Dr. Wentworts Ercx on the Satellites of Mars 35 
therefore, I employed the naked eye only, and, not having an aperture small 
enough to extinguish /vm, I estimated his limiting aperture from that of « Lyre, 
which I found to be 0.01 inch. 
Now, on the 15th September I estimated Mars seen through an aperture 
of 0.01 inches to be fully a Struvéan magnitude greater than a Lyre seen through 
the same aperture ; and therefore the limiting aperture for Mars would have been 
0.005 inch. 
But the aperture for the satellite was 1.40 inches; now these apertures are to 
each other, inversely, as 1,000 to 3.6. 
This ratio of aperture was further confirmed, by viewing the planet and the star 
B through neutral tint coloured sun glasses, which admitted the use of the telescope 
and larger apertures ; it was also confirmed by viewing both objects reflected from 
the first surface of a prism, as is done when looking at the sun. 
Now, assuming the reflecting powers of the planet and the satellite to be similar, 
their diameters will be inversely as their limiting apertures, that is as 1,000 to 
3.6; so that if we take the diameter of Mars as 4,000 miles, we have as the dia- 
meter of the satellite 3.6 x 4 = 14 miles; or, at the then distance of Mars, the 
disc of the satellite would subtend an angle of about 0.08 seconds. 
In the case of Vesta, as with the satellites of Saturn, I found that, 
"owing to the exceeding minuteness of the discs, there was very little loss of 
light in using a magnifying power of 300, which had been employed on the 
satellite of Mars. With this power I found the limiting aperture for Vesta to 
be 0.25 inch; but if Vesta, on 8th of October, the day of comparison, had been 
in the place of Mars on the 15th September, her light would have been increased 
45 times; and therefore her limiting aperture would have been reduced from 0,25 
inch to 0.037 inch. | 
Thus the apertures for Vesta, seen under the same circumstances as the Satellite 
of Mars, and for the Satellite, are 0.037 and 1.4 inches, or very nearly in the ratio 
of 1 to 40; so that if Vesta reflects light as does the Satellite, then the diameter 
of the Satellite would be 2,th of the diameter of Vesta. 
Now, according to the best authorities, the diameter of Vesta is supposed to be 
230 miles, which, at present distance, would correspond to a dise of 0.7 seconds, 
and this cannot be far from the size of the actual dise visible in the telescope. 
Taking the diameter as 240 miles we have as the diameter of the Satellite, on 
the assumption of equal reflection, 6 miles. It is a very curious coincidence that 
the mean of these two values, 14 and 6, is exactly the estimated diameter given by 
Professor Hall, viz. 10 miles; and also how this value, 6 miles, obtained 
on the assumption of equal reflection with Vesta, agrees with Mr. Procter’s esti- 
mate of 5 miles. 
But I think it will appear that the reflecting power of Mars, or his satellites, and 
that of Vesta are by no means equal. For if, on the 18th September, Mars had 
