221 
the Cause of coloured concentric Rings. 
for, if it can give an irregular figure to them by transmitting 
its irregularly modified rays, it follows, that when these rays 
are regularly modified it will be the cause of the regular 
figure of the rings. Nay, it proves more ; for if it modifies 
the figure of the rings by transmission, it modifies them no 
less by reflection ; which may be seen by following the 
course of the rays 6 , 7, 2, 4, ,5; for as they do not pass 
through the defective place of the lens, they can only receive 
their modification from it by reflection. This opens a field of 
view to us that leads to the cause of all these intricate phas- 
nomena, of which in a second part of this paper I shall avail 
myself. 
XXVI. Of the Action of the third Surface. 
When a double convex lens is laid upon a plain metalline 
mirror that happens to have an emery scratch in its surface, 
we see it as a black line under the rings that are formed over 
them. This shows, that when a defect from want of polish 
has not a power to reflect light in an irregular manner, it 
cannot distort the rings that are formed upon it. 
When I laid a good 21-feet object glass upon a plain slip 
that had some defects in its surface, the rings, in every part 
of the object glass that was brought over them, were always 
disfigured ; which proves that a reflection from a defective 
third surface has a power of forming distorted rings, and that 
consequently a reflection from one that is perfect must have a 
power of forming rings without distortion, when it is com- 
bined with a proper second surface. 
When the defective slip of glass, with a perfect lens upon 
it, was placed upon a metalline mirror, I saw the secondary 
set affected by distortions of the rings that were perfectly like 
MDCCCVII. G g 
