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. As is 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 



