DEVELOPMENTS IN ASTRONOMY 
PLASKETT. 261 
the fine geometrical markings on Mars, commonly called canals, has 
been, during the last opposition of 1909, strenuously and ably sup- 
ported by Lowell and one or two adherents, and equally strenuously 
and ably combated by many astronomers, chief among whom was 
Antoniadi. Asis well known, the majority of astronomers are unable 
to see these fine sharp lines, although plenty of other detail is visible. 
During the last opposition photography was used to a much larger 
extent, but I question whether it has settled the matter. Lowell says 
the principal canals show on his photographs, while others are unable 
to see them. The only way this question can be settled is, as Aitken 
suggested, for Lowell to invite some well-known observers, such as 
Barnard, Burnham, and others, to Flagstaff at the next opposition 
and let the whole question be fought out. 
Another disputed point is the question of water vapor on Mars. 
The detection of this water vapor depends upon the visibility of a 
small band or group of lines in the red end of the spectrum produced 
by the presence of water vapor. Slipher, Lowell’s assistant, photo- 
graphed the spectrum of Mars and then the spectrum of the moon. 
The light from Mars, which is, of course, reflected sunlight, passes 
twice through Mars’ atmosphere and then through the earth’s atmos- 
phere. The light from the moon, which has no atmosphere, passes 
through the earth’s atmosphere only. If now there is water vapor 
in appreciable amount in the atmosphere of Mars this band should 
be stronger in the spectrum of Mars than in that of the moon. Slipher 
found that it was stronger in the Martian spectrum, but unfortunately 
some little time elapsed between the two exposures, and there is a 
possibility that the greater strength of the band was due to change in 
the amount of water vapor in the earth’s atmosphere. Director 
Campbell, of the Lick Observatory, considered the question of 
sufficient importance to organize an expedition, carrying instruments 
to the summit of Mount Whitney, elevation 14,500 feet, at which 
altitude only one-fifth of the earth’s water vapor is above and four- 
fifths below. Any small difference between the moon and Mars 
bands will show relatively more conspicuously than at the elevation 
of Flagstaff, which is about*7,000 feet. His photographs were made 
within a few moments of one another, and with Mars and the moon 
at the same altitudes, and are, hence, directly comparable. I saw 
them myself last summer at Mount Wilson, and I can say that there 
is no discernible difference in the vapor bands in the two spectra. 
The bands are very weak and evidently due to the small amount of 
water vapor present in the earth’s atmosphere above Mount Whitney. 
Campbell comes to the conclusion that there is no spectroscopic evi- 
dence of the existence of water vapor on the planet. Although he 
specifically states that he does not contend that Mars has no water 
vapor he says that it is too slight to be detected by the spectroscopic 
