C 29 ] 
II. Computation of the Lengths of the Waves of Light corresponding to the Lines in 
the Dispersion Spectrum measured by Kirchhoff. By George Biddell Airy, 
Astronomer Loyal. 
Received March 2, — Read March 21, 1867. 
The greatest and most valuable system of measures which we possess of the lines in the 
Dispersion Spectrum, produced by the emanation of light from the Sun, by transmission 
through the Atmosphere, and by the combustion of Metals, is that published by Pro- 
fessor Kirchhoff in the Memoirs of the Berlin Academy for 1861 and 1862. This 
noble series includes about 1600 lines, taken in uninterrupted succession through the 
spectrum, beginning a little before Fraunhofer’s line A and ending a little after G. 
The same apparatus was used throughout, and in the same manner; and the system 
followed in the whole process was so nearly constant that we may speak of it generally 
as one consistent series of measures. This work has been made known to English 
readers by an accurate translation published by Professor Boscoe. 
The measures thus exhibited depend upon the form of the apparatus, the refractive 
and dispersive powers of the glass, the nature of the scale employed, and the value of 
its unit of measure. They are not therefore available for physical research, until they 
are cleared of the effects of these circumstances of experiment. There can be no hope 
of our arriving at, or even commencing, any mechanical theory on the formation of the 
spectral lines, until we have obtained the one natural measure of each, namely the 
length of the Wave of Light corresponding to each. 
I have therefore undertaken the labour of computing from each spectral measure the 
corresponding length of the Wave of Light; and I trust that the result may not be 
unacceptable to the Boyal Society. 
I will now describe the method which I have used. 
When I commenced this work, I could not learn (although I made inquiries) that any 
careful measures had been made of the Diffraction Spectrum since the original measures 
of Fraunhofer. I therefore determined to make Fraunhofer’s measures of the prin- 
cipal lines the basis of my computations. From Gilbert’s ‘ Annalen der Physik und 
der Physikalischen Chemie,’ XIV. Band, Leipzig 1823, page 559, I took the following 
values of the Length of a Wave, omitting the value for B, which apparently was not 
determined in the same series of measures with the others : — 
in. 
FR c =Length of wave for C, in Paris inch, =0-00002422, 
Fr d = 
55 
r> 
55 
= 0-00002175, 
Fr e = 
55 
„ E 
55 
= 0-00001945, 
MDCCCLXVIII. 
F 
