20 
Mr. Potter on ’Photometry in 
Calculations from Fresnel’s formula from Sir David Brew- 
ster’s paper, Phil. Trans. 18.30: 
Incidence. Crown glass, reflects, of every 100 rays. 
40« 4-910 
45® 5-366 
50® 6-136 
It appears from the above that Dr. Faraday’s experiments, 
making every allowance for different kinds of glass being used 
in the different experiments, do not yield that desirable veri- 
fication which Professor Forbes found amongst his results. 
However it will be seen that we are not to seek for expert^ 
menta crucis at the above angles of incidence, or at any rate, 
with such low refracting substances. Dr. Faraday’s experi- 
ments with the heavy optical glasses would have furnished 
them if we had had the refractive indices of those glasses ; for 
in high refracting bodies the discordance of Fresnel’s formula 
with experiments is palpable, for it gives results frequently 
one-half more, to twice as much as experiment. 
In the Professor’s next sentence I have to complain of a 
want of candour. My first introduction to physical optics 
was the discovery by experiments, of the law of reflection by 
metals, of a nature quite different from the suppositions of the 
scientific world, as well as of myself previously. Sir Isaac 
Newton had stated his opinion, such as had been received 
ever since ; Bouguer most likely had it'presented to him in 
his experiments with mercury, but blinded by the commonly 
received theory, could not see it : for he complains, that after 
all his precautions the fine dust fell so quickly on his mercury 
as to hinder the reflection at the higher incidences. Sir 
William Herschel had examined his splendid specula by 
photometry without falling upon it. Is it to be wondered at, 
that I should feel greatly proud of this discovery, my first 
effort in physical optics ? 
Professor Forbes says, ‘‘ With respect to reflection at the 
metals, I believe I may assert that I have verified the remark 
of Mr. Potter, that metallic reflection is less intense at the 
higher angles of incidence. I have attempted to ascertain 
whether it reaches a minimum, and then increases up to 90° 
of incidence, as Mr, Maccullagh supposes; but I have not 
obtained decisive results. The quantity of heat reflected by 
the metals is so much greater than Mr. Potter’s estimate for 
light, as to lead me to suspect that his photometric ratios are 
all too small, which would nearly account for their deviation 
from Fresnel’s law.” 
Here my discovery comes to be merely a remark.” I 
