the Inflection , Reflection , and Colours of Light. 251 
All this follows mathematically, on the supposition that the 
parts of light are acted upon in proportion to their sizes; and 
to say the truth, I see no other proportion in which we can 
reasonably suppose them to be influenced ; for such an action 
is not only conformable to the universal laws of attraction and 
repulsion, but also to the following arguments. If the action 
be not in the simple ratio, it must either be in a lower or in a 
higher ; let it be in a lower, as that of the square root, then 
the size of the red would be to the size of the violet as the 
squares of the forces; that is, as 1625625 to 1572009: a 
difference evidently too great ; and, a fortiori, of the cube or 
any other root. On the other hand, if the action were in a 
higher ratio, as that of the square, then the particles would be 
as the square roots of the forces, or nearly as 35.70 to 35.39, 
a difference evidently too small ; for if the size of the red 
particles were only T 3 ^ths greater than that of the violet, and 
the velocity of both were equal, the momentum, and conse- 
quently the intensity of the red, could not so much exceed 
that of the violet as we find it does, and as seems to me to be 
proved by the experiment of Buffon (on accidental colours), 
who found, that after looking at a white object, when he shut 
his eyes, it first became violet, then blue, or a mixture of blue 
and the other colours, and last of all red ; so in the impression 
of the white, compounded of the impressions of all the other 
rays mixed together, the violet was first obliterated or weakest, 
and the red last or strongest. To this reasoning on the in- 
tensity of the particles as owing to their size, I see only two 
objections that can be made. The one is, that the intensity 
is increased when the rays are thrown into a focus ; but we 
must recollect that the rays in this case are mixed, and their 
Kk 2 
