222 Dr. Herschel’s Experiments for investigating 
those in the primary set ; which proves that a polished defect 
in the third surface will give modifications to the rays that form 
the rings by transmission as well as by reflection. 
XXVII. The Colour of the reflecting and transmitting Surfaces 
is of no consequence. 
I laid seven 54- inch double convex lenses upon seven 
coloured pieces of plain glass. The colours of the glasses 
were those which are given by a prism, namely, violet, 
indigo, blue, green, yellow, orange, and red. The rings re- 
flected from each of these glasses were in every respect alike ; 
at least so far that I could have a black, a white, a red, an 
orange, a yellow, a green, or a blue center with every one of 
them, according to the degree of pressure I used. The lenses 
being very transparent, it may be admitted that the colours of 
the glasses seen through them would in some degree mix 
with the colours of the rings ; but the action of the cause that 
gives the rings was not in the least affected by that cir- 
cumstance. 
I saw the rings also by direct transmission through all the 
coloured glasess except a dark red, which stopped so much 
light that I could not perceive them. The colour of the 
glasses, in this way, coming directly to the eye, gave a strong 
tinge to the centers of the rings, so that instead of a pure 
white I had a bluish-white, a greenish white, and so of the 
rest ; but the form of the rings was no less perfect on that 
account. 
XXVIIL Of the Action of the fourth Surface. 
We have already seen that a set of rings may be com- 
pletely formed by reflection from a third surface, without 
