620 The American Naturalist. [July, 
invariably moved towards the light. When the screen was interposed 
and all other light than that from the lamp excluded the same result 
was obtained. They were introduced into the trough at its middle 
point, still the result was the same. Of 39 individuals introduced at 
the end of the trough most distant from the light 54 per cent. passed 
the middle line towards the light, 43 per cent made the entire trip of 
the length of the trough. Only one of the 39 remained at the starting 
int. 
When introduced at the middle of the trough 57 per cent. of 58 — 
Daphniæ made the entire trip in two minutes. In all 67 per cent. 
moved towards the light, 12 remained at the starting point and three of 
these were caught in floating bubbles, 6 individuals, or the remainder, 
moved away from the light. But these scarcely affect the general 
result. . 
From the difference in the distances of the two ends of the trough 
from the source of the light the authors calculate that the difference in 
the intensity of the light at the two ends of a Daphnia .1 mm. long ex- 
posed to the unobstructed rays is somewhere between .0027 and .0031 
of the intensity of the light at one end of its body. But when the screen 
is used the difference at the thicker end of the wedge will be as great. 
as .0034 of the intensity at one end of the Daphnia and will increase to 
about 17 per cent. at the edge of the wedge. Consequently, in as much 
as the decrease in intensity an account of the increasing thickness of 
the screen is greater than the increase due to greater proximity of the 
light the authors conclude that the effect of variations in the intensity 
of the light must be ruled out of court leaving as the only possible 
cause of the movements of the animals the direction of the light rays- 
The difference of opinion, it is pointed out, has been due to a failure to 
distinguish between phototaxis, or the response to the direction of rays, 
and photopathy, or the response to variations in intensity. 
The relation between intensity of light and rapidity of 
movement.—In the same paper these authors just mentioned endeav- 
or to answer the question, “ Do positively phototactic organisms move 
more rapidly toward their optimum intensity of light than toward an 
intensity below the optimum ? ” 
The method by which the authors attacked this problem was similar 
to the one just mentioned. The trough was similarly placed at a 
horizontal distance of 50 cm. from the burner. In order to guage the 
intensity of the light an index was attached to the gas cock so that one 
fourth and full light could be obtained immediately without the use of 
