203 
Parr .— The Response of Pilobolus to Light . 
is beyond doubt. Furthermore, the present experiments show conclusively 
the existence of a definite relation of frequency to this change in the living 
protoplasm which produces response. 
That the square root of the frequencies appears as a function of the 
reaction as calculated in the present paper is a striking fact in view of 
Nernst’s results on the relation of frequency to response in electrical 
stimulation, where he found the response to vary inversely to the square 
root of the frequency (cf. Nernst, 1899 ; Verworn, 1913). Present observation 
would indicate that the light stimulus follows directly in the line of 
electrical stimulation. 
The writer desires to make further investigation into the relations of 
response in these two fields of research. 
Summary. 
1. Pilobolus responds to the light of all the regions of the visible 
spectrum. 
3 . The presentation time decreases gradually from red to violet. There 
is no indication of intermediate maxima or minima. 
3. The presentation time does not vary in direct ratio with the 
measured value of the energy of the light in the different regions of the 
spectrum. 
4. The presentation time varies in inverse ratio to the square roots of 
the wave frequency. 
5. The product of the square root of the frequency times the pre¬ 
sentation time, decreases with the decrease in the energy value of the 
spectral regions, and is an approximate constant for a given light- 
source. 
6. The spectral energy in its relation to the presentation time may be 
expressed approximately in the Weber-Fechner formula, if the wave- 
frequencies be made a function of the constant. 
7. The relation of the spectral energy to the presentation time may 
also be approximately expressed in the Trondle formula, the wave- 
frequencies being made a function of the constant. 
The writer wishes to express her appreciation and gratitude to 
Professor Charles F. Hottes for inspiration and encouragement during the 
development of this problem, which was undertaken at his suggestion. 
She also wishes to thank Professor Jacob Kunz and Mr. Karrer, of the 
Physics Department, for their interest and assistance in the control of the 
delicate instruments used in the measurements of light. 
University of Illinois, 1916. 
