OE SE Ee a LR ee et eS LOE EE ee We Me Ne ety 
eS ye OT ie eee Tea ee 
ee ee. Le eR ae ee ee eR ee eR RI ee 
E. S. Holden—Note on the satellite Tethys. 51 
For November 20, 84.5 Wash. m. t., log. r, = 0°9787 and 
log. p, = 09554. The mean distance of Saturn is 953889 
(log. +, = 0°9795) and the log. of its mean distance from the 
earth is log. p,=0°9789. Thus the brilliancy of Saturn was 
greater on November 20 than at its mean opposition in the 
ratio of 1118 to 1, or H, = 1118 H,. Thus Zethys = 
0000071 H, or in words, Zethys in this particular part of its 
orbit has seventy-one millionths of the brilliancy of the ball of 
Saturn at mean opposition. 
Zollner has determired (op. cit. p. 145) the relative brilliancy 
of Saturn’s ball at mean opposition of Capella to be Saturn= 
0-431 Capeila ; (log. 9°6345). Hence Tethys=0-000030 Capella ; 
(log. 5-440). ; 
From this we can determine the stellar magnitude of Tethys. 
If the light of a first magnitude star (as Capella) be assumed 
as 1-000 and if the light-ratio be 8 (0-40) and if m be the mag- 
nitude of the star on Argelander’s scale, then &"-'= the light 
of the star in terms of the light of the first magnitude star as 
unity. For us 6"-! = 0-000030 or m = 12°8 approximately. 
[On Struve’s scale m’ = 11°8. On Herschel’s scale m” = 12°. 
The resulting stellar magnitude of Tethys on Argelander’s 
scale being as we have seen 12°8 it should be just visible with 
a telescope of a little over four inches aperture.* This I tested 
on November 23d, and I find that with four inches aperture 
Tethys was just barely visible at elongation (Kast) with a power 
of 200 diameters when Saturn was in the field. When Saturn 
was put out of the field it was just steadily visible. With a 
power of 400 it was better seen, never totally disappearing. 
With an aperture of five inches on the finder and a power of 
thirty it was not seen. The appearances were the same to both 
Andeison and myself, These observations give a rough check 
on the preceding accurate ones, as the two methods agree better 
than could be expected. It also affords some evidence as to the 
eyes of the two observers. 
Tf the relative brilliancy of the various satellites among them- 
selves be measured, the foregoing observations afford a ready 
means of deducing their brilliancy in terms of a standard star 
like Capella and hence in terms of any standard star. 
U.S. Naval Observatory, Washington, 1878, Nov. 26. 
. ogson: Mon. Not. R. A. S., vol. xxi, p. 34. It is important to remember 
that in the application of Pogson’s formula there is considerable uncertainty 
owing to the varying effects of different magnifying powers, and to o' ; 
