184. S& P. Langley—Determination of Wave-lengths 
tion of energy, a further consideration of the relations existing 
between the two charts is necessary. The law of dispersion of 
the prism causes the Sungate of energy in its spectrum to — 
be quite different from vont have been observed with a 
the same in both. The area between 
any two ordinates of the curvet 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 aig of 
the normal curve required to the 
o cane at any two wave-lengths, 
shall be equal to that asda between the same wave-lengths © 
in the latter, and from this condition we can deduce a con- 
struction for effecting the required transformation. 
* J. W. Draper, Phil. Mag., ne — p. 104, 1872. 
+ See this Journal, for March, 
t See J. Miller . Pogg. Ann alm, “oh id Rls ee Pogg. Annalen, vol. ly, 
y off upon a line, (fig. 4 ie “convenient ainteosk and divide it info 
equal ‘spaces to represent oat normal w e-length scale, and upon a line OD, at 
cee angles to the first, lay off the i deena and divide it into the same 
mber of parts, spaced eootie to the mi? of dispersion of the prism, as ™ bets 
Sevidength seale ma mg 2 n the bottom of the  oocgea chart (Plate 11, 
Journal, vol. xxv). m 
e 
e proper wave-length, Redrecomte n both lines at rik eum which lie nearest 
each other, figure, where oe ordinates are shown; through e inter 
section of ¢ detiae ing o ordinates voici the curve “EF, and upon CD," 
curve of distribution of ener: 
a. 3, ery small wavelength interval on the prismatic scale; ¢, 
same interval on the ‘nornall scale, and 6 and d Sag average np Be of the be 
aded part of the figure 
senting, therefore, the portion of the total sane included betw 
n these ee 
ef is a portion of the curve EF, fig. 4. Then, according to the condition f trans 2 
formation 
cd=ab 
whence ,Ord::e:a. 
From geometrical considerations, 
C:¢é: 
ng ae 
where @ is the angle which the chowd EF, ne the intersections of the two 
ntly 
pairs of ordinates makes with AB; conseque 
tan @ 
pas, 
