266 M. Melloni on the Itadiatmis of Incandescent Bodies^ 



main invisible to the observer, vvlien the spectrum arises from 

 luminous ajjencies so little enerwtic as are those of the first 

 periods of incandescence. 



To a perfectly sensitive eye, the variations of length would 

 evidently have taken place in the direction of the more refran- 

 gible rays only, and all the spectra would have commenced at 

 the extreme limit of the red rays. 



It results from all these observations, that when the incan- 

 descence of a body becomes more and more vivid and brilliant 

 by the elevation of its temperature, there is not only an aug- 

 mentation in the intensity of the resulting light, but also in the 

 variety of elementary colours which compose it ; there is, too, 

 an addition of rays so much the more refrangible as the tem- 

 peratui-e of the incandescent body is higher. In this there is 

 therefore established an intimate analogy between the pro- 

 gressive development of light and that of heat. Indeed, as 

 soon as I had convinced myself of the immediate transmission 

 of every variety of radiant heat through rock salt, I availed 

 myself of that valuable property to study the refraction of heat 

 from different sources ; and I discovered that radiations coming 

 from those of a high temperature contain elements more re- 

 frangible than those which are derived from sources that are 

 not so hot. 



After having pointed out how very important for the theory 

 of the identity of calorific and luminous radiations, is the ana- 

 logy or rather absolute equality which exists between his ex- 

 periments and mine on the successive appearance of the ele- 

 mentary rays, Professor Draper passes forward to investigate 

 the law of the increase of the luminous and calorific radiations, 

 according as the temperature of the source of heat is elevated. 



Bouguer has shown that a difference of one-sixtieth in the 

 quantity of light which acts on the eye is insensible, and that 

 thus this fraction constitutes the limit of perceptible variations. 

 From this it arises, that we easily perceive differences in the 

 intensity of light when they exceed even to the smallest extent 

 the sixtieth part of the entire quantity. Let there be placed, 

 for example, an opake cylinder between the red-hot platina, 

 or other source of light, and a sheet of white paper arranged 

 at such a distance that its surface may be illuminated all over 

 by the light of the shining body, except on those parts where 

 the shadow of the cylinder is thrown. Then let there be a 

 lamp furnished with a metallic tube in which there is a small 

 opening ; let the rays of the lamp which go out through the 

 aperture fall on the paper enlightened by the luminous source 

 of which we wish to measure the intensity ; let the lamp be 

 gradually approached until the shadow of the cylinder is no 

 longer sensible ; let this experiment be repeated in each of 



