

Cases of Interference and Diffraction. 381 
Mixed plates belong to the same class, the case being 
best defined as laminary diffraction by a great number of 
irregularly distributed transparent disks. If the patches of 
the mosaic were of uniform size, the halo would be fairly 
sharply defined and separated from the direct image by a 
dark space, which would become wider as the size of ‘the 
elements of the mosaic decreased. Though I have succeeded 
in obtaining very perfect halos in some cases, separated by a 
dark area of considerable size, the variation in the size of the 
elements usually causes the halo to take the form of a disk, 
the centre of which is occupied by the direct image. 
Jf the plates are held close to ‘the eye and a distant lamp- 
flame viewed through them, the flame will, for example, 
appear purple and the surrounding halo green. If a small 
sodium flame is employed, parts of the mosaic will show it 
much blurred, and surrounded by a halo, while other parts, 
where the retardation is a whole number of half-waves, show 
it perfectly sharp and distinct. The distribution of the light 
in the halo depends on the form of the elements of the 
mosaic. By pressing the plates firmly together and sliding 
one over the other, the circular air-bubbles can be deformed 
into ellipses. The light in the ring will be more or less con- 
centrated on opposite sides of the halo. If the ellipses were 
drawn out indefinitely, we should pass over to the grating, 
and the points of concentration would become first-order 
spectra, the rest of the halo disappearing. 
I observed a very curious and interesting example of this 
concentration of light in a halo a number of years ago, while 
copying some diffraction-gratings on bichromatized albumen, 
The original grating was puted: on glass, 14,400 lines to the 
inch, a spacing so fine that copies were only obtained with 
considerable difficulty. 
Some of the films were found to have frilled in the process 
of washing, the buckling of the film following the grooves of 
the grating to a certain extent. The albumen surface was 
seen by the microscope to have frilled into oval patches of 
varying length, but of fairly constant width, the width being 
equal to three lines of the original grating. In fig. 2 we 
have a diagram illustrating this ‘condition. This plate when 
held before the eye showed a ring of wide aperture surround- 
ing a brilliant source of light, with four distinct concentra- 
tions, two very bright and two quite faint. The appearance 
reminded one most forcibly of a solar halo with parhelia or 
mock suns. A photograph of this curious diffraction pattern 
was made by directing a camera towards a brilliant point 
Phil. Mag. 8. 6. Vol. 7. No. 40. April 1904. 2D 
