ON THE NATURE OF LIGHT AND COLOURS. 369 



% The colours of mixed plates constitute a distinct variety of the colours of 

 thin plates, which has not been commonly observed. They appear when 

 the interstice between two glasses nearly in contact, is filled with a great 

 number of minute portions of two different substances, as water and air, 

 oil and air, or oil and water ; the light which passes through one of the 

 mediums, moving with a greater velocity, anticipates the light passing 

 through the other; and their effects on the eye being confounded and 

 combined, their interference produces an appearance of colours nearly 

 similar to those of the colours of simple thin plates, seen by transmission ; 

 but a much greater thicknesses, depending on the difference of the refrac- 

 tive densities of the substances employed. The effect is observed by hold- 

 ing the glasses between the eye and the termination of a bright object, and 

 it is most conspicuous in the portion which is seen on the dark part beyond 

 the object, being produced by the light scattered irregularly from the sur- 

 faces of the fluid. Here, however, the effects are inverted, the colours 

 resembling those of the common thin plates seen by reflection ; and the 

 same considerations on the nature of the reflections are applicable to both 

 cases. (Plate XXX. Fig. 450.) 



The production of the supernumerary rainbows, which are sometimes 

 seen within the primary and without the secondary bow, appears to be 

 intimately connected with that of the colours of thin plates. We have 

 already seen that the light producing the ordinary rainbow is double, its 

 intensity being only greatest at its termination, where the common bow 

 appears, while the whole light is extended much more widely. The two 

 portions concerned in its production must divide this light into fringes ; 

 but unless almost all the drops of a shower happen to be of the same mag- 

 nitude, the effects of these fringes must be confounded and destroyed ; in 

 general, however, they must at least cooperate more or less in producing 

 one dark fringe, which must cut off the common rainbow much more 

 abruptly than it would otherwise have been terminated, and consequently 

 assist the distinctness of its colours. The magnitude of the drops of rain, 

 required for producing such of these rainbows as are usually observed, is 

 between the 50th and the 100th of an inch ; they become gradually nar- 

 rower as they are more remote from the common rainbows, nearly in the 

 same proportions as the external fringes of a shadow, or the rings seen in 

 a concave plate.* (Plate XXX. Fig. 451.) 



The last species of the colours of double lights, which it will be neces- 

 sary to notice, constitutes those which have been denominated, from 

 Newton's experiments, the colours of thick plates, but which may be 

 called, with more propriety, the colours of concave mirrors. The anterior 

 surface of a mirror of glass, or any other transparent surface placed before 

 a speculum of metal, dissipates irregularly in every direction two portions 

 of light, one before and the other after its reflection. When the light falls 

 obliquely on the mirror, being admitted through an aperture near the 

 centre of its curvature, it is easy to show, from the laws of reflection, that 

 the two portions, thus dissipated, will conspire in their effects, throughout 



* Young's Exp. and Obs. relative to Physical Optics, Ph. Tr. 1804, p. 1. Potter, 

 Math. Considerations on the Rainbow, Tr. Camb. Ph. Soc. vi. 141. 



2 B 



