76 



KNOWLEDGE. 



March, 1915. 



connected in so wonderful a system were, in 

 reality, a series of dots in close apposition, it would, 

 indeed, be a remarkable fact, and one strikingly 

 suggestive of an artificial origin. Is it not more 

 comprehensible and credible to regard them as 

 continuous lines ? 



It has frequently been stated, and possibly 

 believed by the uninitiated, that the canals have 

 actually been resolved into less regular formations 

 by means of apertures larger than those generally 

 employed by Professor Lowell and others who 

 record observations of canals. It is said that 

 drawings made with larger telescopes, though 

 revealing objects which Lowell has apparently 

 failed to see, yet do not show any trace of the hard 

 lines which Lowell habitually draws. In their 

 place appear hazy, uncertain bands, or in some cases 

 nothing at all. 



It is time that these matters were fully explained 

 in " Knowledge." 



It may occur to the reader that the whole 

 matter might be cleared up by selecting an 

 arbitrator, who should examine Mars under the 

 best conditions obtainable, and declare what he 

 saw. This would seem to be a reasonable sug- 

 gestion; but it is impossible, because there is a 

 disagreement as to what are the best conditions 

 under which to view Mars. Under certain con- 

 ditions no one of any observational ability can fail 

 to see the "canals." Under other conditions they 

 cannot be seen at all. It is one of the purposes of 

 this article to discuss what are the best conditions 

 under which to view the planet. 



For the benefit of those who are not accustomed 

 to planetary observing, I may explain that it is 

 not altogether a simple matter. An unpractised 

 eye finds it difficult to see anything except the 

 mere bald image of the object. It is only by long 

 practice by an able and experienced observer 

 that fine details can be seen at all. 



A great deal also depends, as will appear, upon 

 the quality of the observer's vision and upon 

 climatic conditions. 



It is well known that the latter affects planetary 

 observations to a very marked extent. This is 

 by reason of the fact that the air is not homo- 

 geneous in its refractive properties, owing to 

 differences of temperature. 



On some nights which are quite fine and clear, 

 and would appear to a layman to be very suitable 

 for observational purposes, " seeing " is so bad that 

 fine details are entirely obliterated or blurred out 

 of recognition. 



Small telescopes are far less affected in this 

 respect than are large ones. The larger the tele- 

 scope, the more susceptible is it to adverse climatic 

 conditions of this nature. This must be admitted 

 by everyone. Telescopes of great aperture can 

 only be used with advantage for fine detail under 

 practically ideal conditions, which are rarely, if ever, 

 encountered in the localities in which these great 

 instruments are situated. 



I recall a night of exceptional transparency at 

 Mr. J. H. Worthington's Observatory on the 

 heights of Hampshire. Jupiter, viewed with a 

 four-inch telescope, gave a comparatively steady 

 image ; but, with an aperture of ten inches, it 

 was less steady, but good for an English climate. 

 Viewed with the twenty-inch reflector, it was 

 on that night practically hopeless so far as fine 

 detail was concerned. The brilliancy or trans- 

 parency of that night is testified to by the fact 

 that in the early hours of the morning Delavan's 

 comet was observed low down in the eastern 

 sky, which was already bright with the approaching 

 dawn. It was picked up by Mr. W. H. Steavenson, 

 and this was the first time the comet had been 

 seen by anyone on its reappearance after con- 

 junction with the Sun. 



It is thus apparent that it is inadvisable to use 

 excessively great apertures under imperfect climatic 

 conditions. Any further separation of detail 

 which is obtained by using greater apertures is 

 absolutely outweighed by the increase in the 

 unsteadiness of the image. 



Best results on Jupiter were on that evening 

 obtained, not with the twenty-inch telescope, 

 but with the ten-inch Cooke ; and I am 

 sure that this difference was not entirely due to 

 the fact that the ten-inch is a refractor, while the 

 twenty-inch is a reflector. It should be remem- 

 bered that the climatic conditions prevailing at 

 Mr. Worthington's observatory, which is situated 

 at a height of seven hundred feet above sea level, 

 are exceptionally favourable for England. 



The disadvantage of using excessive apertures 

 under imperfect conditions can be disputed by no 

 one accustomed to the use of large instruments for 

 work on planets. It is a matter of everyday 

 experience, and, were this article intended for 

 astronomers alone, I would not have thus laboured 

 the point. 



In setting up an observatory for planetary work, 

 altitude should be one of the first considerations, 

 for at high levels the atmosphere is least disturbed. 

 The greater the altitude, the better the seeing. 

 Large apertures may thus be used with advantage 

 more frequently at high levels than at low. It is 

 agreed by all that the observatory which enjoys 

 the finest conditions is that of Professor Lowell, 

 Mars Hill, Flagstaff, Arizona. Lowell makes all 

 his observations there. This observatory is situated 

 on the Rocky Mountains at an elevation of no less 

 than seven thousand two hundred and fifty feet. 

 Most of his work on Mars has been done there with 

 a twenty-four-inch refractor, which is exceptionally 

 free from imperfections. Even in the magnificent 

 atmosphere at Flagstaff, Lowell finds that, save on 

 exceptionally fine nights, best results are obtained 

 when he is not using his full aperture. He stops 

 his aperture down to eighteen, and sometimes even 

 to thirteen, inches in accordance with the " seeing." 

 Those observers who do not draw canals use 

 telescopes of large aperture under imperfect climatic 



