442 



NA TURE 



[June 9, 1910 



observations are shown in the chart here reproduced 

 (Comptes rendus, vol. cxlix., No. 20, p. 837), Fig. 3. 

 The most noticeable change since 1907 was in the Syrtis 

 Major, which he found had returned to its form of 1864 

 and 1877. The Lac Mceris, too, had reappeared as a 

 large, indefinite dark patch, and a multiple island was seen 

 in the eastern part of the Mare Cimmerium. 



The Solis Lacus region also presented many striking 

 features, and, among others, M. J. Comas Sola devoted 

 special attention to it (Bull, de la Soc. astron. de France, 

 November, 1909, p. 497). In his opinion, the recent opposi- 

 tion " peut #tre consid6r6e comma la d6route definitive du 

 rcseatix gionietriqiic des canaux." 



It is in regard to these all-important " canali " that the 

 battle of observers rages most intensely. Among European 

 observers, at least, there appears to be consensus of 

 opinion that the term should be used in a more restricted 

 sense, or should only be employed as a generic term 

 embracing several species. There is too great a-diyersity 

 between the broad, persistent, half-tone patches and the 

 narrow, evanescent streaks, glimpsed for one fraction of a 

 second to be lost the next, for them all to be grouped 

 under the one designation. M. Antoniadi strongly insists 

 on this point {Comptes rendus, vol. cxlix.. No. 20, p. 836), 

 and classifies eight varieties. Even then he does not 

 include the fugitive right lines, visible only for the fraction 

 of a second, which he considers may be illusions ; but he 

 very definitely negatives the existence of any geometrical 

 reseaux, of which he finds no trace. At intervals of 

 exceptionally good seeing he sees considerable structure, 

 visible for several consecutive seconds, on the continental 

 areas, and this he describes as " a grey irregular marbhng, 

 complex and cloudy, such as only an artist could render." 



The Rev. T. E. R. Phillips, observing at Ashtead with 

 his 12-inch Calver reflector, was led to substantially the 

 same conclusions (the Observatory, No. 416, p. 463) as 

 M. Antoniadi regarding the canals. 



The necessity for the classification of these features is 

 also advanced by M. Desloges, who suggests {Comptes 

 rendus, vol. cxlix., No. 17, p. 664) three species, and also 

 directs attention to numerous changes observed during this 

 opposition. The fine canatix of his third class w'ere 

 apparently the most affected by the seasonal changes, 

 and M. Desloges finds it difficult to disbelieve their 

 objective existence ; one argument advanced in its favour 

 is that they all appear to start in small gulfs, just as the 

 broad, indubitable, dark bands, of the first and second 

 classes, generally have their origins in the larger gulfs. 



An encouraging feature of the opposition, which in future 

 developments may lead to a settlement of this vexed ques- 

 tion of " objective " and " subjective " phenomena, is the 

 advance made by photography in the recording of the 

 planet's markings. On Prof. Hale's striking photographs 

 (Monthly Notices, vol. Ixx., No. 2, p. 175) the contrast 

 between the dark and light areas is remarkable, the bolder 

 features standing out with a distinctness usually seen only 

 on carefully prepared drawings. 



Results of great interest were also obtained by MM. de la 

 Baume Pluvinel and Baldet {Comptes rendus, vol. cxlix., 

 No. 20, p. 838) at the Pic du Midi Observatory, where the 

 conditions are especially favourable for such observations. 

 The observers intend to make a detailed study of the 1350 

 images recorded on their set of eighty plates, but, from a 

 brief survey, they are able to state that anyone conversant 

 with Martian topography would immediately recognise 

 nearly all the features observed visually. The canals of 

 the first order, the broader bands such as the Indus, the 

 Ganges, Araxes, Cyclops, Euphrates, &c., are all recog- 

 nisable, but there is no trace of the geometrical network 

 of fine canals recorded visually by many observers. 



Whilst in London recently, Prof. Lowell pointed out 

 that while many of the recent photographs form striking 

 pictures by reason of their strong contrasts in the large 

 areas, the treatment which brings out these contrasts is 

 not that calculated to show also the finer details. 



Thus the evidence for the actual existence of the canali- 

 form " canals " is still " mixed." A number of experi- 

 enced, careful observers still proclaim, with no lack of 

 decision, that they exist ; others just as emphatically state 

 that they are, at the most, but the physiological integra- 

 tion of the elements of a mosaic groundwork which covers 



NO. 2 119, VOL. 83I 



the planet. Prof. Frost states that the 40-inch refractor 

 at Yerkes is " too powerful " to show them, and Prof. 

 Hale refers to Prof. Barnard's description of 1894 (Monthly 

 Notices, vol. .Ivi., No. 4, p. 166, 1896) as describing exactly 

 what he sees with the 60-inch reflector at Mount Wilson. 



This question of aperture is not a simple one. Thus 

 Prof. Lowell has repeatedly stated that a large aperture 

 is not infrequently a positive barrier to the seeing of such 

 fine details as occur on planetary discs. Attached to his 

 24-inch refractor he has a system of diaphragms, and the 

 first operation in making an observation is to determine 

 what aperture is most suitable for the conditions obtain- 

 ing at the moment. A similar procedure was followed by 

 Dawes, whose observations in the 'sixties of last century 

 did so much to forward areography. When discussing the 

 work with Sir Norman Lockyer — who also, at that time, 

 was making valuable drawings of Mars — Dawes repeatedly 

 referred to the conditions of seeing as " a 5-inch night " 

 or '' a 6-inch night," &c. .Asked for an explanation, he 

 stated that he often found it necessary to reduce his aper- 

 ture, which normally was 8 inches. 



We also learn from Sir Norman Lockyer that when his 

 drawings were discussed at the Royal Astronomical Society, 

 some doubt was expressed because some of tTie details 

 shown thereon were not shown on the drawings made at 

 the same epoch by the observer using Lord Rosse's re- 

 flector ; yet when the Leyden drawings arrived, later, these 

 details were confirmed. 



Thus Prof. Frost's somewhat enigmatical statement may, 

 logically, be understood to convey a meaning other than 

 that which has generally been ascribed to it, and the 

 failure of the 60-inch reflector to show the straight, hard, 

 sharp lines may not be conclusive evidence of their non- 

 existence. 



So far, the employment of the photographic plate has 

 not provided the hoped-for solution of this special ques- 

 tion, because the exposures necessary are too long. 

 Each image on the plate is an integration, the moments 

 of fine seeing are overlaid by periods of tremor, and, 

 by their very nature, fine lines would be the first to 

 disappear ; it is a case where negative evidence is of 

 little value. Nor does it seem logical to say that these 

 lines do not exist because their appearance can be 

 explained otherwise — physiologically, for instance. Their 

 recognition in the same positions by independent observers, 

 at different times, points to the existence of some material 

 objects, and their changes with the change of season 

 exclude the proposition that they are completely solid 

 markings. Even the suggestion that they are alignments 

 of darker spots does not prove that they are disconnected 

 items. In desert areas the streams dry up, leaving " water 

 holes " — apparently disconnected if viewed from a great 

 distance — and these holes are surrounded by vegetation 

 throughout the dry season, becoming, therefore, isolated 

 objects ; but the river bed is there, and in due season — as 

 on Mars — is filled with water and edged by vegetation. 



But their great size, their prolific distribution, and their 

 rectilinear character, even when seen away from the 

 planet's central meridian, are phenomena which are difficult 

 to explain in the case of the Martian canals ; and the 

 problem yet remains. 



A suggestion made by Dr. Aitken, of the Lick Observa- 

 tory, might possibly solve this vexed question to some 

 extent. Prof. Lowell's unanswerable argument is that, as 

 the " canals " are so near the limit of vision, it is only 

 the very finest atmosphere that they can be seen. AW 

 observers agree as to the first part of this statement, and 

 Dr. Aitken suggests that the second part might be put tc 

 the test bv arranging that such experienced protagonist^ 

 as Prof. W. H. Pickering, M. Antoniadi, and Pro' 

 Barnard should foregather at the Flagstaff Observato: 

 and. with Prof. Lowell, observe Mars during the ne.\ 

 favourable opposition. The 24-inch refractor is, as Prof 

 Lowell has demonstrated, a superb instrument, and foi 

 astronomical observations of this character the Arizor, 

 atmospheric conditions- are une.xcelled. The suggestion 

 a most excellent one, and, could the arrangements b> 

 made, the meeting would no doubt lead to an illuminatior 

 of what, at present, is a very obscure problem. 



There are some problems in astronomy which seem t( 

 be indeterminate. First, we get a positive solution in on^ 



