on the Affections and Properties of Light. 36 1 
member that they allude to the degree of distance to which the 
rays are subject to the action of bodies. I shall only add an 
illustration of this property, which may tend to convey a clearer 
idea of its nature. Suppose a magnet to be placed so that it 
may attract from their course a stream of iron particles, and 
let this stream pass at such a distance that part of it may not 
be affected at all ; those particles which are attracted may be 
conceived to strike on a white body placed beyond the magnet, 
and to make a mark there of a size proportional to their num- 
ber. Let now another equal stream considerably adulterated 
by carbonaceous matter, oxygene, See. pass by at the same 
distance, and in the same direction. Part of this will also be 
attracted, but not so far from its course, nor will an equal 
number be affected at all; so that the mark made on the white 
body will be nearer the direction of the stream, and of less size 
than that made by the pure iron. It matters not whether all 
this would actually happen, even allowing we could place the 
subjects in the situation described ; the thing may easily be con- 
ceived, and affords a good enough illustration of what happens 
in the case of light. 
Pursuant to the plan I before followed, I now tried to mea- 
sure the different degrees of reflexity, &c. of the different rays; 
but though the measurements which I took agreed in this, that 
the red images were much larger than the rest, and the green 
appeared by them of a middle size, yet they did not agree well 
enough (from the roughness of the images, and several other 
causes of error), to authorize us to conclude with any certainty 
“ that the action of bodies on the rays is in proportion to the 
“ relative sizes of these rays/’ This, however, will most pro- 
