12 



PROFESSOR FORBES'S RESEARCHES ON HEAT. 



such a surface, as it is well known that polish becomes more specular for rays of 

 light consisting of longer undulations, the inequalities of the surface first becoming 

 insignificant for red light. 



25. In this I was not deceived. My purpose not being to investigate fully 

 the subject of diffuse reflection, I confined my attention to the establishment of 

 the general fact. Employing an apparatus which I have not yet described, but 

 which bears a great analogy to that figured in the Society's Transactions, vol. xiv, 

 Plate XIII., and described in art. 51 of the Third Series, I observed the intensity 

 of reflection of heat from different sources at a single polished surface of flint- 

 glass, and at a similar surface depolished with emery. I obtained at considerable 

 incidences the following striking results as to the increased susceptibility of heat 

 to be regularly reflected at a rough surface, when it is of low temperature or re- 

 frangibility. 



Ratio of the Intensities of Heat reflected by a POLISHED and a ROUGH Surface of 



Flint-Glass. 



So far then the character of the action of depolished surfaces is consistent. The 

 stifling effect (which diminishes both the reflected and refracted ray) of a rough or 

 laminated surface-, diminishes mitli the refrangiHlity of the incident heat. That the 

 same thing takes place in the Reflection of light we know ; it is probable that it 

 does so in its transmission likewise, though this has not been so distinctly ob- 

 served. Most impure substances transmit a ruddy gleam, vapour of water does 

 so whenever it is not colourless,* and every practical optician knows, that in a 

 great majority of media the violet end of the spectrum is first absorbed. 



20. A more minute analysis of the influence of surface upon heat is what we 

 now propose. And three questions present themselves for immediate solution, (1 .) 

 If deficiency of polish produce a variation in the proportion of not less than 3 to 1 

 in the quantity of transmitted radiated heat from different sources, can we employ 

 salt plates with the ordinary degree of polish, and yet consider them as equally 

 transparent for every kind of heat, as M. MELLONI'S discovery has hitherto en- 

 titled us to do ? (2.) Is the effect of roughness common to other substances as 

 well as rock-salt ? (3.) The operation of depolishiiig with sand-paper is nothing 

 more than the making of an infinite number of distinct grooves on a polished 



* Edinburgh Transactions, vol. xiv. p. 371. 



