134 LESLIE 13. AREY 



successive experiments. At first this seemed to be the correct 

 procedure, but rigorous controls showed that such precautions 

 were unnecessary. When interpreting the results of experiments 

 conducted in the hght it is not the absolute amount, but rather 

 the relative distribution of pigment that serves as a basis for 

 decisions. In experimentation in the dark, absolute differences 

 in the degrees of pigmentation could give rise to errors in judg- 

 ment, for the pigment, gathered into compact masses in the cell 

 'cups' might mask or apparently reverse the effect of temperature. 

 Fortunately, how^ever, the retinas, as judged from the expanded 

 light condition seem, on the whole, to be very equally pigmented 

 and the width of the contracted pigment zone, therefore, gives 

 a fair index of the effect of temperature in the dark. When work- 

 ing in the light especially, it was found to be very desirable to 

 have the experiments at contrasted temperatures conducted 

 simultaneously in order that advantage might be taken of iden- 

 tical light conditions, for as will be shown, light intensity is an 

 important factor in obtaining the maximum expansion of pigment. 



Experimentation in the dark was conducted as follows. A 

 tireless cooker, lined with black paper, was used as a dark cham- 

 ber, on account of the minimal loss of heat incurred by it during 

 the course of an experiment. If such an apparatus be previously 

 brought to the temperature of the introduced jar of water, an 

 experiment can be continued for several. hours without further 

 attention. In all determinations mentioned in this paper which 

 were conducted in the dark, precautions were taken against the 

 possible influence of light during the few seconds necessary for 

 excision and transference of eyes to the fixative. A long series 

 of careful comparisons showed identical results whether the 

 operation was carried out in total darkness or in light of just 

 sufficient strength to permit the operator to see the animal 

 and his instruments. Indeed, the results obtained by operating 

 in an ordinarily lighted room showed no recognizable differ- 

 ences in either pigment, rods, or cones from those secured by 

 working in darkness. 



A detailed description follows of the conditions found in each 

 of the four fishes: 



