198 



KNOWLEDGE. 



[Sei'tembeu 1, 1896. 



fork or stretched wire whicli can give a certain note when 

 it is struck, is able to take up and absorb the note from 

 the vibratintj air around it when that note is sounded in 

 its neighbourhood. 



It may be noticed that the light from the electric arc 

 when seen near at hand has a distinctly bluish colour ; 

 but this same light when viewed from a distance appears 

 yellowish, as certain of its rays have been absorbed by the 

 water vapour in the air on its passage to the eye. For a 

 similar reason the sun is now considered to be a blue star ; 

 his light, which would appear intensely white, and rich in 

 blue rays especially, if it could be seen from beyond our 

 atmosphere, appears yellowish after it has passed through 

 that atmosphere and has lost some of its most refrangible 

 constituent rays. 



The ell'oct of absorption in producing colour is seen from 

 the fact that powdered bodies generally appear white. This 

 is accounted for when we consider that a powder consists 

 of particles arranged at all angles, so that the light falling 

 upon it meets various surfaces and is mostly reflected 

 before it has passed below the surface. Thus the white 

 light reaching it, is not deprived of some of its constituents 

 by selective absorption, as it would be if it penetrated the 

 substance and was then reflected. In this way powdered 

 red glass appears white. For a similar reason the froth 

 of coloured liquids, such as brown ale, appears pure white. 

 The light is reflected from the surface of numerous small 

 bubbles, and does not pass much through the liquid itself. 

 Thus also a cloud is very opaque to light, the light falling 

 on it being reflected at the surface of the numerous globules 

 of water. To this is due the brilliantly white appearance 

 of large fleecy clouds in bright sunshine. 



Some substances absorb equally all the rays of light. 

 Such substances, of which soot is an example, appear 

 black. The reason why a flower like a white lily appears 

 white is that the fluid contained in its cells does not 

 absorb one sort of rays more than another, but allows all 

 to pass with comparative freedom. White light then re- 

 flected from its surface, or from a little below, is not 

 deprived of any of its constituents, but remains white. 



The effect of reflection from internal surfaces accompanied 

 by absorption in producing colour, can be seen by pouring 

 a coloured liquid, carefully freed from floating particles, 

 into a white porcelain basin. Light is reflected from the 

 sides of the basin, passes through the liquid, and its colour 

 is seen. If now the sides of the basin be covered with 

 some black substance, no light will be reflected from them 

 and the Uquid will appear black ; no hght comes to the 

 eye from the interior, and the surface of the liquid reflects 

 all the rays equally. If nest we place in the black-looking 

 liquid a white powder like chalk, its colour is at once 

 restored, light being now reflected from the interior at 

 the surfaces of the chalk particles. 



From the above considerations we can understand to 

 what causes the colour produced on mixing pigments is 

 due. A mixture of blue and yellow paints has a green 

 colour because that is the only colour transmitted by both 

 pigments. The blue paint absorbs the red, orange, and 

 yellow rays, allowing the others to pass through it ; the 

 yellow paint absorbs the blue, indigo, and violet. Thus 

 green rays alone are permitted to pass through both, and 

 the result is that the mixture appears of that colour. 



Some substances appear of one colour when viewed by 

 reflected light, and another when seen by transmitted light. 

 Thus the light reflected by gold is yellow ; but a leaf of 

 gold made so thin that light can pass through it appears 

 of a green colour. This appearance of different colours on 

 reflection and transmission is also seen with many of the 

 aniline dyes. Tne colour of the light due to reflection is 



then made up of those rays which are not admitted at all, 

 but sent back at the surface, together with that light which 

 has been reflected from a certain depth below the surface, 

 and has thus lost some of its constituent rays by internal 

 absorption. The light to which the colour is due when 

 the substance is viewed by light which has passed through 

 it, is that which has been deprived of some rays by reilec- 

 tion at the first surface, and again of others by absorption 

 in passing through. Hence the difference of colour when 

 viewed in the two difierent ways. 



Variations in colour perception no doubt depend on 

 varying sensations in our own eyes as well as on 

 changes in the light itself. Some curious experi- 

 ments have been made with a view to testing our 

 different sensations as to colour. It has often been 

 noticed that a bright scarlet uniform will appear perfectly 

 white in a good photographic dark room with ruby 

 glass windows. With regard to such effects, Ilerr II. W. 

 Vogel described recently in IJerlin some experiments he 

 had made. He used oil lamps and fitted on to them pure 

 red, green, and blue colour screens. It was found that when 

 the white light was entirely shut out, no sense of colour was 

 perceptible to the observers, and objects in the room 

 appeared of various shades of black and white. He found 

 that when a set of colours was lit up by red light, the red 

 pigments appeared white or grey, and this changed at 

 once into yellow, not into red, when blue was added to the 

 light under which they were viewed. Thus a colour was 

 perceived which did not exist in either of the sources 

 of light used. The colour sensation produced by a source 

 of light also depends partly on the intensity of the illu- 

 mination. From these and similar experiments, Herr 

 Vogel comes to the conclusion that our opinion as to the 

 colour of a pigment depends upon our perception of the 

 absence of certain constituents from the light reflected 

 from it. Thus a surface which has a red colour is only 

 perceived as red by us when light of other colours shines 

 upon it, and we observe its incapacity for reflecting these 

 colours. 



When a solution of quinine is viewed in sunlight, a 

 remarkable blue shimmer is noticed extending for a short 

 distance beyond the surface at which the light enters. A 

 similar effect is noticed with many other substances, the 

 colour being different in different cases. The phenomenon 

 is known as fluorescence, as it is well observed in the 

 mineral fluor-spar, and is due to the fact that light is 

 absorbed by the substance and is again given out as light 

 of a different colour. For instance, rays of high refrangi- 

 bility towards the blue end of the spectrum may be taken in 

 and given out as yellow rays of lower refrangibility. In 

 the case of quinine, invisible rays beyond the violet are 

 absorbed and blue or violet rays are emitted by the solution. 

 In all cases of fluorescence a degradation of the rays 

 takes place ; those given out are of lower refrangibility 

 than those which disappear on absorption. By painting a 

 screen with a solution of sulphate of quinine, the spectrum 

 beyond the violet can be made visible, as those vibrations 

 which are too rapid to affect our eyes are changed into 

 others of lower refrangibihty, which can be perceived when 

 they fall on the retina. The curious blue colour of the 

 solution of quinine extends only a short distance into the 

 liquid, because those rays which are capable of producing 

 it are soon absorbed, and the light which passes onward 

 through the fluid is destitute of such rays. The colour 

 produced in cases of fluorescence has a different origin 

 from that of the ordinary surface colour of substances, for 

 the rays absorbed do not disappear as light, but their place 

 is taken by other rays of a different sort. 



The nature of the vibrations which constitute light still 



