ON LIGHT. 291 



(74.) In order, then, clearly to understand the nature 

 of this phenomenon, it must be divested of this source 

 of complexity, and studied in reference to light of one 

 single colour or refrangibility or, as it is called, " homo- 

 geneous" light, pure red or yellow, for instance. But be- 

 fore proceeding further, something more must be said of 

 the whole class of phenomena referable to this head. 

 And first, these colours are not dependent in any way 

 on any colorific quality of the liquid of which the 

 bubbles consist. Any sufficiently thin film, of any kind, 

 suffices to produce them. They are seen in the oily 

 scum on the surface of a stagnant pool. They are seen 

 on the brilliant scales of old glass in stable windows, or 

 on the wings of gaudy-coloured insects, or even on 

 polished steel. Bubbles may be blown of a variety of 

 liquids nay, even of glass. However highly coloured, 

 their intrinsic colour disappears when reduced to such 

 extreme tenuity as is requisite for the purpose in ques- 

 tion. But all exhibit the same hues in the same invariable 

 order. Nay, more it requires no medium at all to pro- 

 duce them, but only an interval between two surfaces. 

 They are seen in the crack of a thick piece of glass 

 which does not extend quite through its whole substance. 

 They are seen when a piece of mica is partially split and 

 one of the laminae lifted up, following as a series of 

 coloured lines the limit of the commencing fissure. 

 It may be said that though no solid or liquid medium is 

 here present, there is air between the divided surfaces. 

 But under the exhausted receiver of an air-pump, there 



