. II.] GENERAL NOTIONS. 37 



surface of bodies placed within the same space and already at 

 the common temperature. 



Thus the reflection of heat does not disturb the equilibrium 

 of temperatures, and does not introduce, whilst that equilibrium 

 exists, any change in the law according to which the intensity of 

 rays which leave the same point decreases proportionally to the 

 sine of the angle of emission. 



50. Suppose that in the same enclosure, all of whose parts 

 maintain the temperature a, we place an isolated body M, and 

 a polished metal surface R, which, turning its concavity towards 

 the body, reflects great part of the rays which it received from the 

 body; if we place a thermometer between the body IT and the re 

 flecting surface R, at the focus of this mirror, three different effects 

 will be observed according as the temperature of the body J/ is 

 equal to the common temperature a, or is greater or less. 



In the first case, the thermometer preserves the temperature 

 a ; it receives 1, rays of heat from all parts of the enclosure not 

 hidden from&quot; it by the body M or by the mirror ; 2, rays given out 

 by the body ; 3, those which the surface R sends out to the focus, 

 whether they come from the mass of the mirror itself, or whether its 

 surface has simply reflected them ; and amongst the last we may 

 distinguish between those which have been sent to the mirror by 

 the mass J/, and those which it has received from the enclosure. 

 All the rays in question proceed from surfaces which, by hypo 

 thesis, have a common temperature a, so that the thermometer 

 is precisely in the same state as if the space bounded by the en 

 closure contained 110 other body but itself. 



In the second case, the thermometer placed between the heated 

 body M and the mirror, must acquire a temperature greater than 

 a. In reality, it receives the same rays as in the first hypothesis ; 

 but with two remarkable differences : one arises from the fact that 

 the rays sent by the body J/ to the mirror, and reflected upon the 

 thermometer, contain more heat than in the first case. The other 

 difference depends on the fact that the rays sent directly by the 

 body M to the thermometer contain more heat than formerly. 

 Both causes, and chiefly the first, assist in raising the tempera 

 ture of the thermometer. 



In the third case, that is to say, when the temperature of the 



