484 S. O. MAST 



Consequently, it is e\ddent that we have here a method by means 

 of which the stiniulatmg effect of hght differing in wave-length 

 can be compared and the relative stimulating effect ascertained. 

 And if this is known it is a simple matter to calculate the rela- 

 tive stimulating efficiency provided the relative energy of the 

 different wave-lengths compared is known. 



The simplest method of procedure is to keep the quaUty of the 

 light (white, e.g.) in one beam constant while that in the other is 

 changed and then to adjust the illumination from the white Ught 

 in the one beam for each change of wave-length in the other until, 

 in each case, the organisms proceed along the same path. The 

 stimulating effect of the different wave-lengths tested will then 

 be directly proportional to the various illmxiinations from the 

 white light required to make the organisms, under each of the 

 different conditions, proceed in the same direction. For ex- 

 ample, if for the green it requires twice as much light from the 

 white source to make the creatures take a given course as it does 

 for the yellow^, then the stimulating effect of the green is twice as 

 great as that of the yellow, and if the yellow has twice as much 

 energy as the green then the stimulating efficiency of the green is 

 four times as great as that of the yellow. This method is ap- 

 plicable to organisms which, in light from a single source, do not 

 tend to travel parallel with the rays, but tend to deflect to the 

 right or the left, e.g., Volvox, as well as to those which do not tend 

 to deflect, provided the extent of the deflection is equal under the 

 various conditions. 



In the experiments described in the following pages, two 80- 

 watt, gas-filled, street series, tungsten lamps with vertical coiled 

 filaments were connected in series with a 40-volt storage battery 

 and enclosed in light-proof boxes each containing an opening of 

 such a size and so situated as to produce a small horizontal 

 beam of light. One of these beams passed through a Hilger 

 constant deviation spectrometer and a lens which focused the 

 filament on the sht, the other passed through a Lummer-Brod- 

 hun rotating sector. The whole apparatus was so arranged that 

 the two beams of light crossed at right angles in the field of 

 observation as represented in figure 1. 



