156 Proceedings of the Royal Society 
two offices performed by the nerves — first, that of informing the 
mind of the fact of the contact or impression being made ; and, 
second, that of giving more or less minute information as to the 
locality of the sensation. Professor Weber experimented upon the 
latter power, by testing the least distance apart at which two 
objects touching the skin of any part of the body could be felt as 
two distinct sensations ; and, as you are aware, this tactile power 
bears no constant proportion to the mere power of feeling a sensa- 
tion of contact. For instance, the back of the hand is perhaps 
more sensitive to a simple contact than the tip of the finger, but 
Weber found that the points of contact are required to be fourteen 
times further apart at the back of the hand than at the tip of the 
finger, before they can be distinguished as separated. 
Now, a very strong analogy exists between these two functions 
of ordinary sensation and corresponding offices of the retina. 
Objects seen obliquely are not strikingly different in brightness 
from the same seen in the direction of the optical axis, but the 
power of definition (apart altogether from mere optical causes) 
varies immensely. I attempted to investigate this defining power 
for different parts of the retina by a method exactly analogous to 
Weber’s — namely, by inspecting two white spots on a blackened 
card, and determining, for different angles of obliquity and direc- 
tion, the greatest distance from the eye at which these spots could 
be detected to be double. But I soon found that, when the vision 
is very oblique, there is a puzzling feeling of uncertainty as to the 
result ; and it occurred to me to assist the judgment by substituting 
for the white spots objects of contrasting colours. 
On attempting to put this idea into practice, I made the im- 
portant discovery, that when coloured objects are inspected under 
oblique vision, the colours are at the same time reduced in inten- 
sity, and changed in character : thus, scarlet becomes successively 
orange, yellow, and whitish-yellow, according to the obliquity; 
green , of a medium character, tends to become white, and violet to 
become blue. 
In experimenting upon the subject, it is best to place the coloured 
object obliquely on the nasal side of one eye, the other eye being 
closed ; much smaller angles of obliquity bring about the phenomena 
when seen on this side of the eye, and we get rid of any complicity 
