LIGHT OF DIFFERENT WAVE-LENGTHS BY FISH 101 



3. In all her measurements of the energy reaching the 

 two colored stimulus plates (pp. 53-54) Dr. Reeves used 

 a new blue filter. Since the filter transmitted more light 

 than the old filter, a slit was used with it to reduce the 

 intensity of the blue to that obtained with the old filter 

 to which the fish were accustomed. The width of the 

 slit, once adjusted, was not varied. A water-cell on the 

 blue side served as an infra-red filter, if indeed such a 

 filter were needed, since no such rays should have been let 

 through by the blue filter. The energy which reached the 

 blue plate during the measurements with the thermopile is 

 therefore the same as that which reached the fish from 

 the blue plate during their tests. The two might differ some- 

 what on account of the unequal thickness of the strata 

 of water involved but in both long rays should be wholly 

 excluded. 



But in measuring the energy reaching the red plate two 

 methods were used to vary the intensity of the red light — a 

 red solution alone and a slit alone. Since red solution and 

 aquarium water both act as infra-red filters in greater or 

 less degree, whatever energy reached the red plate through 

 the one should reach the fish through the other though in 

 less intensity. Infra-red was presumably reduced under 

 the two conditions. When a slit alone was used to vary 

 the red intensity there was no infra-red or long-ray filter 

 present on the red side in the energy-equation measure- 

 ments. By this method infra-red rays would reach the 

 red plate unhindered. When the energies had been thus 

 equated and the fish were then tested the aquarium water 

 served as an infra-red filter on both sides. The energies 

 presumably equated with water cell on one side only 

 would then no longer be equated for the fish. 



It appears that both methods of varying the intensity 

 of the red permit the energies of the two plates to be equated 

 but that only one of them, that in which a red solution 

 was used, retains approximately those equated energies 

 in the stimuli which reach the fish through the aquarium 

 water. 



Table IV, p. 54, presumably includes trials in which 

 both methods of equating energy were used. The table 



