CONDITIONS OF HABITABILITY OF A PLANET. 



85 



of the great public observatories of the world have been turned 

 upon the planet, and the most skilful and experienced astronomers 

 have not been ashamed to devote tlieir time to it. 



There is no need to attempt to review the immense mass of 

 observations that have been accumulated in the last thirty-five 

 years. We may take as representative of the two parties in the 

 controversy Mr. Lowell himself, who has observed Mars with 

 such perseverance for the last eighteen years, on the one side, 

 and on the other, M. Antoniadi, an architect by training and 

 an astronomer by genius, who has even a longer record to show. 



In the opposition of 1909, Mr. Lowell was observing Mars 

 from his observatory at Flagstaff, Arizona, a site carefully chosen 

 by himself for the good definition obtained there, while 

 M. Antoniadi had the use of the great 33-inch refractor of the 

 Meudon Observatory, near Paris. The former showed the planet 

 as covered with a perfect network of " canals," which he describes 

 as " narrow regular lines of even width throughout, running 

 with geometric precision from definite points to another point 

 where an oasis is located." These canals are drawn as following 

 the arcs of great circles, and sometimes extend almost half 

 round the planet, disregarding all inequalities of surface, and 

 Mr. Lowell speaks of them as being so strai.L>ht that in a 

 drawing they have to be put in by the aid of a ruler, a freehand 

 line not being straight or uniform enough. M. Antoniadi, on 

 the other hand, though he shows " canals " of a kind, shows 

 them as streaks, that is to say, they have not the hardness, the 

 narrowness, or the uniformity of Mr. Lowell's representations. 

 They are not mere geometrical lines, but have characteristics of 

 their own ; there is no trace of any geometrical network, looking 

 like the figure of a proposition in Euclid, and M. Antoniadi is 

 quite clear that such network does not exist. Yet his drawings 

 show an immensity of fine detail, much of which escaped the 

 scrutiny of Mr. Lowell. 



Within the last few years it has been found possible to 

 enlist the services of photography in this connection. The 

 difficulties of doing this can only be appreciated by those who 

 have actually attempted it. First of all, the size of the image 

 of the planet depends upon the focal length of the telescope, 

 and at a good opposition the diameter of the image of Mars 

 formed by a mirror or object glass is just one ten-thousandth 

 part of that focal length. In other words, a telescope one 

 hundred inches long, that is 8 feet 4 inches, would give an 

 image only one-hundredth of an inch in diameter, a mere 

 pinpoint. If, however, we desire the image to be only one- 



