224 Dr. Herschel's Experiments for investigating 
upon the defective place of the coach-glass. The rings of 
the secondary set reflected by it were nevertheless as perfect 
as those of the primary set. It occurred to me that these 
rings might possibly be reflected from the lowest surface of 
the perfect slip of glass, especially as by lifting it up from the 
coach-glass I still continued to see both sets. To clear up 
this point, therefore, I took away the slip, and turning the 
defective place of the coach-glass downwards, produced a set 
of perfect rings between the lens and the upper surface of the 
coach-glass, and brought it into such a situation that a 
secondary set must be reflected from the defective place of 
the lowest surface. This being obtained, the rings of this 
set were again as well formed and as free from distortions as 
those of the primary set. 
Upon a plain metalline mirror I laid down two lenses, one 
a plano-convex, the other a piano concave, both of 2,9 inches 
focus, and having the plain side upwards. When two 21-inch 
double convex glasses were laid upon them, the secondary 
sets of both the combinations were of equal size, and perfectly 
like their primary sets ; which proves that the refraction of 
the fourth surface is either not at all concerned, or at least 
has so little an effect in altering the size of the rings that it 
cannot be perceived. 
The result of the foregoing experiments, relating to the 
action of the several surfaces, is, 
I. That only two of them are essential to the formation of 
concentric rings. 
II. That these two must be of a certain regular construc- 
tion, and so as to form a central contact. 
III. That the rays from one side or the other, must either 
