37 8 Mr. Brougham’s Experiments and Observations 
larged by any length of exposure to the light and heat in solar 
observations, they produce irregularities round the image. Such 
at least I take to be the explanation of the phaenomenon, ob- 
served at Paris by M. de Barros during the transit of Mer- 
cury in 1743, and recorded in Phil. Trans, for 1753. But 
there is another more serious impediment to the performance 
of reflectors, and which it is to be feared we have no means of 
removing. In making the experiments of which the history 
has been given, on viewing attentively the surface of the spe- 
culum, every part of it was seen covered with points of colours, 
formed by reflexion from the small specular particles of the 
body. I never saw a speculum free in the least from these, so 
that the image formed in the focus must be rendered much 
more dim and confused by them, than it otherwise would be. 
III. The last conclusion which may be drawn from these 
experiments, is a very clear demonstration in confirmation of 
what was otherwise shewn, concerning the difference between 
coloured images produced by reflexion, and those made by 
flexion. This complete diversity is most evident in the expe- 
riments with specula, the colours produced by which, in the 
form of fringes and rings, ought, as well as the others described 
as images by reflexion in Obs. 11, to be the same in appear- 
ance with those formed by pins ; w r hereas no two things can 
be more dissimilar. 
It remains to examine the 6th proposition : for this purpose 
I made the following observations. 
Observation 1 . Having procured a good specimen of Iceland 
crystal, I split it into several pieces, and chose one whose sur- 
face was best polished. I exposed this to a small cone of the 
sun’s light, and received the reflected rays on a chart; nothing 
