March, 1915. 



KNOWLEDGE. 



77 



conditions, and always, whatever the " seeing," 

 use their full aperture. They do not see the canals ; 

 and it is doubtful whether LoweU, with all his 

 experience and ability, could see them in like 

 circumstances. 



Another point regarding telescopes is that, in 

 order to counteract the effects of the secondary 

 spectrum, the greater the aperture of a telescope, 

 the greater in proportion to that aperture must 

 be its focal length. With telescopes of very large 

 aperture it has been found a mechanical 

 impossibility to make the focal length sufficiently 

 great to counteract this defect. LoweU gets over 

 the difficulty to a certain extent by the use of 

 colour screens. 



The idea that the reason why "canals " are not 

 seen by users of certain large telescopes is because 

 they are resolved into irregular component parts has 

 now received its final death-blow. During the 

 recent opposition of JMars a successful attempt was 

 made at the Flagstaff Observatory to see the canals 

 with an aperture of forty inches. With this article 

 are published for the first time two drawings of 

 Mars made with the forty-inch Alvan-Clarke 

 reflector on the same evening by Professor Lowell 

 and Mr. A. E. Slipher respectively. It is a great 

 testimony to the steadiness of the air at Flagstaff 

 that the canals could be seen there with an aperture 

 of that magnitude. The canals were not seen \\'ith 

 the greatest facility, but unmistakably appeared 

 as thin, unbroken, straight lines. 



It has been said that Lowell, though he draws 

 exceedingly fine details in the canal system, 

 altogether omits faint objects of a comparatively 

 coarse nature, which appear in the dark regions 

 of the planet on the drawings of others who see no 

 canals. This in some cases may conceivably be 

 true ; but it cannot be argued from this that 

 Lowell's drawings, corroborated as they are by 

 numerous drawings of other observers, are there- 

 fore fallacious. Lowell, with his exceptionally 

 acute vision, has devoted his life to the observ- 

 ation of fine planetary detail. The canals are to 

 him the important features on Mars, and he draws 

 them. The delicate shadings in the darker regions 

 do not so greatly interest or occupy him. 



It seemed to me that an investigation into the 

 physiology of vision might have some light to 

 throw on this and other astronomical problems. 

 I found that the present state of our knowledge 

 in this direction was rather rudimentary. There 

 is, however, one fact which is incontrovertible : 

 that there is a distinction between aciiieness of 

 vision (sometimes called acuity), which is the power 

 enabling a man to read fine print in the distance 

 or decipher fine planetary detail, and sensitiveness 

 to impression, which is the power of appreciating 



faint contrasts, enabling a man to pick out faint 

 contrasts of a comparatively coarse nature in the 

 darker regions of the planet Mars. 



These two qualities, though frequently confused, 

 are quite distinct. The possession of one of them 

 in a high degree is no guarantee of the possession 

 of an abnormal share of the other. In discussing 

 this matter with Dr. Edridge-Green, who has, 

 I suppose, contributed more to our knowledge on 

 the subject of the physiology of vision than any 

 other living man, I learnt from him that acuity 

 was dependent upon perfection of the eye, while 

 sensitiveness to shade and colour contrasts was 

 dependent upon the discriminating power of the 

 mind. Colour-blindness is thus analogous to tone- 

 deafness. 



Then with regard to photography. No one will 

 deny that in the matter of discrimination of fine 

 detail the eye is immeasurably superior to the 

 camera. The value of the camera is that what it 

 records is incontrovertible, and only needs correct 

 interpretation. It would appear practically im- 

 possible to obtain photographs of objects so delicate 

 as the ' ' canals. ' ' Yet this feat has been accomplished 

 at Flagstaff, and the more prominent "canals" 

 (including doubles) have testified to the fact of 

 their existence by recording their image on the 

 photographic plate, to the satisfaction of persons 

 experienced in reading photographs. Canals can, 

 of course, be seen only on the very finest photo- 

 graphs. Lowell's photographs of Mars, and Saturn 

 also, stand absolutely unrivalled in excellence, 

 though, since Lowell led the way in planetary 

 photography, others have obtained good results 

 in this direction. I refer, of course, to Professor 

 Barnard's photographs, which, though of supreme 

 excellence, cannot be said to excel those obtained 

 by the Flagstaff observers.* 



There are many misstatements which have been 

 put forward on the subject of straight lines on 

 Mars, but it is impossible to deal with all of them 

 in a single article. The following is possibly worth 

 mentioning as an example. 



I have seen it stated that the " canals " are never 

 seen steadily, but are merely glimpsed in flashes, 

 generally lasting for about one quarter of a second. 

 This is not true. Professor Lowell has assured me, 

 during a conversation on this subject, that at Flag- 

 staff the larger " canals " are frequently held abso- 

 lutely steadily. It was, said Lowell, not easy to say 

 for how long the more difficult features in the " canal 

 system " were seen, but that they came out clear and 

 sharp in moments of best seeing. As they were seen 

 always in exactly the same place, there could be 

 no doubt as to their objectivity. Mr. Worthington 

 also tells me that this was also his experience 

 when observing " canals " at Flagstaff. 



♦ I have in my possession some photographs of Mars taken by Professor Lowell during the opposition of 1907, 

 which he was good enough to give to me during his recent visit to England. They show indications of the canals. 

 Unfortunately they will not bear reproduction. They are open to the inspection of any reader sufficiently 

 interested to communicate with the Editors of " Knowledge." 



