connexion *(x>ith Physical Optics, 21 
think it should pass for a discovery amongst Professor Forbes's 
remarks. 
The comparison of the former and the latter part of this 
sentence is a curiosity. In the first part he considers that he 
has experimentally verified my law, that metals reflect less 
light at higher incidences, and in the second he finds reason 
to suspect that my ‘ratios’ are all too small, which would 
nearly account for their deviation from Fresnel’s law. Now, 
by Fresnel’s formula more light would be reflected at higher 
incidences. How are we to understand the Professor’s mean- 
ing of the word verification ? 
1 consider Bouguer’s observation that he found the dust 
fall so fast on his mercury as to hinder the reflexion at higher 
incidences, to be more proof of the correctness of my law, than 
Professor Forbes’s experiments with steel and silver mirrors. 
It is well known that silver will not polish so as to make tele- 
scopic mirrors, and steel is very difficult to polish for that 
purpose. If the Professor’s mirrors are the labours of the 
cutler and silversmith, I would not value the results they give 
at a straw, however skilfully and carefully made. With opti- 
cians, the most difficult thing they attempt, is to obtain a per- 
fectly flat surface, and so essential in my experiments did I 
esteem it, that I used my mirrors as the oval mirror in a 
Newtonian telescope, and proved their accuracy by the di- 
stinctness with which double stars were exhibited in it. I 
stated with respect to the steel mirror, of the figure of which 
I was less certain than of the others, as follows : “ As I had 
only ground and polished it in the common way for flat sur- 
faces, I was not certain that it might be truly plane, and 
thought necessary to prove it on some astronomical objects. 
Accordingly, on the 19th February, with it and a 5^ inch 
speculum of my own workmanship, of about 50 inches focal 
length, and with a power of 100, I saw a geminorum beau- 
tifully and distinctly defined ; and with a power of 150 saw y 
leonis to be double at the first view, which I think will be 
allowed to be a sufficient test of its surface being nearly plane.” 
Professor Forbes will perceive that my conception of the 
mode to be pursued in order to discover laws of nature was 
a good deal different from his own. 
With respect to the quantity of light reflected by metals 
being greater than I had found it, I have need only to give 
Bouguer’s results, and that of Sir William Herschel, side by 
side with my own, and then to leave the scientific world to 
judge of the weight to be allowed to the Professor’s experi- 
ments. Bouguer, in a preliminary discussion, says he found 
754? rays to be reflected by mercury of every 1000 at llj°in- 
