286 W. LeConte Stevens—Lecent Progress in Optics. 
small angle to a plane silvered mirror which served as reflector. 
From an electric are lamp the light was sent through an appro- 
priate slit and prism, so that a selected spectral band of violet 
fell normally on the prepared plate in the dark room. The 
developed negative presented the alternate bands, in perfectly 
regular order more than a half-millimeter apart. Various tests 
were applied to guard against error in interpretation, and the 
existence of such stationary waves was proved beyond all 
doubt. 
These waves, moreover, when polarized ight was employed, 
furnished the means of determining the direction of vibration 
with relation to the plane in which the light is most copiously 
reflected when incident at the polarizing angle, and thus of 
subjecting to experiment the question as to whether the plane 
of vibration is coincident with this plane of polarization or is 
perpendicular to it. The former of these views was held by 
Neumann and MacCullagh, the latter by Fresnel. Let a beam 
of polarized light fall upon the mirror at an angle of about 
45°. If the vibrations in the incident beam are parallel to the 
mirror, and hence perpendicular to the plane of polarization, 
those of the reflected and incident beams will be parallel to 
each other and hence capable of interference. But if the 
vibrations of the incident beam are in a plane identical with 
that of incidence, and hence in the plane of polarization, the 
vibrations of incident and reflected beams are in mutually per- 
pendicular planes and hence cannot interfere. Wiener obtained 
interference fringes when the light was polarized in the plane 
of incidence, while that polarized in the plane perpendicular to 
this gave no trace of interference. The theory of Fresnel was 
thus confirmed experimentally. Again, the familiar phenomena 
of Newton’s rings show us that on changing media there is a 
change of phase of the incident light, else the central spot 
where the two surfaces come into optical contact would be 
white instead of black. But there has been difference of 
opinion as to whether this change of phase occurs at the upper 
surface of the air film, where the light passes from glass to less 
dense air or at the lower surface where it passes from air to 
more dense glass. In the latter event there should be a node 
at the reflecting surface. Replacing the silvered plane surface 
by a lens in contact with the photographic film, Wiener 
obtained circular fringes with no photographic action at the 
center, showing the nodal point to be at the point of contact, 
and thus again confirming the theory of Fresnel. 
[To be continued. | 
