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that I cannot get the effect by making the film on glass and then pressing it down hard upon speculum metal or mercury although I think the contact is very good in the case of the speculum metal. Possibly, however, it is not. Gelatine films on metal give good colours by direct reflexion but not by diffused light: only faint ones. It would seem that the collodion film must be of variable density or full of fine particles. HoAvever, I leave it to you. I send by express two of the plates used." Probably it was preoccupation with other work (weighing of gases) that prevented my giving attention to the matter at the time.
Wishing to repeat the observation of the diffusely scattered colours, I made some trials, but at first without success. On application to Prof. Wood, I was kindly supplied with further advice and with a specimen of a suitably coated plate of speculum metal. Acting on this advice, I have since obtained good results, using very dilute collodion poured upon a slightly warmed silvered plate (plated copper) warmed again as soon as the collodion was set. That the film is no longer a thin homogeneous plate seems certain. Wood speaks of " frilling," a word which rather suggests a wrinkling in parallel lines, but the suggestion seems negatived by the subsequent use of " mesh." I should suppose the disintegration to be like that sometimes seen on varnished paint, where under exposure to sunshine the varnish gathers itself into small detached heaps. At any rate there is no apparent change when the plate is turned round in its own plane, showing that the structure is effectively symmetrical with respect to, the normal of the plate.
As regards Rowland's suggestion as to the origin of the colours, it does not seem that they can be assimilated to those of " thick plates." The latter require a highly localized source of light and are situated near the light or its image, whereas the colours now under consideration are seen when the plate is held near a large window backed by an overcast sky, and are localized on the plate itself, the passage from one colour to another depending presumably upon an altered scale in the structure of the film. The formation of well-developed colour at the various parts of the plate requires that the structure be, in a certain sense, uniform locally. The case is similar to that of coronas, as in experiments with lycopodium, only that here the grains must be very much smaller.
When examined by polarized light the behaviour of different plates is found to vary a good deal. We may take the case where sunlight is incident normally and the diffuse reflexion observed is nearly grazing. In the case of the specimen (on speculum metal) sent me by Prof. Wood, the light is practically extinguished in one position (a) of the nicol, that namely required to darken the reflexion from glass. In the perpendicular position (/3) of the nicol good colours are seen, and also of course when the nicol is 'removed from the eye. 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.