RADIANT HEAT. 329 



Sir I). Brewster considers Sir W. HerschePs experiment on the 

 refraction of ''culinary" heat by lenses to be very unsatisfactory, as 

 before noticed. He recommends a lens composed of zones, so as to 

 have no greater thickness in the middle than towards the edges, a 

 construction which he has described in his "Optics," p. 322, (Cabinet 

 Encyclopedia,) and made of glass, which unites the highest refractive 

 power with the smallest absorptive power for heat. 



It is also important to find, as sources of heat, bodies which do not 

 become luminous till at extremely high temperatures. 



6.) The researches of M. Melloni have also been extended to this 

 part of the subject. — (Annates de Chimie, December, 1831, p. 388.) 



From known observations on the spectrum, he remarks that there 

 exists, on opposite sides of the maximum, isothermal points — one in a 

 colored part, the other without the red end of the spectrum. 



On causing the different rays to pass through a plate of water, and 

 noting the effect on the thermo-multiplier, the heat of the violet ray 

 was undiminished, but its isothermal totally intercepted. 



That of the indigo slightly diminished; its isothermal not totally 

 intercepted. 



Proceeding in this way with the other rays, he found in general 

 that the portions of heating power intercepted in the colored rays, 

 and those which are transmitted in their isothermal rays, increase in 

 proportion as they approach the position of the maximum, where, of 

 course, upon the whole, the interception is greatest; or, in other 

 words, the rays of the calorific spectrum undergo an interception by 

 water in proportion as their refrangibility is less. 



He gives a table of the numerical results. He views his results as 

 precisely according with and explaining those of Seebeck. With a 

 w^ater prism the heating orange and red rays are more intercepted 

 than the yellow; in this, therefore, the maximum appears. 



Conclusion. 



We have thus far taken as close a survey as is consistent with the 

 limits of a report like the present, of the successive and varied re- 

 searches which have been made with the view of tracing the laws of 

 radiant heat. In the present state of our knowledge it must, upon 

 the whole, be avowed that we have little to contemplate but an 

 assemblage of facts, or alleged facts, determined with more or less 

 accuracy; few, indeed, with any great precision — many resting upon 

 very vague evidence, and in several instances the results of different 

 observers exhibiting a wide discrepancy, or even direct contradiction; 

 whilst, with very few exceptions, any general laws can hardly be said 

 to be established with that certainty which can substantiate their 

 claim to be received as legitimate physical theories. 



In offering suggestions for the advance and improvement of this 

 branch of science, the first and most essential point to which atten- 

 tion ought to be directed is the improvement, or rather invention, of 

 the means of obtaining accurate indications of radiant heat down to 

 its most minute and feeble effects. In reference to this point good 



