
Cases of Interference and Liffraction. ote 
owing to the fact that the collodion-film and the solution. 
have nearly the same refractive index. As soon as the silver 
begins to deposit, the colours reappear and increase rapidly 
in intensity. The bath should be rocked, the process being 
similar to the development of anegative. A little experience 
will enable the moment of maximum brilliancy to be correctly 
judged, when the plate should be immediately removed from. 
the solution, washed, and dried. It is well to provide the 
plate with a cover of glass mounted over it at an angle of 
20°, the whole forming a prismatic box. The object of 
inclining the cover is to get rid of the light reflected from 
it, which would otherwise dilute the interference colours. 
Plates prepared in this way show a wonderful blaze of colour 
and make excellent preparations for the lantern. 
The Colours of Mixed Plates. 
Interference colours of this type were discovered by Young;. 
and described in the ‘ Philosophical Transactions’ for 1802. 
They were subsequently studied by Sir David Brewster. 
Verdet and other writers on Optics have classified them with 
Newton’s thin-film colours, and have given treatments which 
are not very rigorous and fail to show where the energy 
goes to. 
The colours are very easily obtained by pressing a little 
white of egg between two pieces of plate-glass, separating 
the plates and squeezing them together a number of times 
so as to forma froth. The plates are to be pressed firmly 
together with a rotary sliding motion just before the froth. 
becomes sticky, enclosing a film made up of air and albumen: 
in the form of a mosaic. The colours are best seen by 
holding the plate towards a window or other bright source of 
light on a dark field. Certain wave-lengths will be found to 
be absent in the directly transmitted light. Young’s expla- 
nation was that the path-difference between a ray passing 
through an air-space and one passing through the albumen 
was an odd number of half wave-lengths for such colours as 
failed to appear in the transmitted light. Neither Young nor- 
subsequent writers, so far as I have been able to find, show 
what becomes of these absent colours, though both Young 
and Brewster observed the coloured fringes which appeared 
in the dark background to one side of the source of light. 
Brewster published a paper in the ‘ Philosophical Trans- 
actions’ for 1837 in which he referred the colours to dif-. 
fraction, though his treatment was not very complete, and 
concerned chiefly the case of diffraction hy a transparent 
lamina bounded by a straight edge. Verdet objected to this 
