226 HENRY LAURENS AND S. R. DETWILER 
light and darkness. But Flitter would call them all cones 
because their centripetal termination is similar to that found in 
the cones of man. 
The designating a visual cell as a rod or as a cone on morpho- 
logical grounds is not therefore useless, as Putter claims, but, 
as he also points out the structural basis (form of inner and outer 
segments), is brought into line with the functional by what we 
know of the respective functions of the visual cells in man, viz., 
threshold, visual acuity, ability to see movement, and vision of 
color. The rods are visual elements with a low threshold, but 
with possibilities of summated conduction, due to the connection 
of more than one of them with a single bipolar cell; the cones 
are visual elements with a high threshold and isolated conduc- 
tion, based on the histologically found type of connection. 
Putter, in speaking of the conditions in the nocturnal birds, 
admits that the visual elements have knob-like endings, and that 
the visual cells are morphologically typical cones, although they 
have assumed what he regards as the most distinctive character- 
istic of rods. Piitter reverses himself here and is, as well, incom- 
plete, because, as Ramon y Cajal ('94, p. 104) points out, in 
these retinae there are rods ending like those of mammals, while 
the cones which have almost entirely similar endings, reach 
deeper and come into connection with a different set of bipolars, 
so that there is a further morphological differentiation here 
between rods and cones. 
It does not seem at all certain to us that Hess ('10 and '13), 
by his work on the turtle and on birds, has disproved or weakened 
the general truth of the duplicity theory. He claims to have 
demonstrated an adaptation in the turtle retina, where there 
are cones only. The claim that he and Katz and Revesz (13) 
make, that adaptation in diurnal birds is a function of the cones, 
does not seem warranted, owing to the fact that rods are present 
in considerable numbers, as Hess himself describes, particularly 
in connection with the presence of visual purple. The phenom- 
enon, similar to the Purkinje phenomenon, which they state 
to have observed, may therefore, and most naturally, be a 
function of the rods and not of the cones. Katz and Revesz 
