50 LECTURES TO SCIENCE TEACHEES. 



Sir Charles Wheatstone. He prepared them himself. This 

 is a deep blue or purple by transmitted light, but it is an 

 exceedingly thin film, and by reflected light it has a bronzy ap- 

 pearance. Here is another which is green by reflected light 

 and red by transmitted light. In these cases we see that we 

 have a substance which does exercise a preferential selection 

 for one kind of light as compared with another in the act of 

 reflection. But the light which is so selected for preferential 

 reflection is not at all the light which is chiefly transmitted ; 

 on the contrary, it is the very reverse. If we analyse the 

 light transmitted through this red stratum, or through a solution, 

 we shall find that in the green part of the spectrum the 

 substance is more intensely opaque than elsewhere ; that is 

 to say, the film must be excessively thin, or the solution exces- 

 sively dilute, in order that any light at all strong enough to 

 be seen may get through in the green part of the spectrum. 

 The substance is intensely opaque as regards the green, but 

 moderately opaque only as regards the other parts. A 

 solution of this colouring matter does not present this 

 coloured reflection at all. The colouring matter must be 

 excessively concentrated, as it is when a solution of it is dried 

 on glass, in order that this reflection should be shown, and 

 then the kind of light which is more especially reflected in 

 that manner agrees with the kind of light which is intensely 

 absorbed. Those parts of the spectrum which are absorbed 

 with this enormous intensity, so that the dry film is with 

 regard to them as opaque as a film of metal of the same 

 thickness would be, or thereabouts, are reflected as copiously 

 as they would be by a metal, and the colours which are only 

 moderately absorbed are reflected very much as they would 

 be by the glass, and accordingly in the reflected light there 

 is a predominance of those colours which are intensely 

 absorbed. The most remarkable example, that I know of, 

 of the connection between intense absorption and powerful 

 reflection, takes place in the case of crystals of permanganate 

 of potash. These crystals have a bronzy look by reflected 

 light when freshly taken out of the mother liquor, so that 

 the surface is not spoilt by tarnishing, as soon happens from 

 exposure to the atmosphere ; the sides of the crystals have a 

 metallic brilliancy, and reflect green light. Now that light 

 agrees with the light reflected from a metal, not only in 

 its copiousness, but also in certain other properties. Jf I 



