76 



NA TURE 



[May 26, 1887 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he under- 

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 rejected manuscripts. No notice is taken of anonymous 

 communications. 



[The Editor urgently requests correspondents to keep their 

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 and novel facts,"] 



Sunlight Colours. 



Will you permit me to say, in relation to the very interesting 

 lecture on SunHght Colours, reported in Nature, vol. xxxv. 

 p. 498, that Capt. Abney does not seem to have quite appre- 

 hended my meaning, when he represents me as stating in a 

 previous lecture at the Royal Institution, that the sun was 

 "really blue outside our atmosphere," for I nowhere in the 

 lecture used those words, nor intended to convey the idea which, 

 without qualification, they must give the reader. 



I recognize, however, that if my actual words conveyed it to 

 so fair-minded a critic as Capt. Abney, they must have been 

 open to misconstruction, and I therefore ask permission to recall 

 in explanation an important fact referred to in the lecture, to 

 which he does not allude. It is that the sun is surrounded by 

 an atmosphere of its own, and that the prime modification of its 

 actual colour at the photosphere takes place there. Only the 

 secondary change of colour takes place in the earth's atmosphere. 

 " Outside our atmosphere," accordingly, we see, not the absolute 

 colour of the photosphere, but one already greatly modified 

 toward white. I meant, then, when formally defining the colour 

 of the sun outside our atmosphere, to use such qualified phrases 

 as " tends toward blue," or " bluish," and it was for the colour 

 of the sun itself, i.e. at the photosphere, and before any absorp- 

 tion, that I meant to reserve the word "blue." Let me 

 hasten to add that I also tried — even to iteration — to insist that 

 "blue " here does not and cannot mean a monochromatic blue, 

 but a combination of all the spectral colours, in which those of 

 the blue end appear in such immense predominance that this is 

 the dominant effect. 



Capt. Abney also says : " he " (I) " surmised the result from 

 experiments made with rotating disks of coloured paper. He 

 did not, I think, try the method of using pure colours." 



Capt. Abney will, I think, agree on consideration that these 

 words may be liable to convey to most readers a wrong impres- 

 sion of labours which began nearly fifteen years ago, with studies 

 on the absorption of the sun's atmosphere, resting on direct and 

 elaborate photometric comparisons of the light of its centre and 

 edge. These have been followed by confirmatory measures with 

 the bolometer, giving the relative proportions of the pure 

 colours in the normal spectrum, and the tint has not been sur- 

 mised, but experimentally shown by the actual combination of 

 pure spectral colours. 



The solar studies were supplemented in the four years pre- 

 ceding my lecture by almost unintermittent investigations on the 

 absorption of the earth's atmosphere, in which (though consider- 

 ably over 20,000 galvanometer readings were recorded) T do not 

 recall ever making any observation by the aid of "rotating 

 disks of coloured paper." The paper disks have been often em- 

 ployed in explanation of my method, to roughly show the prin- 

 ciples involved, and to illustrate results, but certainly not as 

 means by which these results were surmised or discovered. 



In a communication to the British Association, published in 

 Nature, vol. xxvi. p. 586, after alluding to the antecedent re- 

 searches of Mr. Lockyer and others, which show that certain rays 

 of short wave-length are more absorbed than those of long, I ex- 

 hibited charts showing how much each ray had grown. One of 

 these, which suffered some curtailment at the hands of the en- 

 graver to fit it to the height of the page, was reproduced in the 

 report of the lecture (Nature, vol. xxxii. p. 42), and it is possibly 

 from this that Capt. Abney derives his impression as to my results 

 in other respects. I can only conjecture that it may be so, since 

 in my professional memoirs there are, not only more accurate 

 charts, but with them warnings that the figures representing the 

 relation of the blue and red end in such drawings, or even in the 

 tables whence they are taken, necessarily give minimum values 

 of the blue. 



The fact that this blueness was first predicated from a long and 

 careful study of the absorption of the sun's atmosphere is a dis- 

 tinct one, and I am entirely disposed to admit that this point 

 was not explained at sufficient length in my lecture, in which I 

 had but an hour to describe the work of twelve years. Being 

 forced to confine myself to an account of some limited portion of 

 this long research, I chose that part of it which dealt with the 

 absorption of the earth's atmosphere, as illustrated by the expe- 

 dition to Mount Whitney, but I thought the facts just stated 

 about the influence of the sun's atmosphere too important to go 

 without explanation altogether, and rehearsed them substantially 

 in other words before entering at length on the subject of the 

 telluric absorption. 



As the observations on the sun's atmosphere are still unpub- 

 lished, it may be of interest if I give here, in anticipation of the 

 final reductions, the approximate results of some made at 

 Allegheny in 1882, and which were supplemented by others 

 which I was enabled to make at South Kensington in the same 

 year by the kindness of Mr. Lockyer. 



This table gives the reduction to the normal spectrum at the 

 points indicated in the first line, where A. desijj;nates the wave- 

 length and IX = one micron = i/iooo of one millimetre. The 

 second line gives the approximate transmission by the solar 

 atmosphere (not alluded to in Capt. Abney's lecture). The third 

 line gives the approximate transmission by the earth's atmosphere 

 alone (numbers nearly concordant with those he seems to employ 

 for this secondary effect) ; and the fourth, the combined effect of 

 the two. It is from such numbers as those in this fourth line 

 that we have deduced the true colour of the sun at Allegheny, by 

 methods to be presently alluded to, and which authorize us to 

 state that its dominant tint before any absorption is not so much 

 "bluish" as "blue." 



040 o'4S o'50 o'55 o'6o o'65 0*70 o'75 

 Transmission by solar 



atmosphere '16 '24 "30 '35 "38 '41 "43 '45 



Transmission by ter- 

 restrial "31 '44 '53 "61 '68 '74 '79 '83 



Resultant transmission 



by both atmospheres '05 'ii '16 '21 '26 '30 "34 '37 

 Reciprocal of last, 

 showing approximate 

 brightness before any 

 absorption 20'2 9's 5'3 47 39 3*3 2"g 27 



Thus we see that of the extreme blue or violet light, whose 

 wave-length is 0"4/^, 16 per cent. {i.e. less than \) only is 

 transmitted by the solar atmosphere, and of this 16 per cent. 31 

 per cent, only is transmitted by the earth's atmosphere. It is of 

 this latter alone that Capt. Abney here takes account, but in 

 consequence of the absorption by both atmospheres, only about 

 5 per cent, of the original violet light reaches us ; or in other 

 words, before the double absorption there was over twenty times 

 as much of this sort of blue in the sun as what we now see. On 

 the other hand, of the deep red light whose wave-length is 075 ii 

 as much as 45 per cent, is transmitted by the solar atmosphere, 

 and of this again 83 per cent, by the earth's ; so that after the 

 action of both atmospheres on this ray 37 per cent, is trans- 

 mitted as against 5 per cent, of the violet. If we take the reci- 

 procal of the numbers in this fourth line we have those of the 

 fifth, which evidently show the relative intensity of the colours 

 at the photosphere (i.e. before any absorption), as compared with 

 that of common daylight. I employed in 1882 an optical 

 arrangement, suggested by Mr. Very of the Allegheny Observa- 

 tory, by which we passed from these figures to the production of 

 the actual resultant tint of the solar photosphere ; not by using 

 pigments or revolving disks, but by the direct combination of pure 

 spectral colours in the above proportions. The resultant colour 

 cannot, I repeat, be exactly defined by any one spectral one, as 

 it was not monochromatic ; but the tint was, to my eye and that 

 of others, best technically defined as that of Herschel's lavender, 

 with perhaps a suggestion of purple ; and certainly I think now, 

 as I thought then, that "blue" is the nearest familiar word to 

 describe it. 



It was with all these facts, and many more, in my possession, 

 that I used the language in question. 



I hope after this statement that I may conclude that Capt. 

 Abney and I have really no serious ground of difference as to the 

 propriety of the term " bluish," or as to what it here means. I 

 would only say that by no latitude of interpretation do I take it 

 as meaning white. S. P. Langley. 



Smithsonian Institution, Washington, D.C., May 2. 



