124 RECENT PROGRESS IN OPTICS. 



Hertz demonstrated objectively the electromagnetic "waves whose exist- 

 ence had been foretold by Maxwell's genius ; and with suitable appa- 

 ratus stationary electric waves are now almost as readily made evident 

 as are those of sound. Hertz's brilliant success stimulated his fellow 

 countryman, Otto Wiener, to undertake the apparently hopeless task 

 of producing and studying stationary light waves. Wiener's admirable 

 work (Wiedemann's Annalen, Band XL, 1890, p. 203) excited great inter- 

 est on the continent of Europe, but it has been singularly neglected in 

 England and America. It is worth much more than a x>assing notice. 



Assume a plane silvered mirror upon which a bundle of rays of mono- 

 chromatic light fall normally so as to be reflected back upon its own 

 path. The superposition of reflected and direct waves causes a system 

 of stationary waves, but under ordinary conditions these are wholly 

 imperceptible. The nodes are formed upon a series of planes obviously 

 parallel to the reflecting plane at successive distances of a half wave 

 length. If now we consider a plane oblique to the mirror, it will cut 

 these successive nodal planes in parallel lines, whose distance apart 

 will be greater in proportion as the oblique plane approaches parallel- 

 ism to the mirror. Although a half wave length of violet light is only 

 one five-thousandth of a millimeter, it is easy to conceive of the cutting 

 plane forming so small an angle with the mirror that the distance 

 between the parallel nodal lines shall be a thousand times a half wave 

 length. Such would be the case if the inclination of the cutting plane 

 is reduced to a little less than four minutes of arc. The nodal lines 

 would be one-fifth of a millimeter apart, and readily capable of resolu- 

 tion if their presence can be manifested at all. Imagine a very thin 

 transparent photographic film to be stretched along the oblique cutting 

 plane, and developed after exposure to violet light as nearly mono- 

 chromatic as possible. Then the developed negative should present a 

 succession of parallel clear and dark lines, corresponding to nodal and 

 antinodal bauds along the oblique plane, the photographic effect being 

 annihilated along an optical nodal line. 



The realization of a photographic film thin enough for such an 

 experiment is quite conceivable when we remember that under the 

 hammer gold is beaten into leaves so delicate that 8,000 of them would 

 be required to make a pile 1 millimeter thick. By electrochemical 

 deposit, Outerbridge (Journal of the Eranklin Institute, Yol. CHI, 

 p. 284, 1877) has made films of gold whose thickness is only one-hun- 

 dred-thousandth of a millimeter, or one-sixtieth of a wave length of 

 sodium light. Wiener obtained a perfectly transparent silver chloride 

 film of collodion, whose thickness was about one-thirtieth of a wave 

 length of sodium light. This was formed on a plate of glass and 

 inclined at a very small angle to a plane silvered mirror which served 

 as reflector. From an electric arc lamp the light was sent through an 

 appropriate slit and prism, so that a selected spectral band of violet 

 fell normally on the prepared plate in the dark room. The developed 



