1 873.] Colours, and their Relations. 79 



rather than on those of the atoms constituting the ultimates. 

 How much depends on the arrangement of the ultimates 

 and their state of aggregation has been rendered evident by 

 Faraday's experiments on gold leaf. This metal, when in 

 very thin layers, is transparent, and the light passing through 

 it is green ; but by heating such films, and so altering the 

 state of aggregation of the ultimates, the colour of the 

 transmitted light becomes ruby-red. It can, however, be 

 restored to green by simply compressing the layer. The 

 light from the surface of the film in both cases retains its 

 rich yellow hue and beautiful metallic lustre. In toning 

 photographs with gold, however, the film, when extremely 

 t/iin, is black, and not till the thickness of the deposit is 

 augmented to an appreciable extent do the yellow tint and 

 the metallic lustre return — the lustre preceding the tint in 

 its reappearance, so that, at a certain stage, the surface 

 presents a certain amount of metallic lustre while it is still 

 black. Other metals besides gold exhibit variations of tint 

 depending on the state of aggregation of their ultimates. 



These phenomena bring us face to face with the question re- 

 lative to the nature of intrinsic colours — the manner in which 

 they are produced by the action of the ethereal vibrations. 

 At one time it was generally supposed that the light falling 

 on any coloured surface becomes separated into two portions, 

 of which one is regularly reflected without change, the other 

 scattered in all directions by the reflective action of the 

 molecules or ultimates of the coloured surface, but deprived 

 of some of its waves by absorption. Another opinion, how- 

 ever, has begun to prevail over this first notion. When it 

 is remembered that what arrives at the coloured surface is 

 simply motive energy, wafted onwards through the ether in 

 waves of definite length, embracing vibrations of various 

 rates, it will be perceived that if any of the motive energy 

 of the ether disappear or become absorbed, it must be im- 

 parted to the molecules or ultimates of the surface on which 

 the waves alight. These, again, cannot take up the energy 

 without being themselves set a vibrating at the peculiar 

 rates which they tend to assume. Moreover, the molecules 

 or ultimates, on beginning thus to vibrate, must excite in the 

 ether, in immediate association with them, fresh vibrations 

 synchronous with those peculiar rates, and these will be 

 propagated by undulations in all directions. It is this 

 secondary set of ethereal vibrations which, according to the 

 second view, produce in us those perceptions which we 

 call the intrinsic colours of bodies. It is not a part of the 

 incident light deprived of certain of its component waves, 



