380 
rorULAR SCIENCE REVIEW. 
due to the illumination of a lunar atmosphere. Newton and 
others of Halley’s friends held a different opinion on this point, 
and Halley was therefore unwilling to insist on his theory ; but 
there can be no question that he accepted it as highly probable. 
But in our day this view has fallen wholly into disrepute, or 
rather it had done so long before our day. The careful study 
of the occultation of stars by the moon has convinced astrono- 
mers that the moon has no atmosphere of appreciable extent, 
certainly none so extended as would be required to account for 
the corona. The study of the cusps of the new moon has con- 
firmed this view. Schroter and others have detected or sus- 
pected signs of a prolongation of the cusps, such as would occur 
if the moon had an atmosphere, and, therefore, a twilight-circle 
of appreciable extent. But the prolongation is so slight as to 
prove — even if accepted as existent — that any lunar atmosphere 
must be of the most limited extent. And, lastly, the most 
delicate of all the observations which have been made to deter- 
mine whether the moon has an atmosphere is undoubtedly 
that which Dr. Huggins made several years ago upon the 
spectrum of a star occulted by the moon’s dark limb. It is 
obvious that the chance of recognising in the spectrum of a star 
the signs of the existence of a lunar atmosphere, causing a 
gradual diminution of brilliancy as the moon traversed the 
star’s disc — sudden as the passage undoubtedly is — was far 
greater than that of noticing a change in the brilliancy of the 
star itself, seen directly. Yet Mr. Huggins could detect no 
sign whatever which indicated the existence of a lunar atmo- 
sphere. 
Even regarding the general question of a lunar atmosphere 
as undecided, we can yet feel no hesitation whatever in regard- 
ing it as demonstrated that the moon has no atmosphere which 
can account for the corona. 
Taking next in order the theory that the corona is an 
optical phenomenon due to the passage of the sun’s rays 
through our ow T n atmosphere, we have considerations of less 
simplicity to deal with ; but yet I think we shall have very 
little difficulty in coming to a conclusion respecting this 
theory also. 
In the first place, let us endeavour clearly to recognise what 
the theory is. This is not so simple a matter as it might seem, 
for those who hold the theory generally that the corona is an 
optical phenomenon are not always in accord as to the way in 
which the optical phenomenon is produced. Some, indeed, 
will not allow our atmosphere to have any part in the matter 
at all ; but while denying that the corona is a solar appendage, 
or that it is due to the existence of a lunar atmosphere, yet 
divide between the sun and moon their theory of the cause of 
