210 Mr. S. P. Langley's Experimental Determination of 



line, but is here called 12 on account of its being the last con- 

 spicuous break in the energy-curve. 



(1-98 and 2*04). Small but definite lines. The last dis- 

 covered by the bolometer. But the observable solar spectrum 

 certainly extends to a wave-length of over 2 M "70. 



Distribution of Energy in the Normal Spectrum. 



The curve ?i = ^)X given in Plate VIII. enables us to mark off 

 a wave-length scale upon the map of the prismatic spectrum, 

 without any extrapolation, between our present extreme 

 points of observation, a deviation of 50° 58' (corresponding 

 to X=0 M "344), and a deviation of 44° 25' (corresponding to 

 \ = 2^356), and also to construct a map in which the wave- 

 length scale is an ordinary scale of equal parts, but in which 

 the degrees of deviation, if represented, would be unequally 

 spaced. Such a chart of the normal spectrum has, as we have 

 already remarked, the advantage of being entirely indepen- 

 dent of any particular prism or grating, and consequently 

 of being directly comparable with all other maps of the same 

 kind. 



If, besides making a map of the normal spectrum, we wish 

 to construct a curve representing the corresponding distribu- 

 tion of energy, a further consideration of the relations exist- 

 ing between the two charts is necessary. The law of disper- 

 sion of the prism causes the distribution of energy in its 

 spectrum to be quite different from what would have been 

 observed with a diffraction-grating*. Disregarding the ab- 

 sorbing action of the apparatus, the amount of heat between 

 two definite wave-lengths, as between the A and B lines, should 

 be the same in both spectra, provided the total quantity of 

 heat is the same in both. The area between any two ordi- 

 nates of the curvef may be considered to represent the amount 

 of heat in the part of the spectrum included between them, 

 and the total area of the curve represents the total amount of 

 heat. If, then, we suppose the area of the normal curve 

 required to be the same as that of the prismatic one, the con- 

 dition to be fulfilled by the former curve is, that the area 

 included between the ordinates at any two wave-lengths shall 

 be equal to that included between the same wave-lengths in 

 the latter, and from this condition we can deduce a construc- 

 tion for effecting the required transformation J. 



* J. W. Draper, Phil. Mag. vol. xliv. p. 104 (1872). 

 t See this Journal for March 1883. 



X See J. Miiller, Pogg. Annalen, vol. cv. ; Lundquist, Pogg. Annalen, 

 vol. civ. p. 14G ; Mouton, Comptes Rendus, vol. lxxxix. p. 298. 



