Prof. Draper on the j)roduclion of Light by Heat. 351 



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 refrangibility are emitted, it is obvious that 

 it by no means follows that the first ray visible should be the 

 extreme red. Our power of seeing that depends on its having 

 a certain intensity. 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 refran- 

 gibihty should first be seen, because they act more energeti- 

 cally on our organ of vision ; and as the temperature rises, 

 the spectrum should undergo a partial elongation in the direc- 

 tion of its red extremity. 



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

 ments coincides exactly with that of M. Melloni respecting 

 heat at lower thermometric points. In his second memoir*, 

 he shows that when the lays from copper at 390° and from 

 incandescent platinum are compared by transmission through 

 a rock-salt prism, as the temperature rises the refrangibility 

 of the calorific emanations correspondingly increases. Those 

 philosophers who regard light and heat as the same agent, 

 will therefore see in this coincidence another argument in 

 favour of their opinion. 



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

 -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 func- 

 tion of vision. 



As the luminous effects are undoubtedly owing to a vibra- 

 tory movement executed by the molecules of the platinum, it 

 seems from the foregoing considerations to follow, that the 

 frequency of tliose vibrations increases with the temperature. 



In this observation I am led by the principle, that " to a 

 particular colour there ever belongs a particular wave-length, 

 and to a i)articular wave-length there ever belongs a particular 

 colour;" but in the analysis of the spectrum made by Sir D. 

 Brewster by the aid of absorptive media, this |)rinciple is indi- 

 rectly controverted ; that eminent philosopher showing that 

 red, yellow, blue, and consequently white light, exist in every 

 part of the spectrum. This must necessarily take place when 

 a prism which has a refracting foce of considerable magnitude 

 is used ; for it is obvious that a ray falling near the edge, and 

 one falling near the back, after dispersion, will paint their 

 several spectra on the screen ; the colours of the one not co- 

 * Taylor's Scientific Memoirs, vol. i. p. 66. 



