212 Dr. Herschel’s Experiments for investigating 
mary set belonging to the secondary one with a white center, 
will instantly make its appearance with a black one. We may 
alternately withdraw and introduce again the strip of card, and 
the center of the primary set will be as often changed from 
one colour to its opposite; but the secondary set, not being- 
dependent on the rays 6 , 7, will not be in the least affected 
by the change. 
If the contact should have been such as to give both sets 
with orange centers, the introduction of the strip of card will 
prove that the set which is primary to the other has really a 
green center. 
Another way of destroying the illusion is to expose the same 
arrangement to a brighter light, and at the same time to in- 
crease the obliquity of the angle of incidence; this will give a 
sufficient reflection from the surface of the glass to be no 
longer subject to the former deceptive appearance ; for now 
the center of the primary set will be black, as it ought to be. 
XXII. Of the reflectmg Surfaces. 
The rays of light that form rings between glasses, must 
undergo certain modifications by some of the surfaces through 
which they pass, or from which they are reflected; and to 
find out the nature of these modifications, it will be necessary 
to examine which surfaces are efficient. As we see rings by 
reflection and also by transmission, I shall begin with the most 
simple, and show experimentally the situation of the surface 
that reflects, not only the primary, but also the secondary sets 
of rings. 
Upon a slip of glass, the lowest surface of which was de- 
prived of its polish by emery, I laid an object-glass of 21 feet 
