HELIOTROPIC REACTIONS ANIMALS AND PLANTS 225 



II. METHODS 



When we wish to determine where the most efficient spot of 

 the spectrum for freely moving animals lies, we must realize that 

 in order to get reliable results we must work with organisms which 

 are both very small and very sensitive to light. The organisms 

 must be small so that a large number can be crowded into a 

 narrow region of the spectrum; if this condition is not fulfilled 

 it is extremely difficult if not impossible to make statements con- 

 cerning the relative efficiency of the different parts of the spec- 

 trum which are of sufficient accuracy. Thus attempts to deter- 

 mine the mo.st efficient spot for the heliotropic efficiency of the 

 spectrum for young fish or larger insects can only yield crude 

 approximations. The second condition is that the animals must 

 be very sensitive to light ; since Loeb has shown in former papers 

 that only in the case of extreme sensitiveness will the animals go 

 directly to the source of light, while if the sensitiveness is small 

 the animals may go in very irregular paths although the sum 

 total of the motions towards the source of light will prevail. 

 It would be impossible to get a definite result with animals of 

 this kind. Thus experiments on the relative efficiency of dif- 

 ferent parts of the spectrum with young fish or other animals 

 which are only moderately sensitive are very unreliable. 



When we are dealing with positively heliotropic animals dis- 

 tributed in an oblong trough exposed to a carbon arc spectrum 

 the animals will move towards the source of light independently 

 of the nature of the rays by which they are struck; provided in- 

 tensity and frequency of the waves are above the threshold of 

 heliotropic efficiency. If the animals are evenly distributed in the 

 trough at the beginning of the experiment they will all move 

 towards the source of light and the result should be that at the 

 end of the experiment the animals should all gather equally at 

 the front wall of the trough and their density should be the same 

 on this front wall in the violet-blue and green provided that the 

 rays are sufficiently effective. On this basis it should be difficult 

 to tell whether the blue or the green is more efficient. Since, 

 however, some scattering of light occurs from the surface of the 



