RECENT PROGEESS IN OPTICS. 125 



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 light 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 inter- 

 ference. But if the vibrations of the incident beams are in a plane ident- 

 ical with that of incidence, and hence in the plane of polarization, the 

 vibrations of incident and reflected beams are in mutually perpendic- 

 ular planes, and hence can not interfere. Wiener obtained interfer- 

 ence fringes when the light was polarized in the plane of incidence, while 

 the polarized in the plane perpendicular to this gave no trace of inter- 

 ference. The theory of Fresnel was thus confirmed experimentally. 

 Again, the familiar x>henomenon of Newton's rings shows 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. Keplac- 

 ing the silvered plane surface by a lens in contact with the photo- 

 graphic 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. 



COLOR PHOTOGRAPHY. 



The conditions being now specified under which stationary light 

 waves are produced, let us imagine common instead of monochromatic 

 light to be transmitted normally through a transparent sensitive film. 

 Then a variety of stationary interference planes are produced. This 

 is the underlying principle of the process employed by Lippmann in 

 Paris, who, in 1892 (Comptes rendus, Tome OXIV, p. 961, and Tome 

 CXV, p. 575), succeeded in obtaining a photograph of the solar spec- 

 trum in natural colors. Upon a surface backed with a reflecting mirror 

 of mercury is a silver bromide albumen film, which has been treated 

 with one or more aniline dyes to render it equally sensitive to waves of 



