THE PLANET MARS IN FEBRUARY 1869. 
40 
arranged with perfect uniformity all round this quadrant. When 
the light falls between the clouds it is supposed to be returned 
after a considerable absorption, corresponding to the shaded 
• spaces. When it falls on a cloud it is supposed to be returned 
after much less absorption — that is, to remain much more 
brilliant after reflection, corresponding to the unshaded spaces. 
And it is at once seen that near the limb all the light is (in this 
imaginary case) derived from reflection at the clouds, whereas, 
near the centre of the disc, the larger proportion is derived from 
reflection at the real surface of the planet. 
There is nothing doubtful in the above explanation except the 
assumed existence of small' clouds — invisible separately to the 
naked eye. But this assumption seems at once more natural, and 
to explain the difficulty better than the sugar-loaves of Zollner. 
It may be, however, that when the sun is near the horizon of 
Mars, heavy mists hang in the air, as happens commonly enough 
with us both in the morning and in the evening. This would 
account equally well for the observed peculiarity. 
I should be glad to hear that any one armed with a telescope 
of adequate power had done something to test the climatic re- 
lations of Mars, and also the diurnal changes in the state of the 
Martial atmosphere. By noticing at what part of the disc the 
features appeared most distinct (allowance being made for real 
differences in the distinctness of the markings), something might 
readily be done in this way. The spectroscope also might be 
rendered very efficiently available in this inquiry. It has been 
already noticed by observers that the winter hemisphere is per- 
ceptibly less distinct on the whole than the summer hemisphere. 
For example, the features marked in the upper halves of the 
figures in Plate XL. may be expected to be less distinct than 
those on the lower. But then, as there are places on earth 
where the winter climate is drier than elsewhere, so it may be 
that parts of the winter hemisphere of Mars may be more dis- 
tinct than others. In considering diurnal changes account must 
be taken of the gibbosity of Mars at the time of observation, 
because, as we have said, the centre of the disc of Mars may be 
far removed from the centre of the illuminated hemisphere. 
Perhaps the most remarkable discovery yet made respecting 
the physical condition of Mars, is that contained in a communi- 
cation addressed to the Eoyal Astrouomical Society, by Mr. 
Huggins, early in the year 1867. From this paper I extract the 
following particulars. 
On several occasions during the opposition of 1867, Mr. 
Huggins was able to make observations of the spectrum of 
the planet’s light, or, to use his own accurate phraseology, “of 
the solar light reflected from the planet.” During these obser- 
vations he saAv groups of lines in the blue and indigo parts of 
VOL. VIII. — NO. XXX. E 
