570 



Intelligence and Miscellaneous Articles. 



Stars. 



Obseryers. 



Huggins. | Greenwich. 



Seech i. 



Vogel. 



Sirius 



+ 

 + 



± 









 

















I 



RiVel 



+ 

 + 



+ 



+ 



Regulus 



r\ Ursae Majoris... 







a. Andromeda; . . . 

 Arcturus 





It is seen that accordance exists in several instances, but there 

 are numerous contradictions. We recognize moreover by other 

 signs that a cause of error exists as yet unknown, and that the me- 

 thods of investigation require to be carefully reviewed. 



Several facts are at once very striking: (1) nearly all the obser- 

 vations at Greenwich give negativo results *, positive results being 

 the exception ; (2) the results on different days are not only very 

 discordant as to magnitude, but sometimes even opposite ; (3) Mr. 

 Huggins's observations give very similar results for a great number 

 of stars ; (4) his results for the comet Coggia are not in accordance 

 with the motion of that comet as known from other sources ; (5) the 

 mean values assigned by the different observers are extremely different. 



Surprised at these irregularities, I put to myself the question, 

 Can there he, either in the mode of observing, or in the instruments, a 

 systematic cause of error, producing the displacement of the line, un- 

 known to the observer ? In order to assure myself, I made the 

 following experiments, endeavouring to avoid the conditions which 

 were suspected to be faulty, and to vary as much as possible the 

 other conditions of observation. 



First, to illuminate the slit, I placed the Geissler tube simply in 

 front of the objective at a distance of about 5 decims., arranging it 

 so that the capillary tube, about 6 centims. in length, was exactly 

 parallel to the diameter of the objective and to the slit of the spec- 

 troscope : the light was sufficiently bright in the ocular of the spec- 

 troscope to permit the lines to be seen distinctly. The spectroscope 

 (a direct- vision one) was formed of a Merz prism composed of five 

 strongly dispersing prisms. In the solar spectrum the lines D of 

 sodium were, with this apparatus, separated by double their breadth. 

 To dilate transversely the spectrum of the star, an achromatic cylin- 

 drical lens was placed near the eyepiece, inside the little analyzing 

 telescope. 



The equatorial being directed to the star Sirius under favourable 

 atmospheric conditions, we were able to see clearly the dark line F 

 of the star simultaneously and in the same field with the hydrogen- 

 * See ' Monthly Notices/ vol. xxxvi. p. 30 et seq. 



