r RADIANT HEAT. 359 



at 700° in nearly equal proportions, while mica .010 inch thick trans- 

 mits live times less of the latter than of the former. 



The second section relates to the dipolarization of heat. Pursuing 

 the methods given in the first series, the author ascertained the pro- 

 portion of heat dipolarized by five different thicknesses of mica. 

 From the numerical results thus obtained he deduces the value of the 

 expression in Fresnel's formula for 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 bearing 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, Novem- 

 ber 30, 1838, the Rumford Medal was adjudged to Professor 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, Professor Forbes communicated some remarks 

 to the Royal Society of Edinburgh on the Intensity of Reflected Light 

 and Heat. 



The theoretical law for the intensity of reflected light, originally 

 proposed by Fresnel, has been confirmed on quite different grounds 

 by the mathematical investigations of Mr. Green and Professor Kel- 

 land. Yet scarcely any attempt has been made toward its verification 

 by direct experiment, except in the critical cases for polarized light 

 originally assumed as the basis of the formula, and a few intermediate 

 photometrical determinations by M. Arago. The uncertainty attend- 

 ing all photometry led Professor Forbes to conceive (about the end 

 of 1837) that perhaps some confirmation might be obtained by ascer- 

 taining 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 sub- 

 ject, and by a suitable apparatus for measuring the angles of incidence 

 he endeavored 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 approxi- 

 mation to Fresnel's law is closer than any as yet exhibited hy photo- 



