220 Dr. Herschel’s Experiments for investigating 
prepared, I laid a slip of glass upon a plain metalline mirror, 
and placed the lens with the polished line downwards upon 
the slip of glass. This arrangement has been shown to give 
two sets of rings. When I examined the primary set, a 
strong disfiguring of the rings was visible ; they had the 
appearance of having been forced asunder, or swelled out, so 
as to be much broader one way than another. The rings of 
the secondary set had exactly the same defects, which being 
strongly marked, could not be mistaken. The centers of the 
two sets, as usual, were of opposite colours, the first being 
black, the second white ; and all those defects that were 
of one colour in the first set, were of the opposite colour in 
the second. When, by the usual method, I changed the 
colours of the centers of the rings, making that of the primary 
white and of the secondary black, the defects in both set 
were still exactly alike, and as before ; except that they had 
also undergone the like transformation of colour, each having 
assumed its opposite. It remains now only to show that this 
experiment is decisive ; for by the established course of the 
rays we saw the secondary set of rings when it had a white 
center by the transmitted rays marked 1, 2, 4, 5, in figure 13 ; 
and when it had a black one, by the reflected rays 6 , 7, 2, 4, 5, 
of the same figure ; but in neither of these two cases did the 
rays come through the defective part of the lens in their 
return to the eye. 
This experiment proves more than we might at first be 
aware of ; for it does not only establish that the second 
surface, when properly combined with a third surface* has a 
modifying power whereby it can interrupt the regularity of 
the rings, but also one whereby it contributes to their formation: 
