342 On Ike Objections to Mr. PrevosVs Explanation [Mav, 



the object, is reflected on it ; and so much as the imperfection of 

 the reflecting ))ower of the plate suffers it to absorb, is radiated by it 

 in the same direction. Here, then, is the compensation; and 

 here 1 thinl< Mr. Murray's objection fails. A blackened surface 

 radiates much, it is true; but it intercepts an equal volume of 

 radiation or reflection from l}ehind. A polished surface radiates 

 less, but it reflects as much as it fails to radiate. So when the 

 writer in the Encyclopaedia says " a hot body ought to cool more 

 slowly when near a large body of inferior temperature than 

 when near a small one," lie forgets that this large body intercepts 

 an equal volume from beyond it, and therefore the hot body ?o 

 placed ought 7/oi! " to cool more, nipidly." Ijc says the reverse'^ 

 the case ; but I apprehend it would be found, by careful experiment, 

 that if the adjacent body be of the same temperature as those 

 whose ravs it intercepts (the screen, for instance, of the same 

 temperature as the wall), the reverse will not. be the case, but the 

 time of cooling will be the saiiie. (N. B- Dr. Wells's lately de- 

 tailed interesting experiments on the formation of dew, in conse- 

 quence of the loss of heat by uncompensated radiation, will be 

 found to be in perfect conformity to this view of the subject.) 



Then in the case of the concave reflector it is true that one 

 hemisphere of converging rays is intercepted from behind, and only 

 a cylinder of parallel rays in front, brought to the focus by reflec- 

 tion ; yet this is a compensation : for the intensity of the rays so 

 brought to the focus is equal to that of the converging rays which 

 would have passed in the same direction through the space occupied 

 by the disc, as might be easily shown by the general laws of radia- 

 tion and reflection. When a second mirror faces the first, all this 

 cylinder of rays is intercepted ; but then all the rays which pass 

 through its focus to its face are reflected in the direction of the 

 intercepted cylinder, and a cylinder is formed of reflected rays, 

 similar to, and in lieu of, the intercepted one. The whole cylindric 

 space comprised between the two mirrors now consists of two sets 

 of rays proceeding in exactly opposite directions from and to both 

 the foci. 



Let us now place a liot body in the focus of the second reflector. 

 This last cylinder now consists of extraordinary calorific rays ; and 

 of course a body (as a thermometer) in the focus of the first mirror 

 receives superabundant heat. Substitute an extraordinary cold 

 body ; then, as all the rays which would have passed through tliis 

 focus, and would have constituted the second cylinder, are now 

 absorbed by a body which has very little to radiate, the radiation of 

 the thermometer towards the first mirror is uncompensated ; and it 

 indicates decrease of temperature. 



In Mr. Pictet's detail of the experiment it was observed that 

 wlien the temperature t)f the ice was lowered by the addition of 

 nitrous acid, the thermometer sunk several degrees lower than 

 before. The tiieory proceeds here with perfect regularity. Ice at 

 -f- 32 radiates something towards the compensation : at — 20, still 



