60 



♦ KNOWLEDGE ♦ 



[Dec. 1, 1885. 



ing to an accurate representation of a landscape. He 

 knows that a field three miles off is green, and he paints 

 it approximately of the green which it appears when, he is 

 in it. If he would, as Ruskiu i-eciimmends, cut a hole in 

 a card, through which he could see it, and try to match 

 its tint exactly from his colour-box, he would find, per- 

 haps not a little to his surprise, that it would be in 

 reality rather grey than green at all. We have said this 

 much to show that the question of colour has a practical 

 as well as a merely theoretical or speculative interest. 

 Let us proceed to deal more at length with the produc- 

 tion of colour by absorption. 



J^ow it is to absorptive phenomena that practically all 

 our ordinary' colours are due. The scarlet of the gera- 

 nium, the green of the grass and trees, the blue of the 

 artilleryman's coat, the grey of the rocks, the gorgeous 

 hues of the cathedral window, and the r mge of pig- 

 ments on the painter's palette, — one and all have this 

 origin. Let us endeavour to gain some idea of 

 absorptive action by the aid of a simple experiment. We 

 must obtain a piece of coloured glass — say blue or red, 

 it matters nothing for our present purpose which. Now 

 let us lay it upon a piece of the dead black paper which 

 we have used in constructiug some of our earliest- 

 described pieces of apparatus, and view the reflection 

 either of a candle or of ordinary daylight from its sur- 

 face. What shall we see I Simply an image of the 

 reflected object in its natural colours, as though, in 

 stead of a deeisly-tinted reflector, we wers employing 

 an ordinary piece of white plate-glass. But if, instead 

 of viewing a reflected image in this way from our piece 

 of, say, red glass, we look through it, we shall see that 

 the source of light, so regarded, will appear red. 

 Now, what has happened ? Has the glass actually 

 changed the essential quality of the white light, 

 or what ? The arrangement shown in Fig 2, 

 p. 16, will easily enable us to try. As there stated, 

 a beam of pure sunlight, H h, dispersed by the prism P, 

 will be spread out on the opposite wall into a superb 

 sheaf of colours, ranging from violet throutrh blue, oreen, 

 yellow, and orange, to red. While this spectrum is 

 glowing in all its glorious gradations of colour, let us 

 cover the hole in the shutter H with our bit of red 

 glass. What a marvellous transformation we behold ! 

 The red, and a good deal of the orange end of the 

 spectrum, is still perceptible, but the more refrangible 

 colours have apparently vanished, .and darkness reigns 

 in their stead. Nay, the experiment may be rendered 

 yet more striking if we admit the sunlight through a 

 horizontal slit at H, instead of a round hole, and cover 

 half of this slit with our coloured glass. Then will 

 two spectr.i be thrown side by side on the screen, and 

 the extent to which the red glass has absorbed the 

 colours above the yellow will be even more strikingly 

 shown. It does not in reality whollij absorb even the 

 blue rays, but it needs more refined apparatus than we 

 arj employing to show this satisfactorilj^ Blue glass 

 has the curious property of chiefly cutting out the green, 

 yellow, and orange parts of the spectrum, but of letting 

 through quite a uotxblo quantity of red. The bearing 

 of this upon the ultimate constitution of light will bo 

 seen as we go on. 



If it bo here asked, What becomes of the light which 

 is absorbed by the coloured glass ? the answer is very 

 simple. We have seen, over and over again in these 

 papers, that light consists of a series of undulations — in 

 other words, to adopt Tyndall's phraseology, that it is 

 only " a mode of motion " ; and what really happens is 

 that these rapid vibrations are converted into .slower ones 



(too slow in fact to affect the eye as light at all), which 

 we recognise as A.eai, and which very slightly raise the 

 temperature of the glass. We are now on the threshold 

 of the explanation of the colours of ordinary natural 

 objects. Take the geranium petal as an example. Looked 

 at in sunlight this is of a vivid scarlet. Why ? Simply 

 because, as we have said, the substance of the petal 

 absorbs all the more refrangible part of the spectrum 

 (converting it into heat) and reflects the scarlet light 

 which is left. And here we may revert to our dark-room 

 and prismatic spectrum. Figs. 2 and 3, pp. 16 and 17, for 

 the purpose of performing a most striking and instruc- 

 tive experiment. Let us take our geranium into this 

 room, and passing it slowlj- up the spectrum, watch the 

 succession of effects. It is needless to s.xy that in the 

 red it glows with an intense red light ; but as it travels 

 upwards this disappears until, wheu we arrive at the 

 green, it is impossible to distinguish the petal fromr, piece 

 of black velvet. Nor is the reason far to seek. The 

 petal can only reflect red light. There is no red light in 

 the green, hence it reflects none at all, and puts on the 

 weird and unnatural black hue of which we have just 

 spoken. 



The colours exhibited by dress materials, woollen, silk, 

 lie, have their origin in absorption. White light falls 

 we will say upon a piece of merino. Some of it 

 (not vei'y much) is reflected from the upper surface as 

 from a mirror in the form of white light, but the 

 rest penetrates the fibres of the cloth and is 

 partly returned to the surface coloured from the under 

 part of the topmost bundle, so to speak, of fibres, and 

 partly penetrates deeper into the material, only ultimately 

 to emerge more highly coloured still. It is the combina- 

 tion of these different shades of colour reflected from 

 different depths in one piece of merino, diluted by the 

 white light directly sent back by its actual surface, which 

 forms what we recognise as the colour of the cloth. Silk 

 b3ing much more reflective than wool, gives back a more 

 intensely coloured, richer, and more lustrous light than 

 does wool ; while in velvet, the fibres are seen endways, 

 so as to suppress superficial reflection altogether, and this 

 enables the internal reflected light to reach the eye 

 practically unmixed with white light, giving an effect 

 of richness and depth of colour familiar to everybody. 

 The colours of metallic surfaces are referable in like case 

 to absorption, but in this case the light returned from 

 the subjacent layers is considerablj- mixed with a flood 

 of white light superficially reflected. The powerfully 

 reflective nature of silver is known to all who will read 

 these lines. Lambert has shown that while white paper 

 only reflects 40 per cent, of the light which falls upon 

 it, silver returns actually 92 per cent, of incident light. 

 Water, albeit limpid and colourless if it is examined 

 when in small quantity, shows a very distinctly blue 

 tinge when regarded in any very large volume. Tlie 

 blue of the Lake of Geneva and the almost black colour 

 of the Atlantic will arise to the minds of many of our 

 readerj as illustrations of this. 



Thrit :^bs!lrption aetuall}' does take pLicc in these c;ises 

 is shown by the experiment (originally, we fancy, du; to 

 Buusen) made by Tyndall, in his homeward voyage rfter 

 the solar eclipse of 1870, ami subsequently described by 

 him at the Royal Institution, of sinking a wuijhted 

 porceliin plate in the Atlantic Oee in, and notieiug how 

 it gr.idually appeared bluer aud bluer as its distanc from 

 the surface increased. Naturally, when the bed of a 

 shallow sea is sandy — and hence more or less yellow — the 

 superficial water looks green from the combination of its 

 own proper light-blue colour, due to absorption with the 



