the Colours of Mixed Plates. 345 



observation is to focus the eye on the film itself, the latter 

 being held at a suitable distance from the observer. As in 

 the former case, it is necessary, if the effects are to. be 

 studied critically, to use a light-source of small dimensions 

 and to place the film at a sufficient distance from it. A film 

 of variable thickness may be readily formed between the 

 surfaces of two lenses similar to those used for observation 

 of Newton's Rings. The thickness of the film at its centre 

 need not necessarily be zero; and, in fact, if egg-albumen is 

 used, there is considerable difficulty felt in forcing the lenses 

 into actual contact at the centre. We shall confine our 

 attention here to the phenomena observed with freshly pre- 

 pared films. 



Observations in White Light : Normal Incidence. 



When the mixed plate is held at a sufficient distance in the 

 line of sight between the eye and a distant source of light, 

 vividly coloured rings localized on the film are seen (pro- 

 vided its thickness is not too great), these rings being the 

 lines of equal thickness on the film. In this case the light 

 reaching the eye is that diffracted through small angles by 

 the air-liquid boundaries which the film contains; and, indeed, 

 it is these boundaries which appear luminous to the eye and 

 not the whole continuous film *. The source of light itself 

 appears coloured, and is of a complementary tint to the part 

 of the film through which it is seen. When the eye is moved 

 a little out of the direct line between the plate and source of 

 light, some remarkable changes occur in the appearance of 

 the film. The colours become feeble and impure, and on 

 moving the eye further out of the direct line, the colours 

 reappear again vividly, the rings having simultaneously 

 expanded and moved outwards. With still further movement 

 of the eye the phenomenon repeats itself, but with much less 

 marked fluctuations in the vividness of the colours ; and if 

 the film were initially so thick as to show a coloured centre, 

 fresh rings also appear and move outwards from the centre, 

 until finally an achromatic centre develops and expands so 

 as to cover the whole area of the film when viewed sufficiently 

 obliquely. When the source of light is fairly powerful (as, 

 for instance, when the film is held normally in the track of a 



* Fig. 4 in Plate IV. is a photograph (much enlarged) of these luminous 

 boundaries in a dark held as observed by the method of the " Fouealt 

 test." Each of the laminar boundaries appears as a pair of brilliantly 

 coloured lines running parallel to each other aud separated by a perfectly 

 black line coinciding with the exact outline of the boundary. The theory 

 of this effect will be more fully considered in Fart II. of this paper. 



