18 



Mr. A. Le Sueur on Observations with the [June 16, 



XVII. " Observations with the Great Melbourne Telescope, in a 

 Letter to Prof. Stokes." By A. Le Sueur. Communicated by 

 Prof. Stokes, Sec. U.S. Received April 18, 1870. 



Observatory, Feb. 27. 



Dear Sir, — I have little more definite to tell you with reference to the 

 star i] Argus ; thinking that a larger dispersion would be of advantage, I 

 have had a supplementary arrangement added to the spectroscope, by 

 means of which a direct prism may be interposed between the collimator 

 and the usual prism. 



With this increased dispersion, the red line keeps its place ; the yellow 

 one turns out to be slightly more refrangible than D. 



The green lines, which, with the smaller dispersion, were very difficult, 

 now become almost unmanageable ; this would seem to throw some doubt 

 on their reality, as mere extra dispersion should have little effect on real 

 lines. 



The direct prism being a small one, does not take in the whole of the 

 pencil when condensed to the limits bearable by the collimator ; but as 

 the arrangements at my disposal do not in any case admit of utilizing 

 the full condensation, the smallness of the prism has not had any material 

 effect. 



On the whole, I am now inclined to think that, with respect to the 

 green lines, the appearance of the spectra is due to a character of light 

 somewhat similar to that of a Orionis, &c. ; a spectrum of groups of dark 

 lines, with spaces more or less free between them, producing the effect 

 (when the light is not sufficient to bear a slit fine enough for dark lines) 

 of a spectrum crossed by bright lines. 



The behaviour of the red line, however (of the blue one, being less con- 

 spicuous, I cannot speak with so much confidence), would lead to the 

 already drawn inference that it is a real hydrogen line. 



I have examined other stars of about the same magnitude as rj Argus ; 

 in the majority of these there is not even a suspicion of condensation in 

 any part of the spectrum ; red stars, R Leporis for instance, give a spectrum 

 not dissimilar to that of rj Argus ; but the red line on none of the stars 

 examined is so conspicuous as in rj. 



The weather since the beginning of this year has been more favourable, 

 so that I am able, by degrees, to increase the amount of work done. The 

 routine work is the review of figured nebulae ; as might be expected, the 

 4 feet gives views considerably different from the C. G. H. drawings ; but at 

 present I have nothing worthy of special mention. 



The light of the nebulae, as they are taken up for general examination, 

 is analyzed with the prism ; of those which have been examined I have yet 

 found none for which it may be certainly said that the light is not of defi- 

 nite refrangibilities. 



In irregular nebulae, the bright knots even, which are so distinctly 



