On the Colours of Mixed Plates. 339 



The subsequent instalments of the paper will contain a 

 description of further observations and experiments on the 

 subject aud the detailed discussion of the theory of the 

 phenomena. 



2. The Diffraction-Haloes due to a Mixed Plate. 



The most instructive case is that referred to above — 

 namely, that of a mixed plate which is held normally in front 

 of the eye, a distant point-source of light being observed 

 through it. The plate should have a thickness as uniform 

 as possible, and to minimise the effects of slight unavoidable 

 variations of thickness it should be brought up very close 

 to the eye. In order to observe the phenomenon at its best, 

 it is useful to work with a fairly powerful source of illumi- 

 nation, which should be completely enclosed except for a 

 small aperture through which the light issues and falls upon 

 the plate held at a sufficient distance from the source. The 

 observations should be made in a darkened room so that the 

 fainter extensions of the diffraction-halo surrounding- the 

 source can be easily seen. A tungsten-filament lamp of,, 

 say, 400 c.p. is a suitable source for observations in white 

 light. For observations in monochromatic light a quartz- 

 mercury vapour lamp with green ray filter is the most 

 convenient source to use, though good results may also be 

 obtained with a monochromator illuminated by the electric 

 arc, or with a bead of salt held on a platinum wire in the 

 hottest part of the flame of a Meker burner. 



A mixed plate of uniform thickness may easily be obtained 

 by spreading a few drops of saliva or of egg-albumen between 

 two plates of glass 5 in. X 2^ in. in size and ^ in. thick, and 

 working up the material into a film of uniform consistency 

 by circular sliding movements of the plates over each other. 

 Examined under the microscope, a film of this description 

 shows a thin layer of liquid enclosing a large number of air- 

 bubbles widely varying in size, irregularly arranged and of 

 shape often departing considerably from circularity (see fig. 1 

 in Plate IV.), but showing no bias towards elongation in any 

 particular direction*. The air-bubbles can be distinguished 

 from the liquid by the slightly diminished intensity of the 

 transmitted light, and also by the presence of very minute 

 drops of liquid on the surface of the plates enclosed within 

 the bubbles. (These can be seen in the micro-photograph, 



* Cases in which the bubbles have distorted forms elongated specially 

 in one direction will be considered below in a separate section. 



