and on the Elementary Colours of the Solar Spectrum. 265 



of Davy and Newton, which give 812° and 635°; but those 

 numbers, and especially the latter, were obtained by methods 

 too imperfect to be trustworthy ; consequently the number 

 977° F. given by our author, must approach very closely 

 the degree of heat which produces the first incandescence 

 of bodies. 



After having studied this first question, already examined 

 by other philosophers, Professor Draper enters on an entirely 

 new field of research, investigating the nature of the colours 

 which are developed by an ignited body as its temperature is 

 increased. 



For this purpose he employed a prism of fine flint glass, 

 setting it vertically at a certain distance from the strip of pla- 

 tina ; but previously having placed in the position the platina 

 was to occupy, a vertical slit of the same size in a piece of 

 metal, through which a beam of the daylight passed. The 

 spectrum resulting from the transmission of this beam through 

 the prism was received on a small telescope furnished with 

 micrometricwires, and carefully examined in its different parts, 

 for the purpose of determining exactly the position of Fraun- 

 hofer's dark lines. The strip of platina was then set in the 

 same place, and he proceeded to make observations on the 

 spectra produced by it at different periods of incandescence. 

 From these it results, that the first spectrum visible in com- 

 plete darkness corresponds to a temperature of 1210° F., and 

 extends from the fixed line B to the line F; the second spec- 

 trum produced by a temperature of 1325°, commences very 

 nearly at the same line B, and terminates at the line d; the 

 third at a temperature of 144'0°, appears to begin a little 

 nearer the line A, and goes to some distance beyond G ; 

 lastly, tlie fourth, corresponding to a temperature of 2130°, 

 approaches much nearer to A, and extends as far as the line I. 

 In otiier words, the spectrum of the strip of platina which cor- 

 responds to the red extremity of the prismatic spectrum, is at 

 first very short, and contains only the less refrangible colours ; 

 but as the temperature rises, the spectrum of incandescence 

 extends towards the violet extremity, obtaining the more re- 

 frangible tints, and at last acquiring all the colours and all the 

 extent of the solar spectrum, except the terminal rays at the 

 two extremities, which escape the observer evidently on ac- 

 count of their extreme feebleness. The same cause (insensi- 

 bility due to a want of luminous energy) makes the first spec- 

 trum appear, at the red end, a little shorter than the last; 

 because the less refrangible rays of that colour are, as is well 

 known, so feeble, even in the solar spectrum, that we are 

 unable to perceive them unless they are isolated in a place 

 that is totally dark : nmch more therefore ought they to re- 



