70 ILLINOIS BIOLOGICAL MONOGRAPHS [70 



It will be noted that in every case the tendency is to orientate to face the 

 waves, but in only one case is even one-half of the school oriented. 



As the writer had no instrument for measuring the light intensity, the 

 Eastman Kodak Company's exposure-meter was used as the basis for the 

 determination of the amount of light present when activity ceased in the 

 evening and began again in the morning, and this method proved satis- 

 factory in as far as it goes. The principle involved in this instrument is the 

 exposure of a piece of photographic paper until it darkens to match a color 

 shade on the dial of the instrument. Since one piece of paper may be used 

 twenty times or more by exposing only a small surface of it at a time, the 

 method of comparing light intensities is fairly accurate after one has had 

 a little experience matching colors. As a result of a series of experiments 

 dealing with the intensity of light at which activity ceases at night and 

 begins again in the morning, the writer finds that the light is slightly less 

 intense when activity begins than when it ends. This conclusion is reached 

 from the following data: 



Table 11 

 TIME REQUIRED FOR PHOTOGRAPHIC PAPER TO MATCH DIAL SAMPLE 

 COLOR, IN THE MORNING AND EVENING AT THE TIME 

 ACTIVITY CEASES AND BEGINS AGAIN 



Since the length of time required to darken the paper is a measure of the 

 light intensity, and since the more light present the shorter will be the time 

 required, one may say that the time required is inversely proportional to 

 the amount of light present. In the case of Labidesthes sicculus it appears, 



