Febbuaky 1, igOl."! 



KNOWLEDGE. 



39 



world some 40 millions of miles away all we caji take 

 note of ai-e conti"asts in tone and colour, while the real 

 contoiu- of objects is masked or invisible. Small or 

 faintly-shaded objects, invisible singly, will produce an 

 effect, if close together, of one large mass, and from 

 our inability to see the in-egulai'ity of their grouping, 

 will appear as round spots or long sti'caks. But con- 

 ditions of seeing vaiy enormously on Mai-s, according 

 to its distance and position, and tlic changing illuminar 

 tion of its' disc ; not to speak of vai-iations in ourselves, 

 our atmosphere and our instininients ; the contrasts, 

 therefore, will vary, moi-e detail wiU sometimes be seen 

 in the patches and streaks, fainter mai-kLngs at their 

 edges will appear and disappeai-, altering thcii- outline 

 and extent. The hazy aspect of Schiaparelli's canals 

 may thus be a nearer approximation to reality than 

 the shai-ply defined, and the doubling may be duo to 

 disappearance of faint shadings between more easily 

 grasped boundai-ies. That the canals were discovered 

 after the opposition of 1877, being only suspected during 

 the most favourable period, that they ai-e shai-pest with 

 coloured screens and comparatively small apertures, while 

 in the great Lick and Washington telescopes they have 

 been seen either as few diffused markings, or not at 

 all, suggest that the fine lines ai-e simply a mode under 

 which faint mai-kings may present themselves to imper- 

 fect vision. There is undoubtedly truth in the 

 apparent paradox that greater distinctness comes with 

 poorer vision, for in the best moments the eye dimly 

 perceives, even where it cannot grasp, divisions in simple 

 masses, curves and bluiTing in n;UTow lines, indeter- 

 minate shadows in clear sjDaces. 



Whether the optical theory accounts for all the 

 variations, including those of the polar caps, the future 

 must decide. Most interesting is Cerulli's appeal to 

 the past history of areography, refen-ing to Flam- 

 marion's valuable collection of drawings, all caarefully 

 copied from originals, in his " Planete Mars." Here 

 we may see how in the first rude telescopes impressions 

 of Martian markings were summed up in one large 

 round spot, or one wide band, which latter was by 

 Cassini and some others seen double. By degrees the 

 easiest features of the southern hemisphere were distin- 

 guished, but appeared so variable that an atmospheric 

 origin was ascribed to them. It is particularly in- 

 structive to compare Knott's drawing of November 3, 

 1862, with Lord Rosses of three days later. Knott's 

 telescope was of 7^ in. apertiire, and the features which 

 in the 6 ft. Eosse reflector appeared as large dark 

 patches on a fainter background, he portrays as nan'ow 

 lines on white — canals on a large scale. Again, in 

 two excellent drawings by Kaiser, a broad band where 

 we now recognise Praxodes, seen at the opposition of 

 1862. becomes, six weeks later, when seeing was more 

 diSicuh,, two naiTow bands with faint shadings between. 

 Other examples of gemination in lines and in spots. 

 contractions and enlargements, etc., may be traced, and 

 through all the series there is a remarkable, but in no 

 wise astonishing, vai4ety of representation. One has but 

 to consider the fugitive faintness of the objects, the 

 imperfections of the instruments, and the personality of 

 the observers, which affects not only their vision but 

 their mode of portrayal. On this last point, which 

 comes out very cleai-ly on an examination of the illus- 

 trations in " La Planete Mars," Signer Cerulli has not 

 perhaps laid enough .stress, nor on the influence of un- 

 conscious imitation. 



Mr. Green, the artist astronomer, used to insist on 

 the importance of the trained hand as well as the 



trained eye in order to obtain true pictures of planetaiy 



detail. 



Is the history of discovery with rogai-d to the large 

 markings in Mars' southei-n hemisphere repeating itself 

 now with the more delicate shadings in the northern? 

 and with better optical means would they also lose their 

 misleading appearance of mathematical regularity, and 

 their in.stabilit.y ? 



The ai'tificial origin of the Mai-tian " canals " can 

 hardly be maintained now that they have been scon 

 to traverse the polar caps, and to appear in Venus, 

 Merctu-y, and two of the Jovian satellites. On the 

 optical hypothesis, on the other hand, this is precisely 

 what we might expect. It is perhaps going too far to 

 suggest that the bands of Jupiter and their varying 

 appearances are strict.ly analogous to canals, since their 

 atmospheric origin is rendered probable by other con- 

 siderations, notably by the planet's low density ; yet 

 there is certainly a startling resemblance between some 

 early drawings of Mai's and i-ecent diagi-ams of Jupiter. 

 Schiotcr's Mars, for instance, on page 77, Fig. 48, of 

 " La Planete Mars " (1892 edition), tempts one to quote 

 Dante: — 



+ " Such would Jove become, if he and Mars 

 Were birds, and cliangcd their pluina;;je." 



We ai-e indebted to M. Flammarion for another line 

 of evidence. He had the happy idea of collecting naked 

 eye views of the moon by different obsei-vers, and in 

 response to his appeal an interesting series appeared 

 in the " Bulletin de la Societe Astronomiquo de France," 

 from January to Jmie of last year. The disc of the 

 moon to the unaided eye is about the same size as that 

 of Mai's in an average telescope, but the conditions are 

 not quite the same, as naked ey(! vision does not admit 

 of straining and mis-focussing to the same extent as 

 telescopic. Nevertheless, the study of these drawings is, 

 as M. Flammarion remarks, a lesson on the value to 

 be attached to observations at the limit of visibility, 

 and no one would have believed that the same thing 

 could have been represented in so many different ways. 

 The reader may judge for himself by personal examina- 

 tion whether these drawings support Cerulli's theory 

 of the canals. He will not fail to observe a tendency 

 to draw tlie Seas of Serenity, Tranquillity, Plenty, 

 and Nectar, as two lines more or less parallel, while 

 the Ocean of Tempests is sometimes a narrow curved 

 line, its eastern border only being seen, in contrast with 

 the brilliant limb. Tyclio in one instaiice appears as 

 a very large bright square. 



Whether the optical theoi-y be correct or no, probably 

 no one will deny the wisdom of Signer Ceridli's advice 

 to regard all Martian maps as temporary guides, sure 

 to bo modified by further investigation. We may add, 

 however, that to refrain altogether from speculative 

 hypotheses would be as luiscientific as uninteresting; 

 the sensational theories about Mars have been a 

 stimulus to much excellent work ; but the scientist 

 remembers that they are only theories, and is prepared 

 to sec' them dispelled by fuller light. 



[The Editors do not hold themselves re.spnnsible for the opinions 

 or statements of corre.spnndents.] 



GRADUAL CHANGE IN OUR CLIMATE. 



TO THE EDITORS OF KNOWLEDGE. 



Sirs. -Is our summer half (in London) getting colder 



f'Qual divcrebbe Giovc, s'etili e Martc 



Fo-ssero augelli, c cambiaaser-i p.cnnc." — Par. xxvii. 1 1, 15. 



