456 



Sir David Gill. 



" The Spectrum of 77 Argus." By Sir David Gill, K.C.B., LLD. ? 

 F.B.S., H.M. Astronomer at the Cape. Beceived May 24, — 

 Bead June 6, 1901. 



[Plate 4.] 



The star ?/ Argus, as is well known, was for a short time almost the 

 brightest star in the heavens. Between 1677 and 1870 its light 

 fluctuated between magnitude and 6 '8, and, since the latter date has 

 gradually faded from 6f to 7| — its magnitude at the present day. 



Soon after the McClean telescope was mounted, and by way of 

 testing its performance, a plate was taken, with the object-glass prism 

 of 8J° refracting angle in front of the object glass, of the area of the 

 sky surrounding rj Argus. 



As this plate showed that rj Argus had a very remarkable bright-line 

 spectrum, an attempt was made to obtain a spectrograph with the slit 

 spectroscope, together with a compaiison spectrum. Within the past 

 few weeks I have been engaged in measuring some of these experi- 

 mental spectrograms — a work that other occupations had until now 

 prevented me from undertaking. 



As the reductions of the measures show that the spectrum of 

 ?/ Argus closely resembles that of the Nova Aurigge, it seems to be of 

 considerable interest, in view of the appearance of Anderson's new 

 star in Berseus, to publish the present results, although in many 

 respects they are not so complete as might otherwise be desirable. 

 Thus I have no doubt that, by sacrificing the definition near Hy and 

 by a longer focal setting and longer exposure, one could get a con- 

 siderable extension of the spectrum in both directions with the 

 objective prism, and, with the slit-spectroscope, obtain a good deter- 

 mination of the velocity of the star in the line of sight by a much 

 shorter exposure and with direct comparison of the brightest star-line 

 with H/3. These further points may, however, remain for future 

 investigation. 



The plate taken with the slit spectroscope is shown in fig. 1 

 (Blate 4). It was exposed as follows : — 



1899. April 14 Exposure 165 minutes. 



„ 15 „ 10 „ /- 



„ 16 „ 150 



„ 17 „ 45 



Total 6 h. 10 m. 



The comparison spectrum of iron was obtained from a single 

 brilliant spark between iron terminals connected with a powerful coil 

 and battery of Ley den jars immediately before the first day's 

 exposure. 



