DAY: PIGMENT-MIGRATION IN EYE OF CRAYFISH. 309 
dilemma of quality or quantity, of color or intensity thus far had not 
been resolved. 
In 1907 Hertel (:07) emphasized a fact of fundamental significance. 
He pointed out that photometric determinations of the strength of 
light are insufficient, because we do not know whether the visible 
energy measured by the photometer represents the total radiant 
energy which falls on the retina from a given illumination. A thermo- 
electric device was used to determine this. He remarked that, if one 
got by this method the amount of energy necessary for perception in 
our eye, then one was justified in putting the light-reactions of a sub- 
jective (light- or color-perception) and an objective (photomechanical 
processes in the retina) nature into closer relation, because both are 
functions of the total energy measured in the same units. After 
determining the amounts of thermal energy necessary to produce the 
sensations of red and blue as such (and they approximated each other 
closely, hovering around 8-10 degrees Celsius), the effect of the colors 
of the same intensity was studied in the matter of cone-contraction 
in the retina of the frog. Red produced less contraction than blue. 
In the similarity of the two energy-values for the perception of color 
and the contraction of cones, the author found support for the view 
that the latter played a réle in the former. Hertel made no observa- 
tions upon pigment changes for colors of equal intensity. 
In the results obtained by Boll (’77), Angelucci (’78), Engelmann 
(’84), Van Genderen Stort (’87), Pergens (’99), Herzog (:05) upon 
vertebrates, and by Bell (:06) and v. Frisch (:08) upon invertebrates, 
one finds a general coincidence of opinion that red light is least effec- 
tive on the eye; while Kiesel (’94), with his claim for infra-red as a 
stimulus, stands alone in the opposition. All except Van Genderen 
Stort regarded blue or violet as the most potent region of the spectrum. 
In direct conflict with Van Genderen Stort’s observations of extreme 
migration under the influence of green, are the negative results of 
Pergens (’99) for the same color. The intermediate spectral regions, 
therefore, are of doubtful status. Notwithstanding the concord of 
opinion relative to the extremes of the spectrum, the subject was still 
open to question on account of the inaccuracy of method involved. 
The subsequent pages contain the results of an investigation of this 
problem after the intensity of the colors had been made constant by 
the aid of the radiomicrometer. 
