June 27, 1878] 



NATURE 



223 



probably resume its course in the middle of September, 

 terminate its voyage in the harbour of Horten." 



The members of the expedition are the same this 

 year as last, viz., Profs. Dr. H. Mohn, meteorologist ; 

 Dr. G. D. Sars, Dr. Danielssen, and Herr H. Friele, 

 zoologists; candidate Torncee, chemist; assistant-candi- 

 date Schmelck, physicist and chemist ; and the landscape- 

 painter Herr Schiertz, as artist. The Voeringen will be 

 commanded this year, as formerly, by Capt. Wille of the 

 Royal (Norwegian) Navy, the second in command being 

 the sailing-master, Capt. Greig. The expedition carries 

 "with it several valuable new instruments for measuring 

 more exactly the temperature of the water at great 

 depths ; some of them have, with great good will, been 

 obtained from the members of the English Challenger 

 expedition. As in the preceding years. Prof. H. Mohn 

 will send to Nature communications from the expe- 

 dition. 



PHYSICAL SCIENCE FOR ARTISTS^ 

 VI. 

 •T^HE diagrams given in my last article should have 

 -■■ made it quite clear that the various sunset and sun- 

 rise colours are due to the absorption produced by dif- 

 ferent thicknesses of aqueous vapour ; that the colours 

 of clouds are due to light falling upon them after ab- 

 sorption by different thicknesses of aqueous vapour ; 

 and finally that the blue colour of the sky in the zenith is 

 due to the fact that the pure gases in our atmosphere 

 exist in that molecular grouping which vibrates in har- 

 mony with the short waves of light. 



The blue sky, however, is scarcely ever a true blue. 

 Between us and it there is ever a misty veil which re- 

 flects to us the white light of the sun, as an examination 

 of it'by a pocket spectroscope will prove to anybody. It 

 is to the variation in the quantity of this misty veil that 

 the difference in the colour in the sky at great and low 

 elevations, in different climates, and in the same climate, 

 when clouds are about to form and when scarcely the 

 germs of clouds are present, is to be ascribed. The 

 thickness of our atmosphere is so moderate that neither 

 the hypothetical red nor the blue molecules of aqueous 

 vapour are competent, except during thunderstorms, to 

 influence its colour as they undoubtedly do near the 

 horizon. 



A glance at Fig. 4 in the last article will explain how it 

 is that sometimes in the case of clouds we find the before- 

 stated order of sunset colours reversed. If, for instance, 

 we imagine a cloud lying along the curve xs', an observer 

 attf willsee a cloud at xhigher above the horizon than one 

 at /, but the cloud at x will have received light through 

 a greater thickness of atmosphere than the cloud at s". 

 The red, therefore, at x will be more fonce. than at s' j 

 the order of colour, though not of brilliancy, will be 

 reversed. 



So far we have considered these colours looking 

 towards the rising or setting sun. Let us now turn our 

 back on that luminary. It will be at once obvious that 

 if, for instance, we take a point on the horizon, there will 

 be an enormous increase in the thickness of atmosphere 

 traversed by the ray ; indeed, we may say that for this 

 point the absorption will be threefold. Hence a consider- 

 able reduction of light, a ruddier tinge, due to the in- 

 creased absorption of the more complex molecules, and 

 a mingling of the ruddier light with the blue sky. 



In the voyage which I made to India in 1871 I scarcely 

 ever missed a sunrise or a sunset, and although the 

 point of sunrise or sunset was almost always the scene of 

 a succession of glories unsurpassed in beauty, the point 

 opposite was, if possible, more interesting, the colours 

 were more subdued, and of a more composite order, but 



' Continued frcm p. 157. 



the work of law went on there, as elsewhere. If any 

 clouds happened to be overhead, their greatest glory, 

 which, as I have already shown, can only be put on when 

 the sun is below the horizon — and the sun rises or sinks 

 much more rapidly there than with us — was the herald of 

 the shadow of the earth on the illuminated sky, which 

 crept on a gigantic, mysterious crescent. That the shadow 

 of the earth could thus be seen was new to me, and I am 

 the more glad, therefore, seeing that many may doubt it 

 still, to substantiate my observation and its explanation by 

 a quotation from Prof. Briicke, one of the most distin- 

 guished members of the Vienna University. Prof. Briicke 

 has been doing on the Continent what I have been at- 

 tempting to do in these articles, and just before my last 

 one appeared I saw in La Revue Scientifique an extract 

 from his forthcoming work " Principes Scientifiques des 

 Beaux Arts." I am delighted to see how much at one 

 we are, but for the moment I shall content myself by 

 giving what he says on the point to which I have referred. 

 Talking of sunset he writes : — 



"We see on the horizon to the east a grey blue stratum 

 rising higher and higher, and stopping at that portion of 

 the sky coloured red : it is the shadow of the earth. 



" The shadow of the earth must always encounter aa 

 unilluminated part of the atmosphere. As this shadow- 

 does not fall on a surface, but on a great number of 

 particles spread abroad in space, it is material, that is to 

 say, it has three dimensions, and we see it, foreshortened 

 — in perspective. 



" Sometimes the regions above it are divided in a 

 radial direction into sectors, some of which are dark, 

 like the shadow of the earth, others red. These resemble 

 in the sky the rays of the aurora borealis, and often 

 change their place and size ; in French they are termed 

 'les rayons de cr^puscule.' They are due to the fact 

 that in the path of the solar rays there are masses 

 of clouds which only give passage to isolated ones 

 here and there, which make their presence felt by the 

 luminous train which they leave among the particles 

 of the atmosphere. Hence arise those red prismatic 

 masses spread abroad in the air east and west. At the 

 zenith we do not remark them, because the vision cuts 

 across them, and the stratum of illuminated particles is 

 not thick enough to render them sensible ; but we see 

 them painted on the eastern sky because we regard them 

 obliquely in the sense of their length ; we see them in 

 perspective. By their nature and their mode of origin 

 they do not differ from the beams which the setting 

 sun throws between the intervals in the clouds, nor from 

 those which it sometimes casts in the morning or after- 

 noon through the clouds, when the peasants say that * the 

 sun is drawing water.' ' Voilk un bouillon qui chauffe.' " 



This paragraph not only supports my view, but it 

 opens up several very interesting points on which, if 

 space permitted, there would be much to say ; one or two 

 words, however, must suffice. 



The rifts to which Prof. Briicke has drawn attention do 

 not always arise from clouds ; in fact, they are not seen 

 in their greatest vividness when they do. One evening 

 I saw them thrown, in a perfectly cloudless sky (in fact, 

 there had been no cloud all day), by the sky-line of 

 Socotra, which island we had passed during the day, and 

 which was below the horizon at the time. Capt. Parish, 

 in command of the Mirzapore, to whom I appealed at 

 the time, took the bearing of these rifts, which, in their 

 sharpness and magnitude, were almost appalling, and put 

 the question beyond all doubt. 



"With regard to the "sun drawing water," artists 

 should note the absence of all colour and the radial 

 direction of the beams, all meeting in the sun's place. 

 For some reason or other many artists are not yet quite 

 clear about this appearance, and compromise matters 

 by making the beams look like a distant rain-shower. 

 There are some notable examples of this in the South 



