34 Dr. Wextwortn Ercx on the Satellites of Mars. 
for us to guess at the size from the brightness, on the assumption that the 
surface of the satellite is of equal reflecting power with that of the planet itself. 
The comparison can, of course, only be effected, by means of an intermediate object 
seen under precisely similar circumstances, with regard to the planet, as the satellite: 
itself. 
Such objects were the stars A and B.* A is the star which on 2nd 
September was mistaken for the satellite; its "position then, with regard 
to the planet, was about 290°; and distance from the limb about three diameters 
of the planet. Subsequent observations caused me to estimate the star A as slightly 
fainter than the satellite. 
Again, on 15th September, at 23 hours G. 8. T., the star B was seen; its position 
then was about 280°; and distance from limb of planet circa, 90’. This star was 
fortunately seen in the field along with the satellite itself, and was judged to be a 
little fainter than the satellite. The comparison was made, two or three times, at 
short intervals. So I concluded that the satellite was decidedly brighter than A, 
and very slightly brighter than B. The next thing was to determine the brightness 
of A and B by means of extinguishing apertures. 
After several experiments, on different nights, I found that the mean limiting 
aperture for A was 2.0 inches ; and for B 1.5 inches. By limiting aperture I mean 
the aperture just sufficient to show the star distinctly. But as the satellite was 
slightly brighter than B I shall assume the limiting aperture for the satellite toe 
have been 1.4 inches. 
Now 1.4 inches aperture, with my eye and telescope, corresponds to 9.5 mag. 
Struve, or 13 mag. h., and such therefore I estimate the satellite to have been on 
the 15th September. 
Having thus ascertained the limiting aperture for the satellite, I next tried to 
obtain the. limiting apertures for Mars himself, and also for Vesta, which was in the 
immediate neighbourhood. 
In the case of Mars, seen through an exceedingly small aperture, I found, 
as might have been expected, that magnifying power was not admissible, for 
it converted the enlarged disc into a nebulous haze. In the case of Mars, 
* The places of A and B are :— a‘ 
AS A233) 1210S PD — Ola (mo: 
B ; RA—22 580; PD—102 42 0. 
The above places are not corrected for refraction, and may be in error to the extent of 5 seconds of time 
and 30 seconds of arc. 
The star A is 10 magnitude Struvé, or 14 mag. h.; it is preceded on the parallel, at 10 seconds, by 
another star of the same magnitude. There is an 8.5 Struve preceding A, at about 67 seconds, and 2 
“minute true north of it. 
The star B is the larger component of a double star; mags. 9.6 and 10 Struvé; distance circa, 70”, 
and positicn angle, 350°. This is the true northern, and faintest, of two or three somewhat similar doubles. 
in a fie'd of perhaps half a degree. 
