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POPULAR SCIENCE REVIEW. 
pression of Mars on the assumption that the internal constitution of the 
planet follows the same law as that of the Earth. 
On the Brightness and Dimensions of the Satellites to the Planets . — 
For some time Professor Edward C. Pickering, of the Harvard College 
Observatory, has been engaged in a series of important photometric inves- 
tigations; and in Part 2 of Vol. XI. of the Annals of the Harvard College 
Observatory , he has published the results of the observation of the brightness 
and size of the satellites of the planets. 
Mars . — The inner satellite of Mars , Phobos, is fully 03 magnitude 
brighter than the outer satellite Deimos, and Deimos is in turn 14-5 magni- 
tudes fainter than the brightness of the central portion of the disc of Mars. 
Deimos seems to be variable in brightness, being fully 05 magnitude 
brighter when on the following side of the planet, than when on the 
preceding side. Assuming that the satellites reflected as much of the 
incident light as Mars itself, Prof. Pickering deduces 6 and 7 miles as 
the diameter of Deimos and Phobos, or less than 0"*05 for their maximum 
apparent diameter. According to Prof. Pickering, a certain amount of 
light is not prejudicial to the visibility of these faint objects. 
Jupiter . — Observations were made of the brightness of the satellites, by 
comparing them with the planet itself, and from the amount of light they 
reflected, their apparent size was calculated on the hypothesis that they 
reflected the same proportion of the incident light as the planet. The 
results were — 
I. Satellite. II. Satellite. III. Satellite. IV. Satellite. 
Photometric diameter 0*873 0*818 0*035 0*615 
Measured diameter . 1*081 0*910 1*537 1*282 
Albedo ... 0*652 0*809 0*455 0*230 
Or, in other words, the satellites of Jupiter reflected only two-thirds, four- 
fifths, four-ninths, and one quarter of the same proportion of incident light as 
the planet itself reflects, while this is known to be only six-tenths of the total 
light falling on the surface. These results are in complete accord with the 
known phenomena presented by the satellites. 
Saturn . — Observations were made of all the satellites of Saturn, with 
results as follows, namely, that the comparative dimensions of the different 
satellites were, as given below, on the assumption that all reflected the same 
proportion of the incident light : 
Mimas 
. 292 miles. 
Enceladus . 
. 370 
v 
Tethys 
. 570 
ff 
Dione 
. 542 
ff 
Rhea 
. 745 
ff 
Titan 
. 1406 
ff 
Hyperion . 
. 193 
ff 
Japetus 
. 486 
ff 
if, therefore, Titan be taken as usually shining like an 8| magnitude star, 
the brightness of the different satellites may be approximately considered to 
be — 
