18 
Mr. Potter on Photometry in 
took to discuss, their observations do not call for any further 
notice in this place. With Professor Forbes, however, I 
have a much graver controversy. 
In regard to Professor Forbes’s ‘ Memorandum,’ we must 
strive to find his meaning in some curious passages. In 
speaking of Fresnel’s formulae for the intensity of reflected 
light, he says scarcely any attempt has been made towards 
its verification by direct experiment.” Before we assent or 
dissent from the Professor’s assertion, we should know what 
he means by an attempt at verification. Attempts, and suc- 
cessful ones too, have been made to find the quantity of light 
reflected at various incidences by transparent bodies, by the 
celebrated Bouguer^ a century ago, and by myself latterly f, 
in a series of experiments which occupied me several months. 
As Fresnel’s formula will not give either Bouguer’s quan- 
tities or mine, our labours must be very unsatisfactory, 
having done nothing towards that desirable verification. 
Professor Forbes’s paper will probably supply this desidera- 
tum. A little further on he says, “ It occurred to me, about 
the end of 1837, that the anomalies of photometrical observa- 
tions being nearly as unsatisfactory as ever, some light might 
be thrown upon this important subject by ascertaining the law 
in the case of heat, the intensity of which we have no difficulty 
in measuring.” 
With respect to his actual experiments he says, “ I have 
this winter resumed the subject. I have had an apparatus 
constructed for securing sufficient accuracy in determining the 
angle of incidence, and I have used reflecting surfaces, both 
transparent and metallic ; the former are wedges of plate- glass, 
by means of which reflection from the first surface only may 
be observed, and the latter are plane specula of steel and silver. ^ 
The prosecution, however, of these apparently simple experi- 
ments, has been attended with unforeseen difficulties; and al- 
though the relative proportions of heat at different angles of 
incidence are now pretty well determined for glass in several 
cases, I am not prepared to say whether the absolute amount 
is exactly the same as Fresnel’s formula would give, assigning 
to heat its proper refractive index. It is satisfactory, how- 
ever, to know, that the approximation to it is much greater 
than direct photometrical measures have yet given, with the 
single exception of two experiments of M. Arago already re- 
ferred to; and that I have reason to believe that the experi- 
mental law which Mr. Potter has given from direct observa- 
tion in the case of light, represents my results much less ac- 
curately than the theory of Fresnel.” This undulatory pas- 
* See Boiiguer’s Traite d’Optiqne or Priestley’s History of Vision. 
f See Edinburgh Journal of Science for J830. 
