504 HOWARD. [Vor. XIX. 
Outer rod segments were first examined in a_ polarizing 
microscope with polarizer and analyzer alone. In this way 
no evidence of double refraction by extinction was found. 
However, certain features were brought out more plainly than 
with ordinary light, viz., the axial core and transverse banding. 
The visibility of the latter seems to depend upon the direction 
of the illumination with respect to the rod itself. The desired 
angle can be obtained for any given rod by turning the revolv- 
ing stage of the microscope. 
\Vhen a gypsum interference plate is introduced between the 
analyzer and polarizer (or Nicol prisms), and the prisms so 
placed as to give a sensitive red of the first order, the outer 
segments of the rod give a definite reaction. In a teased 
preparation of fresh retina a field may easily be found contain- 
ing detached rods lying in various directions. Outer seg- 
ments lying parallel to the a axis, or negative optical direction 
of the gypsum plate, were bright yellow while those at right 
angles to this were bright blue. The colors of an individual 
rod may be reversed by turning the preparation so as to bring 
the long axis of the rod into a line at right angles to its 
former position. (Pl. 5, Fig. 43.) The bright yellow. or 
blue of the outer segments appears in marked contrast with 
the inner segments which retain the general red field color. 
These observations were made on the rods of Necturus, but 
I have also tested the outer segments of rods in the frog, 
hen, guinea pig, mouse, rat, and ox, as well as in the outer 
segments of the cone-like elements of the turtle and snake, 
and I have obtained in all cases the same result. In the rods 
and cones of the goldfish I obtained no reaction, although the 
outer segments are fully as large as some which gave distinctly 
positive results. The smooth dogfish, Mustelus canis, gave 
results similar to those from Necturus. 
The definite reaction obtained in the forms mentioned 
demonstrates that the outer segments (at least in their pre- 
dominating substance) are doubly refractive or anisotropic, 
with the long axis in the positive optical direction; 1. ¢., as 
