one 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 blue- 
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 em. 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-f, 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 em. 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- 
