510 ON" THE COLOURS DIFFUSELY - [424 both positions of the nicol, the fainter light in (a) showing much the same colour as in (/3). It will be noticed that this behaviour differs from that observed by Wood (on another plate) and already quoted. On the other hand, one of the (silvered) plates prepared by me shows a better agreement, more light than before being scattered at a grazing angle when the nicol is in the (a) position, .while the colours in the (a) and (/3) positions of the nicol are roughly complementary. No more than Rowland have I succeeded in getting diffusely reflected colours from collodion films on glass or, 1 may add, quartz, either with or without the treatment with the breath suggested by Wood. The latter observer describes an experiment (p. 174) in which a film, deposited on the face of a prism, frilled under the action of the breath and then afforded a nearly three-fold reflexion. But, as I understand it, this augmented reflexion was specular. The only thing that I have seen at all resembling this was when I treated a coated glass with dilute hydrofluoric acid with the intention of loosening the film. Even when dry, the film remained out of optical contact with the glass, except I suppose at detached points, and gave an augmented specular reflexion, as was to be expected, inasmuch as three surfaces were operative. Two views are possible with regard to the different behaviour of films on metal and on glass. One is to suppose that the actual structure is different in the two cases; the other, apparently favoured by Wood, refers the difference to the copious reflexion of light from metallic surfaces. * The first view would seem the more probable a priori and is to a certain extent supported by Rowland's experiment. I'have not succeeded in carrying out any decisive test. On either view we may expect the result to be modified by the metallic reflexion. As to the explanation of the colours, anything more than a rough outline can hardly be expected. We do not know with any precision the constitution of the film as modified by frilling. And, even if we did, a rigorous calculation of the consequences would probably be impracticable. But some idea may be gained from considering the action of an obstacle, e.g. a sphere, of material slightly differing optically from its environment and situated in the neighbourhood of a perfectly reflecting plane surface upon which the light is incident perpendicularly. Under this condition the reflected light may still be supposed to consist of plane waves undisturbed by the previous passage through and past the obstacle. The calculation, applying in the absence of a reflector but without limitation to the spherical form of obstacle, was given in an early paper*. In Maxwell's notation, /, g, h are the electric displacements. The magnetic * "On the Electro-magnetic Theory of Light," Phil. Mag. Vol. xn. p. 81 (1881); Scientific Papers, Vol. i. p. &18. ° At angles of scattering less nearly grazing there is some light inxxix. p. 128, 1874). I do not know the date of Thoulet's use of the solution, but suspect that it was subsequent to Sonstadt's.