the Colours of Mixed Plates. 343 



■and is determined only by the thickness of the film containing 

 the bubbles. 



When glycerine, turpentine, fats, or oils are used for 

 forming the mixed plates, the tendency- towards elongation 

 of the air-liquid boundaiies in the film becomes excessive, in 

 these cases occurring parallel to the direction of movement 

 at any instant of the enclosing glass plates. With such 

 liquids it requires some dexterity to prepare a film showing 

 the complete circular haloes. Generally only a diametrical 

 streak is obtained which shows different colours at different 

 parrs of its length, and turns round and round as the glass 

 plates enclosing the film are slid over each other with a 

 circular movement in front of the eye. 



5. Effect of Partial or Complete Drying of an 

 Albumen Film. 



When a mixed plate is prepared with white of egg between 

 parallel glass plates and is allowed to stand for an hour or 

 two, some very remarkable changes occur in the structure of 

 the film and in the optical effects produced by it. The 

 bubbles of air in the film which at first lie about indiscri- 

 minately (rig. 1 in Plate IV.) soon draw together, coming 

 into contact over a considerable portion of their edges (fig. 8 

 in Plate V.), and this process continues gradually till the 

 edges everywhere touch each other. ( With very thin films, 

 especially those formed between curved surfaces, this process 

 may be indefinitely retarded.) Ultimately, the edges which 

 have joined up straighten out, and when after a day or two 

 the film has completely dried up, it is found on examining 

 the film under a microscope that the albumen is confined to a 

 number of very fine ridges holding the two glass plates 

 together, the form of these ridges being that of a number of 

 irregular hexagons, pentagons, or quadrilaterals forming a 

 network (fig. 9 in Plate V.). The diffraction-halo seen 

 round a distant source of white light when such a completely 

 dried plate is held normally in front of the eye is entirely 

 different in character from that due to a mixed plate freshly 

 prepared ; in fact, the relative position of the achromatic and 

 coloured portions and the spacing of the rings are completely 

 reversed with the dried film. The halo in this case is much 

 fainter. It shows a broad central area which is achromatic, 

 followed outside by rings of gradually decreasing width which 

 are strongly coloured. The radial fibrous structure of the 

 halo is exceedingly well marked (tig. 11 in Plate V.). 

 In fact, the diffraction-halo seen round the source when 



2 A 2 



