396 Original Articles. [July, 



1834. As these results were attained at a period when the number of 

 facts known was limited, and when the interest in the phenomena of 

 light now excited did not exist, an abstract of them may be presented 

 with advantage. 



Professor Draper's Researches. — The general plan of these investi- 

 gations, which occupied many years, was — 1st, to determine the con- 

 ditions of incandescence of solid bodies ; 2nd, the nature of flames ; 

 and 3rd, their variations at different temperatures. I shall therefore 

 speak of them in that order, premising some remarks on the forms of 

 spectroscopes that were used. 



I. Spectroscopes. — The spectroscope first employed was similar to 

 that used by Fraunhofer, consisting of a distant slit, a prism either of 

 flint-glass, or bisulphide of carbon, and a telescope when visual obser- 

 vations had to be made, but suitably modified for photographic inves- 

 tigations. As is generally understood, this form of apparatus will 

 give very excellent results, but the space occupied by it renders it 

 less manageable than the more recent forms. By its aid, modified, as 

 has been said for photographic purposes, the extra-spectral fixed lines 

 above H, viz. M, N, O, P, were discovered by Dr. Draper, as im- 

 pressed on a daguerreotype plate, and an engraving of them published 

 in the ' Philosophical Magazine,' for May, 1843. He estimated that 

 there were were about 600 of these lines so impressed between H and 

 P. In like manner three lines, a, fi, y, of less refrangibility than A, 

 were also detected. These latter lines were attributed to absorption by 

 the earth's atmosphere,, the others to the atmosphere of the sun. 



But Dr. Draper also made use of another form of spectroscope, 

 which does not seem to be even yet thoroughly appreciated. He replaced 

 the prism with a grating or finely-ruled reflecting surface, the advantage 

 of this being that the absorption of dioptric media was avoided, and 

 the fixed lines, as it may be said, occupied their proper positions in 

 the order of their wave-lengths. The prism separates the lines 

 unduly towards the violet end, and crowds them together towards the 

 red. On the contrary, in the interference spectrum from a grating, 

 the yellow is in the centre, and the light declines away towards the 

 violet end above, and towards the red below. 



With this form of spectroscope the fixed lines were photographed, 

 as in the other instance, and maps of their position published. By 

 its use also, Dr. Draper was enabled to settle the question, which 

 could not be determined so decisively with the prism, namely, the 

 distribution of heat. It was shown that the centre of the yellow 

 space is the hottest, and not a region less refrangible than the red, as 

 is the case in Herschel's prismatic experiments, and that from that 

 point the heat intensity declines towards the violet above, and the 

 red below. 



In these delicate heat experiments, advantage was taken of inves- 

 tigations that had been published by the same author in the ' Philo- 

 sophical Magazine,' June, 1840, under the title of ' Electro-motive 

 Power of Heat ;' and in which some exceedingly delicate forms of 

 thermo-electric combinations had been described. 



II. Of the Incandescence of Solids. — The first formal memoir of the 



