218 CHARLES HARLAN ABBOTT 



to the intensities given in the above table, while the numbers 

 on the ordinates refer to the eight degrees of negativeness, ac- 

 cording to the classification of figure 2. 



In the diagram the reactions of the two individuals of Oniscus 

 (A 23 and A 25) are represented by nearly straight lines, show- 

 ing approximately the same negative response to all intensities 

 used in the experiment. Porcellio rathkei (A 24 and A 26) 

 has the usual variations for the species, but these bear no rela- 

 tion to the intensity. The figure merely illustrates graphically 

 the conclusions given above. 



When tested by the first method described above, Porcellio 

 scaber gave its normal reaction also at a point 5.5 cm. from the 

 diaphragm, where the intensity was approximately 400 CM. 



b. Low intensities. Some species of animals which are nor- 

 mally negative to light are indifferent to low^ intensities, while a 

 few are positive under the same conditions. For this reason, a 

 series of tests with low intensities was arranged. These intensi- 

 ties were obtained by using Mazda and carbon lights of lower 

 power and by increasing the distance between the isopod and the 

 source of light. 



After finding Oniscus negative to intensities of 0.859 CM. 

 and 0.648 CM., a final series was tried with intensities which 

 were too low to be measured directly by the illuminometer, but 

 which could be calculated from illuminometer readings at other 

 points, according to the law of inverse squares. The results in 

 this series, given in table 5, represent the reaction of ten individ- 

 uals each of Oniscus and Porcellio scaber at each intensity, the 

 figures referring to the average reaction for each individual, 

 rather than to the individual responses. 



As the animals were tested when facing the light only, those 

 which went toward the light were perhaps indifferent instead of 

 positive. 



Table 5 shows that Oniscus was negative, even at these low 

 intensities. This was shown so strikingly by the test at the 

 lowest intensity recorded (0.0119 CM.) that a further de- 

 scription will be given of this particular experiment. The 

 illumination of the table was so slight that the sowbugs were 



