228 Dr. Herschei/s Experiments for investigating 
distinct, I saw them retain their shape and colour all the time 
without the smallest alteration. 
The same experiment was repeated with a piece of plain 
glass instead of the metalline mirror, in order to give room 
for the fits of easy transmission, if they existed, to exert 
themselves ; but the result was still the same ; and the con- 
stancy of the brightness and colours of the rings of the se- 
condary set, plainly proved that the rays of light were not 
affected by the thickness of the plate of air through which 
they passed. 
XXXII. Alternate Fits of easy Reflection and easy Transmission , 
if they exist, do not exert themselves according to various Thick- 
nesses oj thin Plates of Glass. 
I selected a well polished plate of coach glass 1 7 inches 
long, and about 9 broad. Its thickness at one end was 33, 
and at the other 31 two- hundredths of an inch ; so that in 
its whole length it differed T ^- of an inch in thickeness. By 
measuring many other parts of the plate I found that it was 
very regularly tapering from one end to the other. This 
plate, with a double convex lens of 55 inches laid upon it, 
being placed upon a small metalline mirror, and properly 
exposed to the light, gave me the usual two sets of rings. 
In the secondary set, which was the object of my attention, I 
counted twelve rings, and estimated the central space between 
them to be about l-i times as broad as the space taken up by 
the 12 rings on either side ; the whole of the space taken up 
may therefore be reckoned equal to the breadth of 40 rings 
of a mean size: for the 12 rings, as usual, were gradually 
contracted in breadth as they receded from the center, and, 
