ON THE INCREASING MAGNITUDE OF ETA ARGUS. 77 



In i871, when I carefully surveyed the whole cluster in which 

 it stands, Eta was of the 7th magnitude, or adopting Gould's 

 Standard of Star Magnitudes, which has been carefully prepared, 

 my estimate of Eta was 6.8 magnitude. On 4th February, 1874, 

 I was surprised to find it J a magnitude smaller than in 1871, or 

 7.4 magnitude, and I have since been in the habit of frequently 

 examining it and comparing it with several stars in its own 

 cluster which are not variable. During the years 1875 to 1882 

 the change seemed to me inappreciable, but in 1883 the estimates 

 varied from 7.5 to 7.8, in 1884 I made it 7.6, in 1885 and 1886 

 about the same, although in the latter year I several times thought 

 it was getting brighter, in 1887 I was away, but the first 

 examination on my return removed all doubt, there was evidently 

 a very decided increase ; adopting still Gould's Magnitudes for 

 Stars of Comparison, I find Eta at the end of May, 1888, was of 

 6.9 magnitude, or almost as great as it was in 1871. These 

 comparisons were made in the usual way, viz., by estimating in 

 the telescope relative brightness of the star images. 



But I have also compared it with six stars in its own cluster 

 (the same stars used by the older method) by means of a wedge 

 photometer, 180 measures have been made, and give the magnitude 

 7.24 as a result. I have also compared it in the same way with 

 six stars in Kappa Crucis cluster, 90 measures have been made, 

 which give the magnitude 7.42. My experience with the wedge 

 photometer has been that red stars, of which Eta Argus is one, 

 are made to appear smaller than they do by direct vision, or in 

 other words, that red light is more absorbed by the wedge than 

 white light, so that the magnitude of a red star by wedge 

 photometer is smaller than it should be, and as appears in the 

 foregoing, where direct comparison makes it 6.9, and the wedge 

 7.24. As all my previous comparisons were direct, and I think 

 also those of other observers, we must take 6.9 as its present 

 magnitude, and comparing this with the mean of my estimates 

 for 1883 to 1886, or 7.64, it appears that Eta is now f of a 

 magnitude brighter than it was in 1883, and increasing rapidly, 

 so that should the present rate of increase continue as we have 

 every reason to believe it will, Eta will soon be visible to the 

 unassisted eye again, which it has not been for 20 years. 



In 1869 Prof. Loomis collected the then existing observations 

 of Eta Argus, and came to the conclusion that its range was from 

 the 1st to 6th magnitude, and its period 70 years, with minimum 

 about 1870, but as you have just heard the minimum did not 

 occur until about 1885, or 15 years after he supposed it would, 

 and the magnitude got down to 7.64 instead of 6. 



The previous minimum occurred long before there were any 

 regular observers of the star's magnitude, and therefore we cannot 



