32 REPORT—1840. 
the retardation divided by the wave-length, either of which 
quantities being assumed, the other becomes known. 
In pursuing this calculation, the author finds, that if the 
numerator (or difference of paths) be assumed to be the same 
as in light, the length of a wave of heat would result three 
times as great as that for red light. 
Upon this he is led into some important considerations bear- 
ing on the theory of undulations as applicable to heat. 
Almost exactly similar numerical results were obtained for 
the heat from an argand lamp, from incandescent platina, and 
from brass heated to 700°. 
At the Anniversary Meeting of the Royal Society of London, 
Nov. 30, 1838, the Rumford Medal was adjudged to Prof. Forbes, 
“ for his discoveries and investigations of the polarization and 
double refraction of heat.’’ And in the report of the council 
announcing the award, a brief but appropriate testimony is 
given to the value of these researches. 
Intensity of Reflected Heat: Forbes. 
On March 18, 1839, Prof. Forbes communicated some re- 
marks to the Royal Society of Edinburgh on the Intensity of 
Reflected Light and Heat. 
The theoretical law for the intensity of reflected light, ori- 
ginally proposed by Fresnel, has been confirmed on quite differ- 
ent grounds by the mathematical investigations of Mr. Green 
and Prof. Kelland. Yet scarcely any attempt has been made 
toward its verification by direct experiment, except in the cri- 
tical cases for polarized light originally assumed as the basis of — 
the formula, and a few intermediate photometrical determina- 
tions by M. Arago. The uncertainty attending all photometry, 
led Prof. Forbes to conceive (about the end of 1837) that per- 
haps some confirmation might be obtained by ascertaining the 
law which prevails with respect to the intensities of heat in the 
corresponding cases ; an analogy which seemed extremely pro- 
bable from the facts already ascertained, relative to the change 
of polarization, &c., before noticed. 
In December, 1837, he made some first attempts, which were 
not altogether satisfactory. In the following winter he resumed 
the subject, and by a suitable apparatus for measuring the angles 
of incidence, he endeavoured to measure the intensity of heat 
reflected from surfaces of glass, steel and silver ; and though the 
results can hardly be yet considered completely accurate, yet in 
the case of glass the approximation to Fresnel’s law is closer 
than any as yet exhibited by photometrical observations : while 
