yu7ie 6, 1878] 



NATURE 



15s 



the aqueous vapour is of the blue kind : that is to say* 

 that the smallest thickness which has any visible effect 

 ■will absorb in the blue. We also know that the absorp- 

 tive effect of aqueous vapour is enormously greater than 

 that of the pure gases. 



I feel bound to show at once that this is no scientific 

 abstraction, and it would be impossible to find two better 

 examples to show exactly what I mean than those afforded 

 by two of the pictures which I have chosen as texts — Mr. 

 Vicat Cole's " Rosenlaui " and Mr. Peter Graham's 

 " Wandering Shadows." Whether I am right or wrong 

 about the molecular states of aqueous vapour, there is no 

 doubt that the quantity of it varies considerably. The 

 clouds in Mr. Graham's picture show us that the air is 

 charged with it, for the simple reason that if it were not 

 there would be no clouds to cast the shadows Avhich he 

 has so exquisitely caught. Look now at t'ne dark hill in 

 the distance ; see how blue the air is between us and it — 

 for it is true that it is the colour of the air, or rather of the 

 aqueous vapour in the air, as Leonardo da Vinci first dis- 

 covered, and not the colour of the hill which Mr. Graham 

 here paints. We are in presence of aqueous vapour com- 

 petent to be set in vibration by blue light, and because 

 it vibrates in this way it appears blue. 



What would have happened if the dark hill had not 

 been there ? If the stratum of aqueous vapour had had a 

 background of bright sky, it would have absorbed the 

 blue light of that sky. By virtue of the principles which 

 I have stated, the sky would have appeared red in con- 

 sequence of the abstraction of blue light. This, by 

 the way. 



Turn now to Mr. Vicat Cole's picture, and see the 

 work of the vapour upon each receding buttress of rock 

 on the left of the valley; the depth of atmosphere is 

 rendered to perfection, but we do not get the blue that 

 Mr. Graham gave us, for the reason that there is less 

 aqueous vapour mixed up with the air. 



Many an artist, I am sure, has noticed that at times 

 there appears to be no atmosphere at all ; all sense of 

 distance is lost ; buttresses such as those painted by Mr. 

 Vicat Cole, although obviously, as may be gathered 

 from the structure of the mountain, at different dis- 

 tances from the eye, seem yet to lie in the same vertical 

 plane. 



I saw this effect myself in its very strongest form last 

 year at Cadenabbia. Looking eastwards from the hotel 

 there, over the lake of Como, one sees Belaggio, the 

 hills between Bellano and Lecco on the other side of 

 what is called the Lecco leg of Como forming a magnificent 

 background ; these hills recede from the eye in a magnifi- 

 cent series of buttresses. Although some of these but- 

 tresses were three or four miles oh the other side of 

 Bellaggio, it was impossible to get rid of the feeling that 

 lake, Bellaggio, background to the furthest buttress, was 

 a painted canvas between us and the water. I called the 

 attention of several friends to this wonderful sight ; they 

 saw it exactly as I did. The explanation is quite simple : 

 although the permanent gases of the air were there, the 

 aqueous vapour was not, at all events, in that form which 

 by its action on light gives us what artists call atmosphere 

 in a picture. To me this afforded the strongest possible 

 proof of the statement I have already made that the ab- 

 sorption of the permanent gases of the air goes for 

 nothing so far as art is concerned. 



As I have already hinted, the molecular form of 

 aqueous vapour with which we have most to do is one, 

 the motions of which lie chiefly at the blue end of the 

 spectrum ; a small thickness of it cuts off the extreme blue, 

 and as the thickness increases even the green may be 

 dimmed by it. 



In order to show how on such a point as this, art, 

 representing an accurate study of natural phenomena, 

 may help science, I will here give the result of some obser- 

 vations which my friend Dr. Schuster was good enough to 



make at my suggestion in the Himalayas and Tibet, with 

 a view to test this very question. 



Theory had led me to expect that with the enormous 

 thickness of air available there, absorption at the red 

 end of the spectrum by aqueous vapour would be seen 

 as well as the absorption at the blue, which is so 

 common with us. Seeing the sun a vivid green 

 through the steam of the little paddle-boat on Winder- 

 mere first led me to inquire into the possibility of 

 aqueous vapour following the same law as that which I 

 think we may now accept in the cases of the vapours of 

 metals. As in these experiments with vapours, absorp- 

 tion of the red end alone was seen, as well as absorptioa 

 at the blue end alone, the assumption that these two- 

 absorptions existed in aqueous vapour at once accounted 

 for the green sun, which, I may remark en passant, L 

 caught again last year through a thin veil of mist at the 

 extreme summit of the pass of the Simplon. 



Here, then, are Dr. Schuster's observations made at 

 Simla when the rainy season had just begun : — 



June 27, 8 A.M. — B (one of the Fraunhofer lines at 

 the red end of the spectrum), beautifully shaded. Light 

 visible in the blue as far as 4040 ; most likely further ; 

 but the telescope cannot be moved to greater deviation. 



9 A.M. — Space beyond B closes up, while in the blue 

 the spectrum is visible, as before. 



II. 15 A.M. — The red closed up still more; the blue as 

 clear as before. 



6.30 P.M. — Sun very near horizon ; spectrum seen from. 

 C to G. (This means that both ends of the spectrum 

 are now absorbed.) 



Dr. Schuster further states that he was at the same time 

 struck by the fact that the peculiar redness of the clouds 

 in the evening, which we observe so often in our climates, 

 was only rarely seen, and, when seen, that the colour was 

 rather yellow than red. He adds, "On making this remark 

 to a friend competent to judge, and who, through a long 

 sojourn in Simla, was enabled to form an opinion, 1 

 heard that the redness of the sky at sunset was often 

 beautifully seen at the end of and after the rainy season." 

 So much for the observations at Simla. I now pass on 

 to some observations made in Upper Tibet, where there 

 is no rainy season. I give them in Dr. Schuster's own 

 words : — " The observations all point to the remarkable 

 clearness in the blue. As I have said, the hygroscopic 

 state of atmosphere, as measured by the wet and dry 

 bulb or barometric pressure, cannot alone account for all 

 the phenomena. I find, for instance, that the presence of 

 vegetation affects the atmospheric absorption in a re- 

 markable degree. In the Kyan Chu plain, for instance, 

 the plateau on which I observed the mirage de- 

 scribed in Nature, vol. xiii. p. 67, objects at ten miles 

 distance look as sharp and distinct as those half a mile 

 off; it is, in fact, impossible to judge of distance. 

 Crossing the Tagalung Pass (18,000 feet), we descended 

 from that plain into the Valley of the Indus. As soon as 

 we reached vegetation, at a distance of only two marches 

 from the above-mentioned plain, and at a height still 

 above 12,000 feet, the whole aspect of the country is a 

 different one. Distant mountains now take the lofty blue 

 colour which gives such peculiar charm to the landscape. 

 In the evenings, especially, you cannot help knowing that 

 there is something between your eye and a distant object, 

 which affects its colour and distinctness, and through it 

 you get a standard for judging distances. Without vege- 

 tation, even at a lower height, as, for instance, in the 

 Valley of the Bagha (Lahoul), you seem to look through a 

 vacuum. In the upper part of the Valley of the Indus, of 

 which I am now speaking, I have not seen that clearness in- 

 the atmosphere which I have invariably seen in Switzerland 

 at a height of 3,000 feet. The strong radiating power of the 

 sun, which stands much more vertical in India, is evidently 

 the cause of this, for it can only be organic matter 

 floating in the atmosphere which can produce such a 



