382 Prof. R. W. Wood: Some new 
source of light, and placing one of the frilled plates 
before the lens. This photograph is reproduced in fig. 3 
(PLY XX1.). 
The arrangement of the colours in the ‘“ mock-suns”’ pro- 
duced in this way is, however, exactly the opposite of the 
arrangement in the real ones, which makes it appear doubtful 
if there is any connexion between the two. It is possible, 
however, that the usual treatment of parhelia could be im- 
proved by considering diffraction as well as refraction, as has 
been done in the case rok the rainbow. 
On the Polarized Fringes produced by the interference of 
two streams of light polarized at right angles. 
In the case of ordinary thin-film interferences the planes 
of vibration of the disturbances reflected from the two surfaces 
of the film are parallel. 
It is possible, however, to prepare a film which shall fulfil 
the requirement that the vibrations reflected from its upper 
surface make any desired angle with those coming from the 
lower surface, The path-differ ence between the two streams 
will vary with the thickness of the film; and if the amplitudes 
be equal we shall have the vibrations compounding into 
circular elliptic or plane ones, according to their phase- 
difference. 
A thin glass or gelatine film, backed by a metallic reflect- 
ing surface, is all that is necessary. The incident sodium 
light should be polarized at an angle of 45° with the plane 
of incidence by passage through : a nicol, and the reflected 
light examined with an analysing nicol. The fringes 
obtained in this way present a most curious appearance, 
reminding one forcibly of a spectrum line with a fainter 
component seen in the Fabry and Perot interferometer. 
Their general appearance is shown in fig. 4, which represents 
the fringes obtained by flowing a plate of speculum metal 
with a rather dilute solution of gelatine, and allowing it to 
dry in a slightly inclined position. 
The Basie way to get them, however, is to blow out the 
end of a rather lar ge olass tube into a large thin balloon of 
tissue glass, picking out a portion, bv the light of a sodium 
flame, rhe shows fairly straight Wterkdremens fringes two 
or three millimetres apart. A small piece of the thin glass 
is laid, with its slightly convex side down, upon a clean 
mercury surface, and sodium light, polarized in azimuth 45°, 
reflected from the surface at an angle of about 60°. On 
