380 Prof. R. W. Wood: Some new 
explanation on the ground that the colours are independent 
of the size and arrangement of the air-bubbles, depending 
only on the thickness of the albumen-film and the angle of 
incidence. The interference phenomena of mixed plates are 
easily explained by the elementary theory of diffraction, and 
they should be classed with laminary diffraction effects, and 
ate with thin film interferences, as is usually tbe case. 
The following treatment may not be new, but I do not re- 
member to have seen it anywhere:—In fig. 1 (PI. XXI.) let AA 
represent the glass plates with the albumen and air elements 
between them. We willassume the thickness of the albumen 
such that green light suffers a retardation of : in traversing it. 
If B is the lens of the eye, and parallel rays traverse the 
plate, the secondary disturbances represented by the dotted 
lines (normally diffracted rays) will be brought to a focus at 
EH; that is, the reduced paths of all these rays are equal and 
the disturbances arrive at E in the same phase, if there be no 
retardation. The disturbances coming from the albumen 
elements are retarded, however, and reach EK half a wave- 
length behind the disturbances coming from the air elements. 
The two sets destroy each other at this point, and green 
light will not be represented here. In general, light will 
be absent at this point if the retardation of a ray passing 
through albumen with respect to one passing through 
the adjacent air-space is (2n+1)5. If the film is fairly 
thick, this condition may hold for a number of colours in the 
spectrum, which will consequently be absent in the image of 
any source of light seen through the plate. The question 
now is: What becomes of this energy? In the case of thin 
film interferences, the wave-lengths absent in the transmitted 
light appear in the reflected. This is not the case with mixed 
plates, which show no colour by reflexion. If we refer to 
fig. 1, it is clear that if we take parallel rays diffracted in an 
oblique direction, the phase-difference introduced by the 
retardations in the mosaic may be compensated by the 
obliquity, the agreement of phase being more or less complete 
for green light in the point F. The case is analogous to a 
laminary orating, which yields coloured central images, the 
absent wave-lengths appearing in the spectra. Mixed plates 
throw the hght absent in the direct i image into a halo or ring, 
which is seen to surround the source of light. Laminary 
diffraction phenomena produced by straight edges, two 
parallel slits, and gratings are very fully treated in Ver det’s 
“Onuicss 
