1883.] 



Capt. Abney. Solar Spectrum. 



137 



II. " The Wave-lengths of A, a, and of Prominent Lines in the 

 Infra-Red of the Solar Spectrum." By Captain W. DE W. 

 Abney, R.E., F.R.S. Received November 20, 1883. 



M. Fievez has recently sent me a map of the solar spectrum from 

 C to A* inclusive, and as part of this region is one which I have 

 been measuring, I have examined the new publication with great 

 interest. Photography and eye measurements do not exactly coincide 

 in the detail of the grouping of the little a group as far as A, and A 

 itself is shown by M. Fievez's map as wanting some details which 

 appear in the photographs. Thns in the photographs there are some 

 seventeen lines, whilst in M. Fievez's map there are but thirteen. 

 Between A and a fchere are several lines of marked intensity in the 

 photograph, which are not shown in the new map. The wave-lengths 

 of the different lines from above " a " to A are not the same as 

 those given by Fievez, when they are taken from comparison photo- 

 graphs of the 1st order of the red and 2nd of the ultra-violet on 

 the same plate, or when checked by photographs of the 2nd 

 order of the red with the 3rd order of the green taken in a similar 

 manner. In my paper, u Phil. Trans.," Part II, 1880, I gave a 

 method of using mirrors by which this could be effected, but since 

 Professor Rowland introduced his concave gratings this is much 

 more readily carried out. He has kindly furnished me with gratings 

 for the purpose, having about 14,400 lines to the inch, with focal 

 distances of 7 feet 6 inches and 12 feet 6 inches respectively. These 

 have been employed in determining the wave-lengths of this part of 

 the spectrum.. Cornu's map was nsed as a reference for the ultra- 



o 



violet wave-lengths, and Angstrom's -map for those in the bine and 

 green. The two maps may be taken as equally exact. 



Description 

 of line. 



X from 

 comparison 

 of 1st and 

 2nd orders. 



X from 

 comparison 

 of 2nd and 

 3rd orders. 



X according 

 to Fievez. 



Remarks. 



"a" 



f 7184-4 

 L 7185-4 



7184-5 

 7185 -4 



7197-7 \ 

 7198 7 J 



f This is shown in Ang- 

 < strom's map as a 

 [ single line X 7184'9. 



Most refran- 

 gible edge 

 of A. 



7593 -6 



7593-7 



7600 -0 



Angstrom gives 7604 

 for the .centre of this 

 line, which of the 

 bands he rook as A 

 is not clear. Lang- 

 ley gave 7600*9 for 

 this edge. 



* " Annales de l'Observatoire Royal de Bruxelles," nouvelle serie, tome V. 



