6o 



NATURE 



\_May 1 6, 1878 



allowed to refer to here. When an artist wishes to 

 introduce a constellation as well as a moon into his pic- 

 ture, he should especially avoid the familiar northern 

 ones, especially the Great Bear, as the moon has never 

 been known to pass through that constellation. Still if 

 my memory serves me, she has been painted in her silent 

 majesty rendering glorious the sky over a village church, 

 represented broadside on with the chancel to the right. 

 She had therefore been caught due north in an unguarded 

 moment. With an English village church (which is 

 bound to be truly oriented) represented in the fashion I 

 have indicated it is also as well to avoid the temptation 

 to show up the stained glass windows of the aisles by 

 introducing a sunset behind them. 



I wish to be as little digressive as possible, but there is 

 one point more which demands a word in passing. If an 

 artist will put the moon or the sun into a picture, he 

 should understand that in nine cases out of ten he lays 

 down an almost perfect scale by which the accuracy of 

 his delineation of landscape may be tested. In spite of 

 the strange physiological effect which gives us the exag- 

 gerated sensation of the size of the sun or moon when 

 they are near the horizon (so that we can compare them 

 with familiar objects), the real variation is practically nil, 

 and for our purpose it is enough to say that both sun and 

 moon steadily subtend an angle of half a degree. Now it 

 is because we can observe this angle and because we 

 know the distance of the sun and moon as well, that we 

 can calculate the sizes of these luminaries. Similarly, if 

 an artist paints Peckham Rise from Camberwell or 

 Mount Everest from the valley of Cashmere (supposing 

 either picture possible— I don't know), and then puts a 

 sun or moon \n, par dessus lemarchi, he gives all the data 

 necessary for the determination of the height and size of 

 the hills in question. 



By the kindness of an American astronomer I can 

 give some statistics of considerable interest on the heights 

 of hills in the United States as roughly surveyed in this 

 ■ypay — that is as determined by pictures in which, by 

 means of the moon or otherwise, the necessary data are 

 provided. The pictures on which they are based were 

 exhibited in 1876 and 1877. One mountain (I think it 

 was in Missouri, but its exact name has escaped me) 

 reached the respectable elevation of 105 miles. The 

 average height in the United States generally, taking the 

 pictures all round, was 43! miles. There was only one 

 artist who had got a hill into his picture less than thirteen 

 miles high, but he only succeeded in doing this once. 



So much then with regard to reflection, and the digres- 

 sions which reflection has suggested. 



So far there has been nothing said about coloiu-. 



It has been known from the time of Kepler that the 

 white light of the sun, and indeed of all bodies which 

 emit it, is not the simple thing we have so far taken it to 

 be. It is really a sensation produced in our eye by the 

 commingling of an innumerable series of different wave- 

 lengths of light, each one of which, taken separately, we 

 are bound to consider as a pure colour from the physical 

 point of view, however we regard the physiological action 

 which gives rise to the sensation. 



Nature shows us in the rainbow, about which I shall 

 have something to say by and by, the breaking up of 



this complex beam of white light into its various elements. 

 The physicist arrives at the same result by employing 

 a prism. A round hole in a shutter, through which a 

 beam of light is allowed to enter into a dark room, and 

 a common lustre inserted in the path of the beam, is all 

 that is required in the way of apparatus to demonstrate 

 the marvellous phenomenon of the analysis of white 

 light into its constituent elements. 



We not only get colour as the result of the analysis of 

 white light, but we get it as a result of molecular struc- 

 ture, that is, some bodies like the sun and a candle give 

 us that kind of light which we can break up into a com- 

 plete series of coloured constituents ; other bodies give 

 us light which is not white, which is coloured to begin 

 with, and so remains coloured to the end of the chapter. 

 Let us now pass on to those principles, the applica- 

 tion of which to the coloured phenomena with which 

 artists have to deal will, I am sure, prove of the greatest 

 interest. Here we must grope our way as well as we 

 may be able in a region where at present the senses are 

 entirely powerless. We are in the world of the infinitely 

 little. We approach one of those questions of mole- 

 cular physics which no doubt in a few years must become 

 one of the chief fields of investigation to men of 

 science. 



If I take a lump of iron, it is in what is generally called 

 the solid state. If I apply heat to it it becomes molten, 

 and we call it liquid. If the heat is still further in- 

 creased, we drive the molten iron into iron vapour, as we 

 more commonly drive water into steam. 



Now we have achieved these results gradually, break- 

 ing down the molecular structure of the iron and of the 

 water until at length we have got the molecular structure 

 down to that of its ultimate fineness ; so that when 

 we have got to the stage of vapour at the end of our 

 labour we have got a condition of things in which the 

 smallest particles, or the ultimate molecules, which go to 

 make up iron and water are there in their individuality 

 and exist as separate points. 



Now, what can this have to do with colour ? 

 It has this to do with it : if I make a lump of iron 

 hot, it gives out light, because its molecular constitu- 

 tion is disturbed or rendered more disturbed by the 

 conditions to which we expose it when we heat it, and 

 this state of unrest is rendered visible to us by the phe- 

 nomena of light. Here we have simply the reason 

 why there is a visible universe at all. If it were not for 

 the condition of unrest of matter, we should never see 

 anything ; and, therefore, so far as the sensation of light 

 is concerned, the visible universe would cease to exist 

 altogether if this condition of unrest were abolished. 



This being so then, let us take each of those molecular 

 o-roupings in the solid iron as being in a state of unrest. 

 Let us take for granted that the phenomena of sight 

 depend first upon the state of unrest ; secondly, upon 

 the state of unrest being communicated to the surround- 

 ing ether, which, as we have already seen, does for light 

 what air does for sound; and that the state of unrest 

 of the molecules of iron having set up an equivalent 

 state of unrest in the ether, or, if you like it better, having 

 set up a chain of vibrations in the ether, this ether 

 is competent to communicate its own vibrations thus im- 

 posed upon it to the optic nerve in our eye. 



