78 
POPULAR SCIENCE REVIEW. 
suspect that Jupiter may differ from the minor planets in respects even more 
important than size and rapidity of rotation. Processes of such a character 
as to alter the appearance of a belt which exceeds the whole surface of the 
earth many times in extent must .be due, one would suppose, to an action 
affecting a large proportion of the planet’s mass. It would be well worth 
while to enquire whether the spectroscope affords any indication of the 
nature of those processes which are now at work upon the leading member 
of the planetary system. 
Eclipses of the Sun and Moon . — There will be an eclipse of the moon par- 
tially visible at Greenwich on January 17. The moon will rise shortly 
before the termination of the real eclipse, but her face will be clouded by 
the earth’s penumbra for some time after. A fortnight later there will be a 
partial eclipse of the sun not visible at Greenwich. 
Measuring Star-discs. — Mr. Proctor suggests a method of measuring the 
discs of those stars which are liable to be occulted by the moon, which 
would be worth trying by those who possess powerful telescopes. He 
remarks that u if the image of a star is put in rapid rotation (which may be 
done in many ways), while the centre of rotation moves in a circle, in a 
period exceeding the maximum duration of luminous impressions on the 
retina, the appearance presented to the eye will be that of an epicycloidal 
coil of light whose rapidly advancing end is always on a black ground. 
Now, if the eye be intently fixed on this coil just before the star’s oc- 
cultation by the moon, it is obvious that at the instant of occupa- 
tion the whole coil will vanish ; but during the brief interval which the 
luminous impression on the retina occupies in vanishing, there will be time 
to notice the appearance of that end of the coil which was last formed. 
Now, if we suppose the rapidity of rotation such that a quarter of a revolu- 
tion was made while the moon was traversing the star’s disc, it is clear that 
along that quadrant there will be a gradual diminution of light down to 
evanescence at the extremity. If the star had an apparent diameter only 
half as large, the arc of fading light would be but half a quadrant, and so 
on. The observation would be rendered easier by the fact that a neighbour- 
ing part of the coil would always give the means of a ready comparison.” 
lie finds that if the star’s image rotated 100 times in a second, a star ten 
times as far off as a Centauri, and having a real diameter as large as our 
sun’s, would give an arc of fading light about 55£° in length. 
New Method proposed for observing the Transits of Venus . — Professor 
Young suggests that the spectroscope should be made use of in observing the 
approaching transits of Venus. The plan he proposes depends upon the fact 
that the approach of the moon to the sun’s limb, at the time of the total 
eclipse of August last, was rendered sensible before the time of actual con- 
tact, by the gradual obliteration of the bright lines belonging to the spec- 
trum of the chromosphere. lie considers that the approach of Venus to the 
sun’s limb just before the transit of 1874 begins, and the retirement of Venus 
from the limb just after last external contact, might be rendered sensible in 
the same manner. There seems little doubt that every method by which 
the phenomena presented during a transit of Venus can possibly be observed 
with advantage, will be well studied by astronomers, and in good time. 
Every astronomer worthy the name will look anxiously and hopefully for- 
