198 Parr.—The Response of Pilobolus to Light . 
This law, which was developed from a psycho-physical basis by Weber 
and Fechner (1882), was later shown by Muller (1878) to have a physio¬ 
logical significance. While Pfeffer showed its application to chemotropic 
stimuli, he expressed a belief that it would be found true in other forms of 
reaction such as phototropism and geotropism. 
Massart (1888) has shown that the irritability of the sporangiophores 
of Phycomycetes between two unequal intensities of light follow the Weber- 
Fechner law in their reaction. 
If for the term ‘ sensation which belongs to the realm of psychology, 
we substitute ‘irritability’ (cf. Preyer, 1874), the Weber-Fechner law may 
be tested in its application to the present problem within the limits of the 
measured intensities of the spectral regions of the two light sources. In 
Table IX are given the measured values of the energy and the presentation 
time, together with the solution of the formula to obtain the value of con¬ 
stant A for each given region. The value of A, however, is seen to 
decrease rather uniformly through the spectral regions, and this strengthens 
the belief already expressed, namely that the two factors, wave-frequency 
and energy combined, give the phototropic response. If the value of A 
in each case is multiplied by the square root of the wave-frequency of the 
region in which the measurements were made, an approximation to 
a general constant for the whole visible spectral region is obtained as shown 
in the last column of Table IX. 
A comparison of the present series of results with those conforming to 
the Talbot law, as found by Nathansohn and Pringsheim (1907), Blaauw 
(1909), and others, shows a considerable difference. If quality of light is an 
essential, as shown by the present experiments, Pringsheim’s methods in the 
light of these results are not quite fair, since the smoked glass used to 
reduce the quantity of light would interfere with its quality. This would 
cut down the different wave-lengths unequally and the heliotropic efficiency 
accordingly would be indefinite. 
Patten working with blow-fly larvae in their relation to different 
intensities of light found in one series of experiments an excellent agreement 
with the Weber-Fechner law. A second series of measurements, however, 
gave different results. He concludes that the conformity with Weber’s law, 
where it does occur, is entirely accidental. The results of the present 
series of experiments as shown in Table IX do not corroborate Patten’s 
conclusion. Blackman (1905) has shown that a slight change in temperature 
causes a corresponding change throughout the physiological system of the 
organism. It is not strange, then, considering the difficulties in obtaining 
identical experimental conditions, that curves of response should vary con¬ 
siderably. The valuable mass of evidence presented by such investigators 
as Pfeffer (1883), Massart (1888), and others must lead to the belief that 
some fundamental factor exists in the protoplasm which gives rise to results 
conforming to the Weber-Fechner law. 
