were nearly five times as many in the former as 

 in the latter, and in the case of red and green, and green 

 and blue, the majority collected in the light having the 

 longer wave lengths. In all these experiments the worms 

 were undoubtedly negative to the light conditions they 

 avoided. 



The red in these experiments contained rays between 

 6io and 710"'^; the blue rays between 550 and 570'"'", 

 those between 700 and 720'^''', and those below 540'^'^; the 



