in the Atmospheres oj certain Fixed Stars. 197 



In the latter paper he writes, " The most remarkable correspondence 

 is in the case of the large group on either side of H s . A slight 

 shift of about a tenth-metre is required to bring the groups into identi- 

 cal positions. However, the close similarity of the whole grouping of 

 the two spectra as they appear on the plate, admits of little doubt that 

 the extra lines actually constitute the spectrum of oxygen. If this be 

 established, the spectrum of the first division of helium stars would 

 be due to hydrogen, helium, and oxygen." 



In his subsequent work* Mr. McClean concludes, " Taking everything 

 into account, the succession of coincidences between the extra lines of 

 /3 Crucis and the oxygen spectrum can only be accounted for on the 

 basis of the extra lines being in the main actually due to oxygen." 



This conclusion does not, as yet, appear to have been fully accepted 

 by spectroscopists, partly because, from the low dispersion used, the 

 lines of the groups are not separately shown. It is very generally 

 known that the instrumental equipment of the Eoyal Observatory 

 at the Cape, has. recently been enriched by a complete equipment 

 for Astrophysical research, the whole being the munificent gift of Mr. 

 McClean, F.R.S. 



The slit-spectroscope, for attachment to the photographic refractor, 

 reached the Cape in the middle of January last, and I resolved that 

 its first published work should deal with Mr. McClean's interesting- 

 discovery. 



As a complete account of the instrument and its observatory will be 

 subsequently published, it may be sufficient for the present to state 

 that the object glass of the photographic telescope has an aperture of 

 24 inches and focal length of 22 feet 6 inches, its minimum focus 

 being, at present, for rays about midway between H/3 and H y . 



The collimator of the spectroscope has an aperture of 2J inches and 

 focal length of 22-J inches, so that a cylinder of parallel rays 2 inches 

 in diameter falls on the prisms, and the latter are of sufficient size to 

 pass the whole of the rays which form the image of the spectrum on 

 the sensitive plate. The instrument is provided with two camera- 

 telescopes of 2f inches aperture, one being of about 36 inches focal 

 length, the other of 16 inches. 



Only the larger of the two camera-telescopes have been employed in 

 the after-mentioned observations. 



There are two cast>-iron prism boxes ; one of them contains three prisms 

 of about 60° each, which for rays near H y produces a deviation of 180°, 

 so that the camera-telescope becomes parallel in the reverse direction 

 to the slit-telescope. The other prism box contains a single prism of 

 62°. The prisms in both boxes are fixed, without screw adjustment, 

 in minimum deviation for H y . The collimator is in the axis of a solid 

 drawn steel cylinder — the latter attaching by a flange at one end to 

 * ' Spectra of Southern Stars' (Stanford, London, 1898). 



VOL. LXV. Q 



