540 
PHYSIOLOGY: LOEB AND NORTHROP 
The proof for the correctness of this view was furnished for the helio- 
tropic curvatures of the hydroid Eudendrium by Loeb and Ewald^ and 
by Loeb and Wasteneys.'^ The authors could show that if the intensity 
of light was lowered the time required to call forth the heHotropic 
curvatures of the polyps had to be increased in such a way as to keep 
the product i t constant. 
A second proof was furnished by Ewald^ who showed that when con- 
stant illumination was replaced by an intermittent one the same effect 
could only be produced when the product of time of exposure and in- 
tensity of intermittent light was the same as that of a constant light. 
Ewald worked on the orientation of the eye of Uaphnia to Kght. This 
crustacean turns its eye to the light and when the eye is under the in- 
fluence of two lights of equal intensity the eye is turned in a direction 
at right angles to the Hne connecting the two lights. By keeping the 
one light constant, the other intermittent (through rotating a disk with 
a sector cut out in front of it) Ewald found that the two lights acted in 
an equal way when the product i t in both cases was equal. 
The experiments of Eudendrium as well as Ewald's experiments are 
tedious and it seemed desirable to have a simpler method for the veri- 
fication of this law. Bradley M. Patten^ in working on the heliotropic 
reactions of the larva of the blowfly (which is negatively heliotropic) 
determined the path of the animals under the influence of two different 
sources of light striking the animals simultaneously. Theoretically 
the animal should creep in such a direction that the intensity of illumi- 
nation on both sides of its photosensitive elements should be equal, and 
Patten could prove that for each ratio of the two sources the path was a 
definite one. By rotating a wheel with a sector cut out before one 
source of light and cutting down the intensity of the other by a slit 
Patten could also show that indeed the heHotropic effect is determined 
by the product of intensity into duration of illumination. 
"Using the apparatus described, one of the beams of light was cut 
down by a diaphragm and the other by an episcotister, so that the light 
coming from one side was a steady beam of low intensity, and that 
from the opposite side an intermittent beam in which bright flashes 
alternated with darkness. The apertures in the sector wheel were ad- 
justed so that the amount of light from each source was equal for a unit 
time. It has already been established that when the larvae are sub- 
jected to equal steady beams of Kght from opposite directions the ag- 
gregate response is almost precisely at right angles to the line connect- 
ing the sources of light. The average angular deflection of 200 trails 
at equality was only 0.09°, when the degrees represented a distance of 
