PROCEEDINGS ' 
OF THE 
NATIONAL ACADEMY OF SCIENCES 
Volumes SEPTEMBER TS. 1917 Number 9 
HELIOTROPIC ANIMALS AS PHOTOMETERS ON THE BASIS OF 
THE VALIDITY OF THE BUNSEN-ROSCOE LAW FOR 
HELIOTROPIC REACTIONS 
By Jacques Loeb and John H. Northrop 
ROCKEFELLER INSTITUTE FOR MEDICAL RESEARCH. NEW YORK CITY 
Communicated. July 25, 1917 
While it was customary to express animal instincts in terms of human 
behavior, one of us many years ago began to replace this anthropo- 
morphic method by the objective and quantitative methods of the 
physicist. These methods were most easily applicable in the case of 
those instincts familiar to every layman in which animals were appar- 
ently attracted or repelled by light. Loeb^ was able to express the ef- 
fect of light in these cases in the following terms: Certain animals are 
compelled automatically to orient their body in such a way that sym- 
metrical, elements of their photosensitive surface are struck by light 
of the same intensity. In that case the tension and energy production 
in the symmetrical muscles of both sides of the body are equal and there 
is no reason for the animal to deviate from this direction of its motion. 
If, however, the symmetrical photosensitive elements (e.g., the eyes) 
receive unequal illumination the tension or energy production of the 
symmetrical muscles is no longer the same and the animal is auto- 
matically turned until its orientation is again such that symmetrical 
photosensitive elements receive the same amount of light. 
It was obvious from the observations that this reaction was a function 
of the constant intensity of light and Loeb assumed that it was a photo- 
chemical effect and that the function was probably the law of Bunsen 
and Roscoe, whereby the effect equals the product i t, where i is the 
intensity of light and t the duration of illumination. ^ 
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