508 



HISTORY OF SCIENCE. 



of the surface. Thus tarnished lead radiated 45, while lustrous lead 

 radiated only 19. 



The researches of Leslie were surpassed both in delicacy and extent 

 by those of the Italian physicist MELLONI (1801 1853), to whom we 

 are indebted not only for much of our knowledge of radiant heat, but 



also for the most delicate instrument for detecting minute differences 

 of temperature. It will be unnecessary here to describe the several 

 steps by which Melloni, working partly in conjunction with Nobili, 

 brought the instrument to its present perfection. We shall describe 

 the instrument as represented in Fig. 232, and as used by Melloni in 

 his classical researches. At A B is seen the arrangement of five thermo- 

 electric couples, composed each of a bar of bismuth (shaded light) and 



