228 HENRY LAURENS AND S. R. DETWILER 
prey, and he thinks that they detect the plants and insects upon 
which they feed by the sense of smeh. Rochon-Duvigneaud 
asks the very apt question in reference to Hess' claim of adapta- 
tion in the fowl, why it is, if they possess the power of adaptation 
almost as well-developed as that of man, that they go to roost 
long before man ceases to enjoy good vision. 
With reference to the duplicity theory and the distribution 
of rods and cones, the work of Abney ('16 and '17) is most 
interesting and important. By determining the minimum 
intensity of light of various wave length which can be perceived 
at the fovea and up to ten degrees from its center Abney and 
Watson ('16) obtained results indicating that in some cases the 
fovea of man is free from rods, which increase rapidly as the 
fovea is left, while in others there is a plentiful supply of rods 
at the fovea, their distribution, at any rate up to ten degrees, 
being very nearly uniform, and, if anything, in excess at the 
fovea. In the first group the light appears colored as long as 
it is visible at all, particularly in the green. In the second group 
the light loses color a considerable time before it is extinguished, 
except in the red. In other words, there is an achromatic inter- 
val. Abney ('17) later examined persons suffering from night- 
blindness, in which the rods are generally believed to be non- 
functional, for extinction of color from the red to the blue. 
The light was found to vanish when its color was extinguished, 
so that the same reduction in intensity of the light was the 
threshold for both light and color, similarly to the cases men- 
tioned above where there are no rods at the fovea, indicating 
that there is an absence of sensitive rods in the whole retina of 
the night-blind. 
If dark adaptation is directly associated with visual purple 
the vision of an animal possessing rods only as compared with 
that of an animal with cones only, both in respect to ability to 
see in light and darkness, after longer or shorter adaptation to 
the one or the other condition, and as well the relative stimulating 
value of spectral lights of equal energy, should -be expected to be 
markedly different, quantitatively as well as qualitatively. Now 
we have in the reptiles admirable subjects for investigating this 
