2 o 6 Dr. Heksciiel’s Experiments for investigating 
formed by transmission, may at last be seen by a reflection 
from some interposed surface, these subsequent transmissions 
or reflections are to be regarded only as convenient ways to 
get a good sight of them. 
With this definition in view, and with the assistance of a 
principle which has already been proved by experiments, we 
may explain some very intricate phenomena; and the satis- 
factory manner of accounting for them will establish the truth 
of the theory relating to the course of rays that has been 
described. 
The principle to which I refer is, that when the pressure is 
such as to give a black center to a set of rings seen by reflec- 
tion, the center of the same set, with the same pressure of the 
glasses seen by transmission will be white.* 
I have only mentioned black and white, but any other alter- 
nate colours, which the rings or centers of the two sets may 
assume, are included in the same predicament. 
XVII. Why two connected Sets of Rings are of alternate Colours. . 
It has already been shown, when two sets of rings are seen, 
that their colours are alternate, and that the approach of the 
shadow of a penknife will cause a sudden change of them to 
take place. I shall now prove that this is a very obvious con- 
sequence of the course of rays that has been proposed. Let 
figure 7 and 8 represent the arrangement given in a preceding 
article, where a 1 6-inch lens was laid upon a looking-glass, and 
gave two sets of rings with centers of different colours : but 
let figure 7 give them by one set of rays, and figure 8 by 
another. Then, if the incident rays come in the direction which 
See Article XI. of this Paper. 
