reflected from some Collodion Films. 425 



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 effec- 

 tively 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 in both positions of the nicol, 

 the fainter light in (a) showing much the same colour as 

 in 03). 



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. 



