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 Netzhtiute welche anstatt 

 in der Dunkelheit in einer moglichst intensiven rothen Beleuchtung 

 verweilt hatten, verhalten sich in Bezug auf die Vertheilung der 

 Pigmentkorner gerade so wie die Netzhaute der Dunkelfrosche." 

 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, i. e. wave-length, of light has upon 

 the visual organ, then the quantity, i. 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 " Hef nerkerze " 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 



