No. 440.] STRUCTURE OF OUTER SEGMENTS. 



545 



In such preparations fields may easily be found containing 

 detached rods with their longitudinal axes lying in various 



Outer segments Km- parallel to the a axis of the gypsum 

 plate, ± 45 to the cross-hairs, showed a bright yellow color, 

 while those at right angles to this were bright blue. The colors 

 of an individual rod could be reversed by turning the preparation 

 so as to bring the rod into a line at right angles to its former 

 position. The inner segments of the rods are not highly refrac- 

 tive. These observations were made on the rods of Kana pipiens 

 but I have also tested the outer segments of the rods or cones, 

 as the case may be, in the mudpuppy (Xecturus). turtle, snake, 

 lizard (Anolis), guinea pig, mouse, and ox. and with wholly con- 

 firmatory results. 



This definite reaction demonstrates that the substance of the 

 outer segments is positively doubly refractive or anisotropic, /. c. y 

 as regards their optical properties the outer segments have axes 

 of maximum elasticity at right angles to their lengths. To 

 obtain an immediate basis for comparison I made similar tests 

 of other tissues. Thus bundles of naked axis cylinders from the 

 innei surface of the vertebrate retina gave light reactions exactly 



