222 



MR. A. MALLOCK ON COLOUR-PRODUCTION 



Are opaque and reflect certain periods 

 and absorb the rest. 



Complete opacity ma}' arise either from 

 the absorption of all wave periods or 

 by complete reflection. 



Fluorescence and Phosphorescence. 



Opaque pigments such as lead chromate, 



etc. 



Lamp-black. 



Silver and most white metals. 



II. 



A relation between the wave-lengths of light and the 

 structural dimensions of the matter which appears coloured : 



Reflection or transmission from or 

 through a striated or laminated 

 structure. 



Scattering or transmission of light by 

 particles of sizes comparable with the 

 , wave-length, but irregularly distri- 

 buted. 



Examples. 



Colours of thin plates. 

 Mother-of-pearl. 

 Lipmann films. 

 Diffraction gratings. 



Pitted surfaces with pits of uniform 

 depth. 



Red of sunset. 



Light, seen through vapours or emul- 

 sions. 



Blue sky and the colours of the sea and 

 rivers. 



Glass coloured with gold. 



Supernumerary rainbows. 



III. 



The colours of polarization in most cases depend both on 

 molecular structure and on the linear dimensions (measured 

 along the paths of the rays) of the bodies which exhibit them. 



With regard to the first class, little is known concerning the 

 intimate structure of matter. It is a fact, however, that light- 

 waves travel more slowly in solids and liquids than in vacuo, and 

 that the velocity is in some unknown way dependent on the wave- 

 length. 



In most cases this is best represented by assuming that the 

 ether is, as it were, loaded by matter, while in some others 

 it would appear that the elasticity of the ether is affected. The 

 difference may be exemplified by a stretched string which has a 

 definite period settled by its length, tension, and mass. If the 

 string is loaded, the period is increased by an amount depending 

 on the added load, but an equal increase in period may be pro- 

 duced by relaxing the tension. If the shape of a transparent 

 body is such that the direction of light after passing through it 

 depends on the velocity of the waves in the interior, as for 

 instance in a prism or sphere, the emergent light will appear 

 differently coloured in different directions. 



In the case of pigments, it is most probable that the individual 

 molecules have a natural period identical with that of some of the 



