234 Sir Norman Lockyer and Mr. A. Fowler. 



The Kensington Photographs. 



An investigation of the spectrum of a, Aquilae was commenced at 

 Kensington in 1890, and, with the various instruments employed up 

 to 1892, fifteen negatives were obtained.* In all these the lines 

 were ill defined, and it was decided to take a special series of photo- 

 graphs " in order to determine whether the haziness of its spectrum 

 lines is invariable."! Since then a considerable number of photo- 

 graphs has been obtained, but although variations have been suspected 

 it is found difficult to estal)lish their reality. One thing seems quite 

 certain, namely, that the lines are always ill defined. 



At the Koyal Society Conversazione in 1894, enlarged copies of 

 photographs of the spectra of a Aquilse and P Arietis were exhibited 

 which indicated that Pickering's view that the haziness of the lines is 

 due to rotation is probably correct. 



Dr. Scheiner's experiment of photographing the solar spectrum out 

 of focus has since been repeated ; but while it was found possible to 

 produce bands in this way, only a few of them agree with those in 

 a Aquilse. Among these coincident bands, are 4031 — 4036 (Mn), 

 4046 (Fe), 4064 (Fe), 4132—4135 (chiefly Fe), 4143-6—4144 (Fe), 

 4226-9 (Ca), 4250-3-4251 (Fe), 4260-2—4260-6 (Fe), 4271-3—4271-9 

 (Fe). 



On the other hand, by taking an out-of-focus enlargement of a 

 negative of the spectrum of P Arietis, the violet being put more out 

 of focus than the blue, the spectrum of a Aquilse is almost perfectly 

 reproduced (see Plate). The difference in width of the bands appears 

 to be sufficiently explained by the gradually increasing dispersion in 

 prismatic spectra as the violet end is approached. "With the instru- 

 ment employed at Kensington a tenth-metre near X 4046 is repre- 

 sented by a distance on the photographs about 1*4 times as great as 

 that corresponding to the same difference of wave-length near A 4384 ; 

 and since the velocity which would produce a displacement of one- 

 tenth metre at X 4384 would produce a displacement of 0-92 tenth- 

 metre at X 4046, the displacements on the photogi'aphs for the same 

 velocity, with the particular instrument employed, will be in the pro- ^ 

 portion of 1 to 1-29 at X 4384 and X 4046 respectively. 



' Classification of the Star. 



This experiment appears to be a sufficient demonstration of the 

 essential similarity of the spectra of a Aquilse and /3 Arietis, so that 

 the former conclusion that the two stars should be classed together i» 

 perfectly justified. 



* ' Phil. Trans.; A, vol. 184 (1893), pp. 685-G88. 

 t Ibid., p. 696. 



