SENSIBILITY OF VOLVOX TO SPECTRAL LIGHTS 355 



v/hich was being tested, the influence of change in ' physiological 

 condition/ or of an increase or decrease in speed of locomotion 

 and of precision in orientation, in successive tests, was eliminated. 



A colony was usually given five trials, in each direction and 

 the respective times taken to swim toward the spectral light 

 averaged, as were those taken to swim toward the white, and 

 the ratio between the two rates taken. From four to sixteen 

 colonies were thus given trials in each light. 



In the results obtained from the determination of the presen- 

 tation time the reciprocal of this value was taken as the stimu- 

 lating efficiency of the light. In comparing the relative rate 

 of locomotion, we likewise take the reciprocals of the time re- 

 quired to travel a certain distance. 



In table 2 the ratios of the rate of locomotion toward the 

 source of the spectral lights to the rate of movement toward the 

 white light are listed in column 3. The relative stimulating 

 values of the various spectral lights calculated by taking the 

 reciprocals of these ratios are listed in column 4, and as percent- 

 ages of X maximum in column 5. The curve is shown in figure 2. 



An extensive series of tests was also made of the stimulating 

 value of the various wave-lengths while the colonies were ex- 

 posed at the same time to the white light and to the spectral 

 light, the particular light whose stimulating value was being 

 tested impinging at right angles on one wall of the aquarium, the 

 white light on the opposite wall. The speed of locomotion to the 

 first fifteen of the spectral lights while the white light was also 

 burning was then ascertained, and the speed of locomotion to 

 the white light alone, as above, and the ratio again taken. These 

 values are shown in column 6, table 2; the reciprocal of them, 

 or the relative stimulating value, in column 7, and the values 

 computed by regarding X maximum as 100 in column 8, which 

 values have also been plotted (fig. 2) . In the region of the spec- 

 trum of highest stimulating effects the values are not very differ- 

 ent from those obtained for the spectral light alone (columns 5 

 and 8). The reason for this is probably that the stimulating 

 effect of the wave-lengths of greatest efficiency, and of those in the 

 immediate neighborhood in the spectrum, is relatively so great 



