DAY: PIGMENT-MIGRATION IN EYE OF CRAYFISH. 307 
phenomenon. The only work ever done on the crayfish was that 
performed by Bell (:06) to ascertain how that animal reacted to 
colored lights and how changes in the retinal pigment affected the 
reactions of the animal to white light. 
Contemporaneous with Boll’s discovery that the pigment changed 
position under the influence of light, was his observation that the 
pigment-epithelium adhered to the retina least after exposure to red 
light, more after yellow and most after green, blue, violet and white 
light. Angelucci (’78) said, “Diejenigen Netzhiiute welche anstatt 
in der Dunkelheit in einer moéglichst intensiven rothen Beleuchtung 
verweilt hatten, verhalten sich in Bezug auf die Vertheilung der 
Pigmentkérner gerade so wie die Netzhéute der Dunkelfrésche.”’ 
Blue, on the other hand, he reservedly stated, elicited greater migra- 
tion than did white light. Engelmann (’84) claimed that the results 
of the work done by Van Genderen Stort, his student, and by himself 
with the use of light filtered through colored glass and by tests with 
spectral light, pointed to the probability that blue was the most 
effective. Three years later, however, Van Genderen Stort (’87) 
advocated green as being the most potent stimulus. That color, he 
observed, caused the pigment to proceed past the ellipsoids of the 
rods, the migration-terminus for ordinary light, out to the external 
limiting membrane. 
In all of the previous investigations the factor of intensity had been 
neglected. It is obvious that if the purpose of experiment be to 
ascertain the effect which quality, 7. e. wave-length, of light has upon 
the visual organ, then the quantity, 7. e. amplitude of the light-wave 
or radiant energy, must be kept the same in each color used. 
Pergens (’99) was the first to realize and give recognition to the 
claims of this argument. Discovering that blue of the spectrum was: 
too feeble for measurement he resorted to combinations of colored 
glass with which to produce monochromatic light. The intensities 
of the colors were equated by means of a Ritchie photometer. Two 
years previous he had obtained results from experiments on Leuciscus, 
in which red stood last in rank of efficiency, green next, then yellow, 
and blue first. Continuation of his work upon this fish with sup- 
posedly measured light yielded different results. With a light- 
intensity of one “Hefnerkerze’”’ he got a graded series of migration 
through red, yellow, green, and blue in ascending order. For lower 
intensities, however, the order became green, red, yellow, blue. The 
green, as indicated by his curves, evoked hardly any migration what- 
ever. When the intensity was diminished until no migration occurred 
