124 



NATURE 



yjune 27, 1878 



Kensington Museum. That phenomena so diverse in 

 their origin and appearance should be mistaken for each 

 other does not say too much in favour of the cultivation 

 of the observational faculties of artists as a rule, 



I shall next refer to two or three other questions which 

 have been dealt with by Prof. Briicke in the article to 

 which I have referred. Prof. Briicke is again with me 

 to a certain extent in tracing the origin of most sky- 

 colour to a defect of the blue light, but he does not make 

 the attempt to run it to earth that I have done, by 

 ascribing it to aqueous vapour ; indeed he considers it 

 rather due, I take it, to the presence of solid particles in 

 the air. Thus, after pointing out that the dawn is generally 

 orange, and the sunset redder, he states that at night the 

 • quantity of molecules capable of troubling the air is gene- 

 rally greater. For my own part, I should be inclined to 

 ask whether, during the night, the molecules of aqueous 

 vapour which absorb the blue have not been driven into 

 higher forms — deiv being one of them — owing to the 

 reduction of temperature. Tiiis would at once explain 

 not only the generic difference between sunrise and 

 sunset colours, which is more marked here than in the 

 tropics, but also the golden instead of red sunsets which 

 accompany the formation of cloud. 



Another point of difference. Prof. Briicke considers 

 green sky as an effect of contrast produced by the quan- 

 tity of red light which enters the eye. I cannot agree 

 to this, first, because I have given a physical reason 

 for the green ; and secondly, because I have observed 

 it without any strong contrast of colour to mislead the 

 eye. The considerable darkening of the green after- 

 sunset is, I believe, purely physiological ; and it is ah 

 effect of so curious a nature, that it raises several inte- 

 resting questions with regard to the manner in which the 

 eye grapples with the middle colours of the spectrum, 

 namely, the orange, yellow, and green, which can be 

 made to change to a certain extent according as the light 

 is more or less intense, which does not happen with the 

 other colours. 



The changes in mountain scenery form the subject 

 of several interesting remarks by Prof. Briicke. As 

 long as distant mountains are illuminated by a high 

 sun, their outlines are not very clear ; because, as he 

 well puts it, the reflection of this light from the 

 lower strata of the atmosphere is then so great that 

 the illumination at the horizon, where mountains are, 

 is as strong as where they are not. He then points 

 out that at night the setting sun fills the sky towards the 

 west with a great brightness which renders the profiles 

 of the mountains between us and the sun much darker. 

 Their contours are neatly detached, but it is not 

 only on the horizon that this is seen ; the various 

 chains are better distinguished, and appear one behind 

 the other like the scenes in a theatre, because the light 

 in which we see them does not come from them but from 

 the interposed air. The sides of the mountains which we see 

 are dark because the other sides are turned towards the 

 sun, but the various thicknesses of air interposed between 

 us and them reflect to us the sunlight ; hence the atmo- 

 sphere of a picture is truly the work of the air. 



Here is what Prof. Briicke says about sunset tints ; I do 

 not follow him in all his explanations : — ''When the sun 

 reaches the horizon and the red tint is developed, the 

 colours of the landscape change in their turn and the moun- 

 tains themselves appear red when we regard no longer their 

 shadows but the illuminated air which lies in front of them." 

 It appears to me this gives too much work to the air ; a rock 

 surface is generally as capable of dispersing red light 

 which falls upon it, as a molecule of aqueous vapour is ; 

 '' still the tint has not the intensity of the alpine colour ; 

 it is a red less intense and more empurpled, which 

 sometimes approaches even the violet or the lilac." 



I shall have a word to say on this, but I will first give 

 Prof. Briicke' s explanation : — 



" Two causes are at work in this latter case ; 

 the first is the mixture of red and blue light. 

 At night when the sky is clear the shadows are 

 coloured a strong blue. The shadow region is 

 illuminated by the blue light of the sky, and appears 

 more pronounced, owing to the contrast of the red- 

 dish-yellow light, as we have already seen. The illu- 

 minated air reflects the blue rays more abundantly than 

 the red ones, and consequently the former have the ascend- 

 ency. If not scientifically correct, it is at least practically 

 so, to suppose the blue light in which we see the moun- 

 tains bathed after sunset to be mixed with purple or lilac. 

 The second cause of the violet tone in the distances 

 to the west is to be found in the frequent contrast. In 

 the west, in fact, a great part of the sky is illuminated 

 by yellow light; often this yellow is a perfect sulphur- 

 colour, which contrast makes objects even in the middle 

 distance, which turn their dark sides to us, appear violet ; 

 thus, looking to the west, dark, unploughed earth appears 

 violet when the majority of terrestrial objects turn their 

 dark sides towards us." 



An observation I made at Cannes last year leads me 

 to think that the whole cause of this purple colour has 

 not been stated in the foregoing. It was near the 

 hour of sunset, and I was looking towards the south- 

 west, delighting in the blue colour at the foot of 

 Les Estrelles — while their crests were being gilded 

 by the sunset — when, almost instantaneously, the valley 

 to the north of these hills was enfiladed by a beam 

 from the sun itself, which threw part of the aqueous 

 vapour in the valley into a frenzy of gold. This gra- 

 dually got ruddier as the sun got lower, and the 

 amount of vapour lighted up between me and the blue 

 vapour at the foot of the hills was at the same time 

 reduced; the blue and the red then melted together 

 into the richest and most beautiful purple that I, at all 

 events, have ever seen. 



We have only, then, to assume that, when we thus see 

 purple, that colour is produced by a mixture of particles, 

 some of which are reflecting to us the blue light of the 

 sky, because they can do no other, while others, again, 

 are reflecting to us the red light of sunset, because it is 

 more powerful than the light from the sky. 



J. Norman Lockyer 



AN ECLIPSE SPECTROSCOPE 



SOME little time ago I communicated to the Royal 

 Society a suggestion for the use of Mr. Ruther- 

 furd's reflection gratings in obtaining photographs of the 

 coming eclipse. The plan suggested was that the grating 

 should be placed short of the focal point of a telescope, 

 and at right angles to its axis, and that the diffracted 

 images of the chromosphere should be received on 

 photographic plates adjusted for the different orders of 

 spectra on either side the axis. I am glad to learn from 

 Prof. Newcomb that the value of this method of obser- 

 vation will probably be tested by Prof. Young, who is in 

 charge of one of the six expeditions already organised to 

 observe the eclipse. The chief defect in this mode of 

 observation lies in the difficulty of determining the posi- 

 tion of the lines photographed, supposing the chromo- 

 spheric spectrum to vary considerably from the ordinary 

 solar one so far as the intensity of the lines is con- 

 cerned ; and as it seemed desirable that these gratings 

 should be utilised for less serious attacks, I have recently 

 been endeavouring to see if the method can be improved. 



The annexed woodcut shows one form of the new 

 arrangement, which has many conveniences. It is a 

 rough model on wood, but will suffice to show the method 

 of use. 



The grating, which is free to rotate, is placed in front of 

 a little telescope of low magnifying power and the stand 

 which carries both is so placed and the grating so 



