COLE. — IMAGE-FORMING POWERS OF VARIOUS TYPES OF EYES. 345 



The intensity of the light from either source was found at the end 

 of the investigations to be only about l.'i") candle meters. There were 

 two reasons why this intensity was not constant throughout the exper- 

 iments. In the first place, the voltage appeared at times to be very 

 inconstant, causing considerable liuctuation in the intensity of the 

 lights. These fluctuations were usually of sufficient amount to be 

 noticeable ; but since Iwth lights were on the same circuit, the relative 

 intensities were but little affected ; however, at such times the lights 

 were tested with the photometer, in order to make sure that they had 

 not changed relatively, and that they were still balanced at the work- 

 ing point. It was found by experience that with small fluctuations 

 their change in relative intensity was inappreciable, it being only in 

 cases of considerable fluctuations that it was necessary to make any 

 readjustment. The other change in intensity was due to a gradual 

 deterioration of the filaments, and therefore was more constant. The 

 light intensity employed at the beginning of the investigation was very 

 close to 5 candle meters, from which, as stated above, it gradually ran 

 down to as little as 1.25 candle meters in the later experiments. The 

 whole change from this cause was probably not so great, however, 

 as sometimes occurred in the temporary fluctuations due to varying 

 voltage. We seem justified in leaving these comparatively small in- 

 constancies out of consideration, since it is not probable that they were 

 of sufficient amount to influence materially the results on any of the 

 animals used. At most, these differences could probably have made 

 only a slight difference in the percentage of reactions and none in their 

 character, since Adams ( : 03) has shown that in the earthworm, for 

 example, it is only at very low intensities, near the point where the 

 animal changes from negative responses to positive, that small differ- 

 ences of intensity have a relatively great effect. At the higher intensi- 

 ties considerable range in the intensity produces relatively little change 

 in the percentage of responses. Furthermore, the animals employed in 

 these experiments were selected for their decided reactions to light of 

 about the intensity used, and it is probable that in no case where posi- 

 tive results were obtained was this near the point where these forms 

 change the character of their response.* 



By means of a microspectral photometer the qualities of the two lights 



* The common European garden snail {Helix pomatia) was found to be so in- 

 constant in its responses that it was not suitable for tlie purpose of these inves- 

 tigations. This may have been because the light used was near the critical 

 intensity for this species, though it is more probable that the changes in response 

 depended entirely upon certain physiological states of the animal independent 

 of the light (see p. 391). 



