174 -Dr. Herschei/s Experiments for Investigating 
I found that the modifying power of the irregularly curved 
surface of the mica, made these configurations visible over the 
same angular space in which I had already seen the rings, 
and lenticular figures. 
These appearances are often so delicate, that they may 
easily escape our notice ; although we should follow them 
with great attention when the eye is moved. This will 
happen, especially when they are extremely minute, and in 
the experiment of the last paragraph of the 50th article, I had 
actually overlooked them ; but on repeating the same after- 
v/ards with a magnifier, I perceived them without much 
difficulty. 
I tried not only the smaller angles of the former experi- 
-tnents, with the same result pf a gradually increased range 
of visibility, but had also recourse to the plate of glass of 
unequal thickness mentioned in the g2d article, the sides of 
which, when produced, would meet in an angle of s' 2 ". Its 
surfaces approach so nearly to parallelism, that the inference 
I had drawn from the trial of smaller angles, when rings on 
the convex mirror were examined, was now verified by an 
application of this plate to the surface of mica ; for with the 
assistance of a magnifier, I saw the coloured forms over an 
angular space amounting to i7Ti 8'28", which is full as 
much as we could have seen with a plain glass. From this 
range, in which the actual angles of elevation of the eye 
above the plane of the glass at each extreme were measured, 
it appears that 5 degrees, which I have before allowed for 
this purpose, is more than sufficient. 
The foregoing three sets of experiments prove, that the 
first of the assertions, into which I have divided the objection* 
