1867.] 



On the Lengths of Waves of Light. 



405 



exalt the power of the magnets, and the other made available for blasting 

 or other purposes. Want of time prevented me carrying this out until 

 now; but since the interesting papers of C. W. Siemens, F.R.S., and Pro- 

 fessor Wheatstone, F.R.S., were read last month, I have carried out the 

 idea as follows : — Two bars of soft iron, measuring 7i in. x 2| in. x | in., are 

 each wound, round the centre portions, with about thirty yards of No. 10 

 copper wire ; and shoes of soft iron are so attached at each end, that when 

 the bars are placed one above the other there will be a space left between 

 the opposite shoes in which a Siemens' s armature can rotate : on each of 

 the armatures is wound about ten yards of No. 14 copper wire cotton- 

 covered. The current generated in one of the armatures is always in con- 

 nexion with the electro-magnets ; and the current from the second arma- 

 ture, being perfectly free, can be used for any purpose for which it may be 

 required. The machine is altogether rudely constructed, and is only in- 

 tended to illustrate the principle ; but with this small machine three inches 

 of platinum wire '01 can be made incandescent. 



March 21, 1867. 



Lieut.- General SABINE, President, in the Chair. 



Dr. Thomas Sterry Hunt and Dr. Thomas Richardson were admitted 

 into the Society. 



The following communications were read : — 



I. " Computation of the Lengths of the Waves of Light corre- 

 sponding to the Lines in the Dispersion-Spectrum measured 

 by Kirchhoff.^^ By George Biddell Airy, F.R.S., Astronomer 

 Royal. Received March 2, 1867. 



(Abstract.) 



The author, after adverting to the excellence and importance of the 

 spectral measures made by Professor Kirchhoff, points out that these 

 measures are not available for physical inquiry until we have deduced from 

 them the length of the light- wave corresponding to each line. The author 

 therefore undertook the work of computing the lengths of the light-waves. 

 For this purpose, he referred to Fraunhofer's direct measures of the lengths 

 of the waves corresponding to certain lines, and, ascertaining the numerical 

 measures in Kirchhoflf's scale corresponding to the same lines, he expressed 

 Fraunhofer's wave-lengths by an algebraical formula, in which the variable 

 (quantity was KirchholFs measure. This formula was applied to each of 

 the lines (about 1600 in number). The wave-lengths were at first ob- 

 tained in parts of the Paris inch ; but all were ultimately converted into 

 parts of the milhmetre. 



VOL. XV. 2 L 



