HARD WICKE ' S S CIE NCE- G O SSI P. 



In the case of most natural bodies, we have seen 

 that the colour is due to an absorption of portions of 

 the light in traversing a small thickness of the 

 substance, and in the subsequent reflection from the 

 •interior. In these cases the portion of the light 

 •reflected from the surface itself is of the same colour 

 as the incident light. With metals and allied bodies, 

 this is not always the case. In the act of reflection 

 at the surface of a metal, a partial selection takes 

 place with regard to the rays. In the light reflected 

 from a sovereign, yellow rays predominate, whilst 

 the interior of a gold-plated vessel shines with a still 

 deeper orange-yellow, owing to the selection exercised 

 by each successive reflection. 



Some metals only exhibit colour on such often- 

 a-epeated reflections. Light thus reflected from 

 .steel becomes blue ; from silver, yellow. 



Another remarkable feature about metallic re- 

 flection, is the amoujit of light reflected ; whereas 

 white paper only reflects 40 per cent., polished 

 silver reflects 92 per cent, of the incident light. 



It is these peculiarities in the nature of metallic 

 surface which render a gilt frame so suitable for 

 ■enclosing a painting, since it isolates it from 

 surrounding objects, and does not intrude its own 

 ■ colour upon the painting, since that colour is of a 

 different character to the colour of the pigments 

 msed in the painting. 



The ordinary colour of metals is then due to the 

 components of white light, which are entirely re- 

 flected . If we obtain a sufficiently thin sheet of a 

 .metal, we can examine the light transmitted through 

 it ; and this, as we should expect, will be of a 

 different colour to the ordinary colour of the metal. 

 In the case of gold, the light at the first surface is 

 .robbed of its yellow rays, which are completely 

 reflected, and the transmitted portion consists of blue 

 or bluish-green rays. Gold can be easily precipitated 

 i 1 the metallic state from its solutions, and being 

 then in a very fine state of division, is capable of 

 transmitting light. Such solutions containing pre- 

 • cipitated gold, appear bluish-green by transmitted 

 li^ht, and orange-red by reflected light. 



There are many other bodies which display similar 

 ■colour phenomena to metals. In the case of a 

 .crystal of permanganate of potash, the light reflected 



from the surface is green. The crystal itself is almost 

 opaque, so that it is not easy to observe the colour of 

 light transmitted through it ;, but we find that the 

 colour of a solution of it is a deep purple, well-known 

 as Condy's fluid. Solutions of the aniline dyes when 

 spread on glass and allowed to dry — so as to leave a 

 thin film of colour — display one colour when we look 

 through them, and another when we observe the 

 light reflected from their surface. 



The ordinary aniline ink (used for writing on 

 graphs) is of a violet colour ; but light reflected 

 obliquely from the surface of such writing is apple- 

 green. Light reflected from the surface of a film of 

 blue aniline is bronze. 



There is another phenomenon attendant on the 

 reflection of light from metallic bodies, which is that 

 the reflected beam is at no angle completely plane- 

 polarised ; i.e. it always consists of two parts, one 

 plane-polarised, and one not ; whereas, with most 

 surfaces, at some particular angle the incident beam 

 is completely polarised. 



So far we have dealt with questions of colour 

 involving absorption. Let us now examine the results 

 obtained by the mixture of coloured lights which is 

 quite distinct from the mixture of pigments on the 

 painter's palette. Various methods have been adopted 

 for this purpose. Lambert and Heluiholtz used a verti- 

 cal plate of glass, with a piece of coloured paper placed 

 horizontally on each side, and observed the union 

 of the reflected and transmitted colours. When one 

 of the pieces of paper is yellow and the other red, 

 their superimposed image is orange. With blue and 

 yellow papers the image is grey. Maxwell's colour- 

 top is one of the most convenient methods of making 

 colour mixtures. It consists of a spindle capable of 

 making rapid revolutions, on one end of which 

 coloured discs of cardboard, seven or eight inches 

 in diameter, are placed. Each disc has a radial 

 slit to allow of other discs being combined with it, so 

 that a composite disc can be formed with several 

 sectors of different colours. When such a disc is 

 rotated at from twenty-five to fifty revolutions a 

 second, the sensation aroused in the eye by one 

 sector has not time to disappear before the images 

 of the other sectors are brought to bear, and con- 

 sequently a complete fusion of the colours takes 

 place. When a yellow and blue disc are combined 

 and rotated, the whole appears a dull grey ; vermilion 

 and a bluish-green also yield a grey, as do purple 

 and emerald green. Now a surface is grey which, 

 while it reflects white light, does not reflect so much 

 as white paper or chalk does. Therefore we may 

 say, that these pairs of colours produce by their 

 mixture the sensation of white light of a low 

 intensity. Two colours which do this are said to be 

 complementary. Other colours when combined may 

 produce the sensation of some intermediate colour in 

 the spectrum. For instance, a disc half red and 

 half yellow, when rotated looks orange. A green 



