W. LeConte Stevens—Recent Progress in Optics. 285 
existence had been foretold by Maxwell’s genius; and with 
suitable apparatus 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 study- 
ing stationary light waves. Wiener’s admirable work* excited 
great interest on the continent of Europe, but it has been 
singularly neglected in England. and America. It is worth 
much more than a passing notice. 
Assume a plane silvered mirror upon which a bundle of rays 
of monochromatic light falls normally so as to be reflected back 
upon its own path. ‘The superposition of refiected 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 reflect- 
ing 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 
parallellism to the mirror. Although a half wave length of vio- 
let light is only 5;!,,th 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 then be 4th 
millimeter apart, and readily capable of resolution if their 
presence can be manifested at ail. Imagine a very thin trans- 
parent photographic film to be stretched along the oblique 
cutting plane, and developed after exposure to violet light as 
nearly monochromatic as possible. Then the developed nega- 
tive should present a succession of parallel clear and dark lines, 
corresponding to nodal and anti-nodal bands along the oblique 
plane, the photographic effect being annihilated along an opti- 
cal 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 
8000 of them would be required to make a pile one millimeter 
thick. By electro-chemical deposit Outerbridget has made 
films of gold whose thickness is only zyq¢yyyth of a millimeter, 
or ,th of a wave length of sodium light. Wiener obtained a 
perfectly transparent silver chloride film of collodion, whose 
thickness was about th of a wave length of sodium light. 
This was formed on a plate of glass and inclined at a very 
*O. Wiener, Wiedemann’s Annalen, xl, p. 203, 1890. 
+ Journal of the Franklin Institute, vol. ciii, p. 284, 1877. 
