RADIANT HEAT. 339 



points already stated, and especially to determine the degree of accu- 

 racy of the instrument. From a comparison of its sensibility with 

 that of air thermometers, the author concludes that 1° of deviation of 

 the needle corresponds to an effect indicated by about jV of a cen- 

 tigrade degree. Without increasing the dimensions of the instrument, 

 by which its sensibility would be impaired, he has been enabled, by 

 the adaptation of a small telescope, readily to measure y 1 ^ of its de- 

 grees, that is, about ^ -J- ¥ of a centigrade degree. 



One of the most interesting points to which the author directed his 

 attention, was the possibility of detecting heat in the moon 1 s rays. 

 These rays, concentrated by a polyzonal lens of 32 inches diameter, 

 and acting on the thermo-multiplier, gave no indication of any effect, 

 so that Professor Forbes considers it certain that if there be any, it 

 must be less than j F -J R 7 of a centigrade degree. 



He repeated Melloni's experiment of the refraction of heathy a rock- 

 salt prism, and was enabled to obtain some approximate quantitative 

 results, giving the index of refraction for heat in this substance, 

 which was a little less than that for light. 



In the course of his second section he describes further experiments 

 relative to the question discussed by Melloni, of the separation of the 

 effects due to heat and light, especially the peculiarity (before men- 

 tioned) attending green light; he tried flames variously colored with 

 salts — giving red, yellow, green, and blue light; but found the pro- 

 portions of rays transmitted by alum, glass, and rock salt to be nearly 

 constant for each substance. 



To this part of the subject Professor Forbes again directed his 

 attention, in a later series of experiments, in which he has obtained 

 numerical results of the highest value. These are detailed in the last 

 part of his third series of Researches on Heat, read before the Royal 

 Society of Edinburgh, April 1G, 1838, and published in the Transac- 

 tions of that body, vol. xiv. To the earlier portion of this memoir we 

 shall refer, under another division of this report. 



The third section relates to the index of refraction for heat of 

 different kinds as compared with that for light in the same medium. 

 The method of observation adopted is indirect, turning upon the de- 

 termination the critical angle of total internal reflection. This was 

 ascertained in rock-salt prism, having two angles of 40°, and one of 

 100°. The sentient surface of the pile is so placed with regard to the 

 prism that it continually receives rays coming from the source of 

 heat, after undergoing two refractions and one reflection, whatever be 

 the angle of incidence, which is effected by a very simple but 

 ingenious mechanical construction. Every kind of precaution to 

 avoid error was adopted. And in this way the author obtained a 

 series of indices "for the mean quality of the heat most abundantly 

 contained in the rays obtained from \arious sources." These values 

 are given in a tabic, and are professedly but approximate. Professor 

 Forbes has, however, subsequently favored me with an unpublished 

 communication, in which he states that while the numbers may be 

 regarded as relatively correct, in order to become absolutely so, they 



