PHYSIOLOGY: LOEB AND NORTHROP 
541 
but 1.5 mm. If the Bunsen-Roscoe law holds for the phototactic re- 
sponse of the larvae, they should orient perpendicularly to the rays of 
light when subjected to the action of steady and intermittent lights of 
equal energy per second. The experimental results based on 136 
trails made under these conditions show an average angular deflection 
of but 0.07° from the perpendicular. These results seem to show that 
in the blowfly larva the phototactic reaction follows the Bunsen-Roscoe 
energy law."^ 
It seemed desirable to extend the proof for Loeb's theory of animal 
heliotropism and especially for the validity of the Bunsen-Roscoe 
energy law to other forms of animals, and we selected for the purpose 
the reactions of the larvae of the barnacle which were already utilized 
by Groom and Loeb^ in their early experiments on the transformation 
of positively helio tropic animals into negative ones and vice versa. 
These larvae move in a straight line towards or away from a single 
source of light, and when two lights of equal intensity are g,iven they 
move in a line at right angles to the line connecting the two lights. 
Thelse animals are small and can be obtained in large numbers. They 
were made to collect in the corner of a dish with a little sea water and 
were then sucked up into a pipette which was blackened with the excep- 
tion of the opening. When such a pipette is put into a glass dish with 
parallel walls whose bottom is black (by putting paraffin blackened 
with lamp black at the bottom of the dish) the larvae will flow out in a 
fine stream and swim when they are positively heliotropic in a straight 
line towards the source of light. They thus form a rather narrow 
white trail on the dark bottom and it is possible to measure the angle of 
this trail with the line connecting the two lights. In this way in each 
observation the trail of thousands of individuals is measured. By using 
one constant and one intermittent source of light and comparing the 
results with those obtained by two constant lights we can test the 
validity of the Bunsen-Roscoe law. 
The method of the' experiments was as follows: abed (fig. 1) is a 
square dish of optical glass with blackened bottom and containing a 
layer of sea water. A and B are two lights, the intensity of which is 
determined by a Lummer-Brodhun contrast photometer. In front 
of each light is a screen with a round hole permitting a beam of Hght to 
go to the dish. The lights and the dish abed are so adjusted that the 
two beams of light striking the sides a b and b c at right angles cross 
each other in the middle of the dish. The light A is fixed while the 
light B is movable on an optical bench. The experiment is made in a 
dark room and the lights A and B are enclosed in a box. At the begin- 
