238 RESEARCH SEMINAR. 



of the blue is produced, not in the horn substance of the feather, 

 but at the dorsal inner surfaces of medullary cells where the inci- 

 dent light passes from the horn substance into the air filling the 

 cavities of the medullary cells. The blue color is probably due 

 to the stronger refraction of the blue end of the spectrum with 

 consequent greater reflection of the blue. 



The greens are usually produced by feathers which do not differ 

 essentially from blue feathers except in having a yellow pigment 

 in the cortex. The yellow pigment absorbs the incident blue, 

 but transmits the green rays. The latter are reflected more 

 strongly than the rays on the red side of the green in the spec- 

 trum, and they consequently give the feather its green color. 



Many variations occur in the shape of blue and green barbs, and 

 the latter very commonly have a high elevation of the dorsal 

 cortex which contains a diffuse yellow pigment. The barbules 

 are usually reduced or absent. 



In some species, we find the green changing to dark dull yellow 

 when the angle of incidence becomes very large. This phenom- 

 enon does not belong to the category of the common metallic or 

 iridescent colors. It depends on purely mechanical conditions. 

 The green disappears when the angle of incidence becomes so 

 great that only the dorsal, yellow cortical elevations of the barbs 

 are in view. 



August y. Rheotropism in Fishes. By E. P. LYON. 



It seemed impossible to the author that pressure could be the 

 .method of stimulation which brought about orientation of fishes 

 in a current of water, according to the usual theories on the sub- 

 ject. The effect of the current would seem to be simply to carry 

 organisms having approximately the same specific gravity as 

 water down stream. No pressure would result unless the animal 

 became -oriented and swam against the current. It therefore 

 seemed that points of reference on the bottom or banks must be 

 as important in stimulating the fish as the current itself. Testing 

 this hypothesis it was found that the fish responded to any move- 

 ment of the bottom of the dish in which they were kept, turning 

 in the same direction as the moving objects. By a series of ex- 

 periments it was definitely made out that by far the largest ele- 



