394 Prof. Draper on the Production of Light by Heat 



the reds, and on the other in the blues, the terminations being 

 equidistant from the centre of the yellow space. 



Now if the rays coming from shining platinum were passed 

 through a piece of glass, on which parallel lines had been drawn 

 with a diamond point, so as to give an interference spectrum, 

 even admitting the general results of the foregoing experiments 

 to be true, viz. that as the temperature rises rays of a higher j*e- 

 frangibility are emitted, it is obvious that it by no means follows 

 that the first ray visible should be the extreme red. Oar power 



depends 



Even 



when it has assumed that extreme brilliancy which it has in a 

 solar beam it is barely visible. We ought therefore to expect 

 that rays of a higher refrangibility should first be seen, because 

 they act more energetically on our organ of vision ; and as the 

 temperature rises, the spectrum shoukfundergo a partial elonga- 

 tion in the direction of its red extremity. 



I may here remark, that the general result of these experi- 

 ments coincides exactly with that of M. Melloni respecting heat 

 and lower thermometric points. In this second memoir* he 

 shows that when the rays from copper at 390° and from incan- 

 descent platinum are compared by transmission through a rock- 

 salt prism, as the temperature rises the refrangibility of the 

 calorific emanations correspondingly increases. Those philoso- 

 phers who regard light and heat as the same agent, will there- 

 tore see in this coincidence another argument in favor of then 

 opinion. 



In view of the foregoing facts I conclude, that, as the tem- 

 perature of an incandescent body rises, it emits rays of light of 

 an increasing- refrangibility; and that the apparent departure 

 from this law, discovered by an accurate prismatic analysis, is 

 due to the special action of the eye in performing the function 

 oi vision. 



As the luminous effects are undoubtedly owing to a vibratory 

 movement executed by the molecules of the platinum, it seems 

 irom the foregoing considerations to follow, that the frequency oi 

 those vibrations increases with the temperature. 



In this observation I am led by the principle, that .« -jr 

 ticular color there ever belongs a particular wave-length, and 

 a particular w-avp-lpno-th th a r a «,^„ k^i^^^o o nnrtio.nlax color, 



» to a p^' 



dto 



, „„, ,»u. c -iciigiii mere ever oeiongs a. fjau^""" - , tf 



but in the analysis of the spectrum made by Sir D. Brewster oy 

 the aid of absorptive media, this principle is indirectly contio- 

 verted ; that eminent philosopher showing that red, yellow, blue, 

 and consequently white light, exist in every part of the spectrun u 

 This must necessarily take place when a prism which has a 

 trading face of considerable magnitude is used ; for it is ob^ i° 



• Taylor's Scientific Memoirs, vol. i, p- 56 



