3IO 



NATURE 



[Fed. 19, 1874 



sant undulations of the air ; and in the seldom-recurring 

 moiTients of stillness so much under good circumstances 

 is visible, that even the best artist cannot draw it all in 

 half an hour, a period during which usually there are but 

 a very few tranquil glimpses, and after which the planet 

 will have materially changed its aspect from rotation. 

 Even were it easier to distinguish what is actually visible, 

 it requires great practice to represent it faithfully ; and 

 whoever has had personal experience of the difficulties of 

 such designs will have but a limited confidence in the 

 various portraits or the supposed changes that they repre- 

 sent. As a further illustration of these difficulties he 

 refers to the representations of the Orion nebula by 

 Rosse, Lassell, Secchi, and Liapounov (he could have 

 added Herschel II.) ; or the portraits given by Bond, and 

 others, of Donati's Comet. He might have cited, had he 

 known of it. Prof Young's remark as to the solar corona 

 (where, however, these difficulties are heightened by the 

 excitement of the moment), that " the drawings made by 

 persons standing side by side differ to an extent that is 

 sometimes really ludicrous, and has induced more than 

 one astronomer who had not himself seen an eclipse, but 

 judged only from the written accounts and sketches, to 

 declare his belief that this whole outer corona is a mere 

 subjective phenomenon." 



The justice of Kaiser's remarks will readily make 

 itself felt, but they do not exhaust the subject ; some- 

 thing may perhaps be added as to the " personal equa- 

 tion " of vision. Independently of mechanical defects in 

 the eye, there are inaccuracies of perception ; and even 

 if the rays have kept an uninterrupted and undeviating 

 course to the retina, they do not always produce corre- 

 sponding impressions on the mind. Whatever may be 

 the cause, we frequently meet with defects in the sense of 

 form, or proportion, or inclination, or even the presence 

 of features which are not the immediate objects of atten- 

 tion. Comparisons of size are often very erroneous ; 

 craftsmen well know the meaning of " a true eye ;" and 

 the expression " I did not see it," is constantly employed 

 with reference to a thousand objects whose repre- 

 sentation on the retina is all the while unquestionable. 

 It is in these respects that celestial photography is inva- 

 luable as recording everything and putting everything in 

 its proper place ; but photography, as Kaiser observes, is 

 inapplicable to the light of Mars. Another point, too, 

 might have admitted of notice. Although we may cer- 

 tainly, with him, be baffled in reconciling Rosse and 

 Lassell, we may bear in mind, as regards the comparison 

 of larger and smaller instruments, Dawes's important 

 remark to the effect that a certain relative proportion of 

 light and power may be essential to the visibility of some 

 classes of difficult objects. 



Without subscribing implicitly to the whole of Kaiser's 

 views, some of which admit of doubt — as, for instance, 

 when we contrast his assertion that the spots are never 

 sharply defined, with the clearness and keenness of outline 

 occasionally recorded by Lockyer and others — we may 

 well admit their general accuracy. But we find it more 

 difficult to accompany him in his inferences as to the 

 planet's physical constitution. 



The earth-light upon the moon having been found by 

 Schroter more conspicuous when it proceeded from the 

 hemisphere of our globe containing the largest amount of 

 land. Kaiser implies that it has hence been inferred that 

 (as it is difficult for us not to imagine other planets con- 

 stituted like our own), the brighter and darker portions of 

 Mars are equivalent to land and water. Whether such 

 an opinion may have been arrived at in this circuitous 

 way or not, it seems highly probable without any reference 

 to lunar appearances. The eminently absorptive power 

 of water is well known ; even a thickness of seven feet 

 will, it is said, diminish the incident light by one-half; 

 and below 700 feet it is quenched in unbroken darkness ; 

 and the quantity of diffused light reflected from its surface 



would be inconsiderable, while the solar image at the 

 distance of the Earth would be too minute, in all proba- 

 bility, to be visible. This reasoning would seem fairly 

 to hold its ground against that of the Leiden astronomer, 

 who does not believe that seas so looked upon would show 

 such innumerable gradations in tone, or be so invariably 

 ill-defined at their edges, while the same telescope gives 

 perfect sharpness to the polar snows. He goes in fact so 

 far as to say that if we may form any conclusion from 

 their aspect, it is, that they cannot resemble seas such as 

 our own. But as to distinctness of boundary, his experi- 

 ence is not accordant with that of other excellent ob- 

 servers, especially Lockyer, who remarks that " the effect 

 of a cloudless and perfectly pure sky both here and on 

 Mars appears to be, that the dark portions of the planet 

 become darkest and most distinctly visible ; the coast- 

 lines (if I may so call them) being at such times so hard 

 and sharp that (as has been mentioned by Mr. LasseU) 

 it is quite impossible to represent the outlines faithfully." 

 A more natural inference, it seems to the writer, would 

 be that these fluid masses contain large areas of very 

 slight depth, that the edges are in many places very 

 shelving, and that possibly they may be the more trans- 

 parent from the absence of salt. Other astronomers, 

 Kaiser tells us, but without mentioning their names, have 

 reversed the idea, and thought the bright parts to be 

 seas, but they do not thus escape his objections on the 

 score of definition, nor account for the dusky tracts which 

 some of the great bright expanses contain. He has per- 

 haps got hold of a more substantial difficulty in the aspect 

 of the north polar region, where the white spot is often 

 encompassed by a widely-extended dark zone with many 

 gradations of tint. The width of this belt, very great 

 when foreshortening is taken into account, is no doubt 

 variable : Beer and Madler ascribed it to the non-reflec- 

 tive power of the damp surface bared by the rapidly melt- 

 ing snow. On the whole, when Kaiser considers that 

 nothing is established with certainty but the existence of 

 an atmosphere and the connection of the polar spots with 

 the seasons, we hesitate to follow him ; and we should 

 prefer the conclusion of Phillips, adopted by Lockyer, 

 that " over a permanent basis of bright and dusky tracts, 

 a variable envelope gathers and fluctuates, partially modi- 

 fying the aspect of the fundamental features, and even in 

 some degree disguising them under new lights and shades, 

 which present no constancy, a thin vaporous atmosphere 

 probably resting on a surface of land, snow, and water." 

 A more protracted course of observation may possibly 

 modify in some way this result, but so far as past investi- 

 gations extend, we may say that nothing has been de- 

 tected inconsistent with it. Could we be actually trans- 

 ported to that far distant surface, we should probably 

 find much to astonish us that we cannot so much as con- 

 jecture here ; it was a sound remark of Schroter's 

 that unity in variety is the universal character of crea- 

 tion ; and the spectroscope of Huggins has already in 

 this instance confirmed it by the detection of absorption- 

 lines the cause of which is utterly unknown. Our future 

 inquiries should be conducted in that impartial spirit 

 which is equally ready to admit the indications of discre- 

 pancy and of resemblance, and which is more anxious to 

 ascertain facts than to seek their premature elucidation. 

 We have as yet read but a part of the inscription on that 

 golden shield : some of it has probably been deciphered 

 correctly ; how much of the remainder may give way we 

 know not ; but the whole, it will never be given to us to 

 understand. 



The extensive researches in which Dr. Terby of Lou- 

 vain has for some time been engaged, and in which he 

 has shown unwearied diligence and perseverance, if em- 

 bodied, as we trust they will be, in one comprehensive 

 result, will give material assistance in disentangling and 

 concentrating our present scattered and discordant mate- 

 rials, and we may look forward with hope to the very 



