394 



NATURE 



\_Marck 1 6, 1876 



conclusion that the relative intensity of the blue and red 

 end of the solar spectrum is subject to great variations, 

 variations which do not seem to stand in a simple ref- 

 lation to the hygroscopic state of the atmosphere or to 

 barometric pressure. My results, while entirely confir- 

 matory of those of Vogel, point to the fact that in the 

 higher regions of the Himalayas, and at the season the 

 observations were made, atmospheric absorption takes 

 place chiefly in the red end of the spectrum. The blue 

 end of the solar spectrum, even when the light of the sun 

 has passed through a cloud, is remarkably bright. 



The following extract from my note-book will place this 

 fact beyond doubt. The observations were conducted at 

 Simla with a spectroscope of eight prisms of about 60°. 

 The direct sunlight was reflected by means of a small 

 mirror into the slit. The slit was generally adjusted until 

 one line between D^ and Dj was distinctly seen. As far as 

 I could judge, all the lines, but not more than those given 

 in Angstrom's map, were seen. The rainy season had 

 just begun, but had not yet appeared in the violent way it 

 did after my departure from Simla :— 



Extract from Note-book, 



June 27, 8 A.M. — B beautifully shaded. Light visible in 

 the blue as far as wave-length 4040, and most likely 

 further, but the telescope cannot be moved to greater 

 deviation. 



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

 the spectrum is as visible as before. 



Red end closes up. Blue perfectly visible. 



1 1. 15 A.M. — The red closed up still more. The blue as 

 clear as before. 



The sky is beautifully blue, but a slight halo seen round 

 the sun. 



July 3, 5.30 P.M. — The atmospheric lines near D seen 

 distinctly. The blue is exceptionally clear and visible as 

 far as H. Sky rather cloudy, and halo round the sun. 



6.30 P.M. — Sun very near horizon. Spectrum seen from 

 Cto G. 



In judging on the visibility of the spectrum, it should be 

 borne in mind that, owing to the great number of prisms, 

 a great part of the absorption in the blue was due to the 

 glass, and that, therefore, owing to the great dispersion in 

 the blue and instrumental absorption, the blue was seen 

 under peculiar disadvantage. The above are only a few 

 out of many observations. I have observed the passage 

 of a cloud in front of the sun without any apparent effect 

 in the blue, while the red end was all cut off. 



I 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 

 the colour was rather yellow than red. On making this 

 remark to a friend competent to judge, and who through 

 a repeated sojourn in Simla was enabled to form an 

 opinion, I heard that the redness of the sky at sunset 

 was often and beautifully seen at the end of, and after the 

 rainy season. 



I now pass to a few observations which I have made in 

 Upper Thibet, a country which lies beyond the range of 

 the rainy season. 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 remarkable degree. In the Kyan 

 Chu plain, for instance, the plateau on which I ob- 

 served the mirage described 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 Taga- 

 lung Pass (18,000 feet), we descended from that plain into 

 the valley of the Indus. As soon as we reached vege- 

 tation, at a distance of only two marches from the above- 



mentioned plain, and at height still above 13,000 feet, the 

 whole aspect of the country is a different one. Dis- 

 tant mountains now take that lofty blue colour which 

 gives such peculiar charm to the landscape. In the even- 

 ings 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 vegetation, 

 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 atmo- 

 sphere 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 evi- 

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

 floating in the atmosphere which can produce such a 

 striking result. That the absence of any rain or deposit 

 of any kind must not be left out of account is clear. The 

 air in the side valleys of Cashmere, although rich in vege- 

 tation, is particularly transparent. Strange enough the 

 principal valley of Cashmere, i.e. the valley of the Jehlum, 

 is generally hazy, although there is a good deal of rain. 



I have seen the planet Mars look almost white ; Jupiter 

 and the other stars at that time had a bluish tint. 



II. — Glaciers. 

 On the maps of Upper Thibet one finds a great many 

 glaciers marked down. From my knowledge of glaciers 

 I would not have given to these frozen masses of snow 

 the name of glaciers. On inquiring further into the matter, 

 I find that from measurements made by Schlagintweit 

 these so-called glaciers have only very little, if any motion ; 

 and judging from what I have seen and heard, I should 

 say they must be only half-formed glaciers. The cause of 

 this seems to me to be the want of pressure above the 

 glaciers. In a country where the snow line is ig,ooo feet 

 high, and in which the mountains are seldom over 21,000 

 feet — for such is the country I am talking of — there cannot 

 be a sufficient pressure to convert the snow into a clear 

 mass of ice. I am however told that there is in Spiti one, 

 but only one, glacier which deserves the name. 



III. — Temperature of the Blood. 



I am sorry that my observations on the temperature of 

 the blood were cut short by untimely breakage of the 

 thermometers, I have taken, however, a few observations 

 in the plains of India, when the temperature of the air was 

 higher than that of the blood. In a temperature of about 

 100° the blood was little above 98'. 



IV. — Parhelia. 

 According to received opinions, parhelia are due to the 

 refraction of light through crystals of ice. If this ex- 

 planation is correct, and there seems to be no reason to 

 doubt it, the following observations are of interest, as 

 they show that even at the equator ice-clouds exist, and 

 that parhelia are more often seen in India than in England. 

 I have only once been lucky enough to see a parhelion in 

 England, and that was since my return from India. In 

 tropics and in the Himalayas I have seen within four 

 months, eight times a rainbow-coloured ring round the 

 sun. Its distance from the sun could only be measured 

 by rough means, but it seemed to me to be larger than the 

 generally given value of 22°, although near it. I subjoin 

 the various observations : — 



1. May 3. — Near Singapore, about sixty miles north of 

 equator, at 5 o'clock p.m., part of a rainbow-coloured ring 

 was seen, with the sun as centre. It stood on the white 

 edge of a dark cloud. 



2, Aug, I.— At Dwara, in the Kulu Valley, almost the 

 exact reproduction of the above phenomenon was seen 

 on a cloud hanging on the side of a mountain. It was 

 during the rainy season, at a height of about 5,000 feet. 

 Weather rather hot. 



