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 relative 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 gave 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 1388 also 135 
and 137 were so close as to be interchangeable. In series II there was 
little uncertainty about thecorrespondence 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 
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