No. 3.] VISUAL CELES, IN VERTEBRATES, 611 
concluded, however, that the appearance was due to optical 
effects alone. It is evident in some of the references to an 
axial structure that quite different objects were in the minds 
of the authors. Kuhnt (see Schwalbe, ’87, p. 105) refers to 
the structure of the outer segment as, ““Kornige axiale und 
eine streifige periphere Substanz.” This description evidently, 
like that of Schneider ( : 02), who uses the term ‘*Achsenstab,”’ 
refers to all the mass of an outer segment inside the peripheral 
fibers, without any further differentiation within these. Dre- 
ser ('86) says, “Ich konnte eine axiale Substanz nachweisen,” 
and goes on to describe it as an ‘“‘Achsenkanal,” which gives 
to a cross section of a rod the appearance of a ring. This 
differentiation he was able to bring out only by certain chem- 
ical treatments and stains. . Bernard (: 01, p. 465) in speak- 
ing of the conditions in a living rod says, that a reticulum is 
condenced into [7. e. to form] the axes of the rods, the reticu- 
lum being replaced by an inflow or absorption of substance from 
the pigment granules, through the walls of the outer segment. 
The fiber of Ritter as described and figured is too small in rela- 
tion to the diameter of the rod to be identified with the axial 
core which I have observed. However, it seems probable from 
the figures of other investigators, that the same appearance 
gave rise to the various descriptions. For instance, an inspec- 
tion of Hensen’s (’67) figures shows that his cross section of 
the fiber are larger than they appear in the longitudinal view. 
In connection with the double refraction of the outer seg- 
ments I wish to call attention to some results which seem 
contradictory to my own. Valentine (’62) investigated with 
polarized light a large number of animal tissues including the 
rods of the retina and the axis cylinders of nerves, and as 
the following quotations show, he found that the reactions of 
these two bodies were not alike but opposite. ‘Die nahere 
Verfolgung des Gegenstandes zeigt, dass die optische Axe der 
Langsaxe der Nerven parallel geht; man also hier einen wahr- 
haft negativen Korper vor sich hat und die ganze Erscheinung 
nur von dem Marke herruhrt” (Valentin, 62, p. 123). “Man 
