332 bulletin: museum of comparative zoology. 



intensity and longer exposure were needed to elicit the migration. 

 From this it was concluded that the latent period was a direct function 

 of the physiological state of the animal. 



The question of this latent period came up in connection with those 

 experiments in which the time of exposure was varied while the 

 intensity remained constant. Since there was a manifestation of 

 inertia in the form of initial latency, might not there also be another 

 manifestation of it at the close of the exposure in the form of momen- 

 tum? Would the pigment, after a five-minute exposure to bJue- 

 violet or after the longer exposures to red, continue to migrate upon 

 cessation of the stimulus? If there had been any appreciable after- 

 effect of the exposure, it would have complicated the comparison 

 of the effects of the two colors; but tests made by exposing to red at 

 50 cm. for five minutes then restoring the animal to the dark for ten, 

 fifteen, thirty, or forty-five minutes, yielded no detectable increase 

 in the migration. 



A further source of error might have come from the apparatus. 

 If there were any leakage of white light, its additional effect on the 

 migration would have been much greater in the long exposures to 

 red than in the short one to blue-violet. Although diaphragms 

 had been interposed (Fig. F) to eliminate such an error, a check was 

 employed which settled the question conclusively. A series of six 

 independent exposures upon a single photographic plate (Seed's 

 "Gilt Edge 27") was made as follows: — ^ at 50 cm. for periods of 

 thirty, sixty and ninety seconds; and at 150 cm. for periods of 270, 

 540 and 810 seconds, the last three periods being respectively nine 

 times as long as the first three. The results are reproduced in Plate 5, 

 Figs. 10 a-/, and are according to the above order. The exposures 

 in each vertical pair (e. g., a and d) are comparable, the one above 

 being an exposure to a strong intensity for a short period, and the one 

 below to a weak intensity for a compensatingly long period. The 

 difference in the actinic effect of corresponding exposures was almost 

 imperceptible. The series was made with the diaphragms in posi- 

 tion as during the experiments with the animals. A test with the 

 diaphragms left out yielded the result that the long exposures at 150 

 cm. showed a greater actinic effect than the corresponding ones at 

 50 cm. Such a delicate test, therefore, proved not only the freedom 

 of my results from any error due to leakage of light, but also the 

 efficiency with which diaphragms may be used to exclude extraneous, 

 reflected light in experiments of a similar nature. 



In order to make sure that the equality of intensity was not dis- 



