364 Mr. Brougham's Experiments and Observations 
found, by a simple contrivance, that they were to one another 
in the very same ratio that the rays by specula follow. 
In the fifth place, I compared the general appearance of the 
two sorts by viewing them at the same time, and was struck 
with their general appearance, unless that these of specula were 
most vivid and distinct. 
These things made me suspect that they were actually caused 
by the thin coat of gums with which the surface of the plate was 
varnished, called lacker. Accordingly I took it off with spirit 
of wine, and found the rings disappear; on lackering it again 
they returned; and in like manner I caused a well finished 
concave metal speculum to form the rings of which we are 
s’peaking, by giving it a thin coat of lacker. This is a clear 
proof that these rings were exactly the same with those of 
thick plates (to use Newton’s expression), for the coat of 
gums is, when thin, pretty transparent, as may be seen by 
laying one on glass plates. 
But this coat is extremely thin, and cannot exceed the 200th 
part of an inch ; so that the colours of thick plates are in fact 
the very same with those of thin plates, except that the two 
kinds are made by different sized plates. We cannot, therefore, 
distinguish them, any more than we do the spectrum made 
by a prism whose angle is 90° from that made by one whose 
angle is 20°. This kind of colours is not the only one I have 
observed of nearly the same kind with those of plates ; we shall 
presently see another much more curious and remarkable. 
III. 
In reflecting on the observations and conclusions contained 
m my former paper, several consequences seemed to follow. 
