108 TROPISMS 



soon as the spectrum is turned on, these organisms are 

 simultaneously exposed to two different beams of light 

 which come from the two mirrors M and M^. When one 

 type of light, e.g., that from M, is much more efficient than 

 the other coming from M lf practically all the organisms 

 are oriented by the light from M and move toward this 

 mirror, collecting in the region R. When the relative effi- 

 ciency of the two types of light is almost equal, the organ- 

 isms move in almost equal numbers to R and R^. By 

 using as a standard of comparison the same region of 

 the spectrum and successively altering the position of 

 the other slit in the spectrum, we were able to ascertain 

 with accuracy the relative efficiency of the different parts 

 of the spectrum for the two forms of organisms. When 

 the two parts of the spectrum which are to be compared 

 are very close to each other, it is necessary to deflect 

 the beams with the aid of deflecting prisms, before they 

 reach the two mirrors. 311 



Experiments on the newly hatched larvae of Arenicola, 

 a marine worm, showed that the most efficient part of the 

 spectrum was in the bluish-green of about A = 495 //*, 

 while for the larvae of Balanus eburneus the most efficient 

 part of the spectrum was found by Loeb and Maxwell, 

 by Hess, and by Loeb and Wasteneys in the region of 

 yellow and yellowish-green. 311 



Mast 348 made similar experiments on these organisms 

 with a method in which the organisms were exposed to two 

 beams of light of different wave length crossing each other 

 at right angles. One light was kept constant while the 

 other was made intermittent by a disk with a sector cut 

 out rotating in front of the light. The size of the sector 

 was varied until the organisms moved at an angle of 45 

 to the two beams. When this happened the heliotropic 



