200 
it slows down gradually, and finally it reaches 
a point of equilibrium which represents a defi- 
nite ratio between the concentrations of the 
reacting substances. 
of, 100/65 
100. 
Time — tours 
Fig. 1. Hypothetical course of the reaction 
P—S (formation of photosensitive substance) 
which takes place during dark adaptation of Ciona. 
The regular change in the reaction time during 
dark adaptation depends on this chemical reaction. 
The curves expressing this change may be dupli- 
cated by plotting a constant fraction (149) of the 
unused P against time as abscissa. 
Tn the case under consideration, the relation 
between the two substances may be represented 
in Fig. 1. The ordinates at the right indicate 
the per cent. of photosensitive substance (S) 
formed from the precursor (P). The ordi- 
nates at the left give the amount of the pre- 
cursor (P) remaining during the progress of 
the reaction. 
The production of the photosensitive sub- 
stance in the sense organ of Ciona undoubtedly 
takes place in this manner when the animal is 
placed in the dark room. Jt will be seen from 
Fig. 1 that the amount of the precursor (P) is 
at first large, then it decreases rapidly, then 
slowly, and finally it reaches a constant mini- 
mum. This is also what happens to the reac- 
tion time, and therefore to the amount of 
photosensitive substance broken down before a 
reaction can occur during the process of “ dark 
adaptation.” 
Since it was assumed that the photosensitive 
material decomposes into its precursor, the 
amount of the precursor formed at each reac- 
tion during dark adaptation, runs, in general, 
parallel to the amount of the precursor still 
unused in the reaction. Therefore, in order to 
serve as an “inner stimulus,” the quantity of 
precursor formed by the stimulating light must 
SCIENCE 
[N. S. Von, XLVIII. No, 1234 
bear a definite quantitative relation to the 
amount already present. This is merely the 
basis of the familiar Weber-Fechner concept, 
that the amount of stimulus necessary to pro- 
duce a perceptible increase in sensory effect 
represents a constant fraction of the quantity 
of stimulus that has gone before. 
VI 
The crucial test of any explanation lies in its 
ability to predict the course of events. Sucha 
test was applied to the hypothesis suggested 
above. 
We do not know with any accuracy the 
course of the reaction taken to form the photo- 
sensitive substance from its precursor. But 
the reverse reaction—the formation of precur- 
sor from sensitive material—has already been 
shown to follow the Roseoe-Bunsen rule. Con- 
sequently, the quantity of precursor present 
depends upon the amount of light energy which 
the animal has recently received. 
If a dark adapted Ciona is repeatedly ex- 
posed to light at sufficiently close intervals of 
time, only a negligible quantity of new photo- 
sensitive material should be formed. The 
amount of precursor produced by the light, 
however, will depend entirely upon the total 
exposure time. Moreover, if it is true that, in 
order to act as a stimulus, the amount of pre- 
cursor formed must bear a constant ratio to 
the amount already present, the reaction time 
should always bear the same relation to the 
reaction times that have preceded it. 
This is indeed found to be the case. Czonas - 
that have been kept in the dark for several 
hours, and are then exposed to light at inter- 
vals of a minute, and their reaction times 
taken, follow exactly the prediction outlined 
above. A curve drawn with time as ordinates, 
and sensitization periods (reaction time minus 
1.76 seconds) as abscissas, has a simple loga- 
rithmie form corresponding to the usual 
Weber-Fechner expectation. If instead, the 
logarithms of the sensitization periods are 
used as abscissas, the resulting curve is a 
straight line. This indicates that the amount 
of energy required to produce a reaction at 
any stage in the repeated stimulation is a con- 
