3Iaj' 30, 1878] 



NATURE- 



12 



any of them did. Although I must plead not guilty to the 

 last count, still I am so persuaded that in the nature of 

 things these opinions were likely to be held that I feel 

 compelled, before I go further into the experimentation 

 with our improvised spectroscope, to draw attention to 

 the kind of knowledge I hope to show can be gained by 

 its use. 



It might at first sight appear that, limiting ourselves to 

 the sun as a great radiator of light, the artist has only to 

 do with white light in his pictures. This is true in one 

 sense, but only in one sense ; for the artist has to deal with 

 sunlight after it has been filtered through — after it has 

 been absorbed by — many substances, notably the aqueous 

 vapour of our air, and after reflection from others. I shall 

 show in the sequel, for instance, precisely how it is that 

 the sun is red at sunset ; at such a time as this the sun 

 practically gives us coloured light because our atmosphere 

 has abstracted from it some of the constituent rays 

 which fell on the upper air. A familiar instance of this 

 may be referred to. The colour of a ripe cornfield, in an 

 autumn sunset, comes from the fact that, for the moment, 

 in consequence of this absorptive eflfect, our sun has been 

 a red star instead of a white one. Cause and effect are there 

 before our eyes, and science connects them, and yet, alas, 

 i have seen pictures in which the grand colour of the corn 

 "has been given in perfection, while the painter has been 

 so ignorant of the cause of it that he has given us a cold, 

 grey, cloudy sunset, instead of a red, cloudless one. 



Further, as all the ordinary colours of natural objects 

 depend upon the way in which they reflect or absorb white 

 light, the colours of all must change at the hour of sunset 

 or sunrise, if the light which they usually transmit to us 

 is wanting in the light which they then receive. An object, 

 for instance, which appears blue at noonday will appear 

 tlack if only the red light of sunset falls upon it. The 

 blue sky over head is really a rich source of light of all 

 colours ; it is not a true blue, it is a mixture of blue and 

 white ; so that sunrise and sunset effects are much more 

 potent when a great bank of cloud overhead leaves only the 

 eastern or western horizon open. A striking thing for an 

 artist to observe under this condition is the difference of 

 luminosity of a red brick house, and such objects as trees 

 and fields ; the house seems glowing with light as if it were 

 red hot, and for the simple reason that it gets as much light 

 of the red kind, which is what it wants, and which it reflects 

 to us, from the setting sun, as it does from the noon- 

 day one ; whereas the trees and grass are no longer 

 supplied with that light which they throw back to us 

 usually, and so appear black for the same reason that 

 lampblack appears black at noonday. 



It has been a favourite theme with many astronomers 

 to enlarge upon the marvellous coloured phenomena 

 which must take place in those planets which are near 

 enough to stars of strongly contrasted colours to 

 receive their light, now from one, and now from the other, 

 producing not only a perfect modulation and combination 

 of the colours of both, but also the strongest contrasts, 

 such as, for instance, the setting of a red sun followed 

 by the opposite rising of a blue one. No doubt such 

 phenomena would be very enthralling, but to my mind 

 the chromatic effects produced by the aqueous vapour in 

 our own air absorbing the various elements in the light of 

 our single white sun when it is not more than ten degrees 

 above the horizon, supply us with a world of beauty with 



which we may well be content, and I for one have not 

 found the beauty one whit less enthralling since I have 

 endeavoured to picture to myself the causes to which it is 

 due. 



All this, however, by way of anticipation : the causes so 



far as I am acquainted with them at work in the circum- 

 -stances I have named, and in ten thousand others, are 



easily to be got at by a little simple experimentation. 

 In my last paper I suggested a simple form of spectro- 



-cope. Here is a figure which will show how the prism 



should be placed and what it will do to the light commg 

 through the slit C. 



Fig. I. -Shewing arrangement of iUt and prism,"] 



We may place a lens behind the prism as shown in 

 Fig. 2, and throw an image on a screen, which may con- 

 veniently be a piece of white paper. 



Fig. 2.— Introduction of a lens to produce an image. 



If we are content to use the lens and screen which 

 constitute the human eye it must be placed near the 

 prism. 



An expenditure of a few shillings is all that is required 

 to enable the origin of one class of coloured phenomena to 

 be investigated, that class, namely, in which the colour is 

 due to the giving out, by the light source, of certain kinds 

 of light only. This money should be expended in buying 

 two little glass or brass tubes, ^ inch and \ inch in 

 diameter, and 5 inches long, a little glass tubmg of very 

 small bore, a few inches of platinum wire, a small quan- 

 tity of red and green fire, and india-rubber tubing to 

 convey gas from an ordinary burner to a table. Of the 

 two tubes a Bunsen burner can be easily constructed 



