No. 3.] VISUAL CEELS CIN. VERTEBRATES. 563 
outer segments are doubly refractive, while no trace of this 
was to be seen in the inner ones. Tests of outer segments or 
rods from the frog with a polarizing microscope, employing 
a gypsum interference plate, determined an optical axis in the 
length of the rod and that the rod has positive double refrac- 
tion with respect to this axis. Preparations of frog retinas 
so disposed as to permit the light to pass through the length 
of the rod, show that in this axis no double refraction takes 
place. 
The rods of amphibians are notable for their size, the dimen- 
sions of some in micra being as follows: 
Outer Segment. Inner Segment. 
Length. Breadth. Length. 
GCN mcsieayeteneee VOL 6 19 
UN 0); ae APS i? Rae 24-28 12 15-16 
TMG Oe a ee a 40-45 8 16-20 
Bufo variabilis °..... 76 8 17 
Sailenaneyavaliely Coe or oc 44 12 18 
Outer Segment.—In form the outer segment is cylindrical 
with a hemispherical slightly bulging distal end. Under high 
magnification its outer surface is marked by parallel striations, 
which deviate from a strictly longitudinal course only in that 
they are very slightly spiral. This appearance is due to super- 
ficial furrows alternating with ridges. The form of the outer 
segment may thus be well compared to that of a column with 
a slightly spiral fluting. The striations according to Schultze 
(724) have some relation with a deeper differentiation in 
structure, for in separated laminae, in addition to the grooving 
of the surface, there is a suggestion of a radial division of the 
substance. 
Another character of the outer segment of the rod is the 
transverse banding, which appears at regular intervals. This 
is seen to best advantage by very oblique illumination of fresh 
preparations (Schultze, *69>, p. 380) and is a surface indi- 
cation of a plate-like structure. The thickness of the plates 
in micra according to the measurements of Schultze (’67) 
and Zenker (’67) is as follows: frog .5;—.6, triton .55—.6, 
