330 bulletin: museum of comparative zoology. 



band lies proximal to the outer limit of the tapetum. The degree of 

 gradation away from the center was often an aid in distinguishing 

 the influence of the different colors, because it was most abrupt for the 

 red and least so for the blue-violet. 



Direct observation of the photokinetic changes supplemented the 

 section-method in the following respects: the same animal could be 

 tested for all the colors; a larger number of observations could be 

 made, on account of greater facility; the unexposed eye served as a 

 check upon the other; and, since the glow could be determined by the 

 colored light at the beginning of the exposure, the same initial con- 

 dition of the eye could be ensured for each test. The chief objection 

 to the method lay in the fact that it involved a judgment by eye and 

 an artificial method of recording the amount of migration. Since 

 each record was made independently of previous ones for the same 

 animal, and since as a rule the large number of animals used at one 

 time prevented the retention in ones memory of the relativ^e values of 

 previous exposures, the chance for being mentally biased in recording 

 observations was practically eliminated. After a little practice the 

 repetition of certain trifling peculiarities of individual eyes in the 

 records ga\'e ground for the belief that a certain degree of accuracy 

 had been acquired in judging and recording conditions. In order to 

 establish it beyond a doubt, however, a check was made in the follow- 

 ing manner : — eleven eyes, eight in one series and three in another, 

 were sectioned for the purpose of comparing the actual amounts of 

 migration with the conditions as observed immediately after exposure 

 and recorded after the animals had been plunged into hot water. 

 In my own estimation the gradation of the migration in the sections 

 practically coincided with the gradation as shown by the records; 

 but in order that an unbiased comparison might be made. Professor 

 Parker also arranged the slides and the records independently of each 

 other in the order of progressive migration. In Table VI are given 

 our respective comparisons of the sections and records for the two 

 series of eyes. In the first series there was no doubt about the mini- 

 mum condition in eye No. 131, the maximum in 136 and an inter- 

 mediate condition in 134. In my judgment 133 and 138 also 135 

 and 137 were so close as to be interchangeable. In series II there was 

 little uncertainty about the correspondence of sections and records. 

 This series is reproduced in Plates 4 and 5, Figs. 7a, 7b, 8a, 8b, 9a, and 

 9b, and shows the recorded observations of the glow together with a 

 photograph of a section of eye 140, 139, and 141, respectively. These 

 comparisons of the recorded estimated condition with the actual 



