284 W. LeConte Stevens—LRecent Progress in Optics. 
light, shines faintly when this is withdrawn. If now the liquid 
air be poured off so that the crystal rises rapidly in tempera- 
ture, it glows brightly. 
Luminescence and Photography. 
Photography, like luminescence, is a manifestation of the 
transformation of energy, most frequently of initial short wave 
length. The production of color by photography is nothing 
new. It was noticed by Seebeck nearly a century ago that 
silver chloride becomes tinted by exposure to ordinary light, 
with accompanying chemical change; that if then subjected a 
long time to red light it assumes a dull red hue, or a dull bluish 
hue if held in blue light. It is likewise possible by proper 
selection of luminescent salts to produce a selected series of 
tints during and after exposure to those rays which are most 
effective in photography. But such colors cannot be made 
fixed and permanent. The problem of securing on the photo- 
graphic plate a faithful and lasting reproduction of the various 
tints of a spectrum thrown upon it, has baffled most of those 
who grappled with this subject. That it has been fully and 
quite satisfactorily solved cannot yet be affirmed, but the last 
few years have brought a much nearer approach to success 
than an equal number of decades previously. Viewed from 
the scientific standpoint, the goal has certainly been touched, 
even if commercial demands are still made in vain. } 
Stationary Light Waves. 
Two quite different methods are to be considered in tracing 
the recent development of this interesting application of optt- 
cal principles. The first is originally due to Becquerel,* 
but lately in the hands of Lippman it has been improved and 
brought much nearer to success than by its originator. It 
depends upon the production of stationary waves of light. 
Everyone is familiar with the formation of stationary waves 
upon an elastic stretched cord, and with the acoustic exhibition 
of stationary air waves in a closed tube by Kundt’s method of 
light powders. That similar loops and nodes must be produced 
under proper conditions by interference of waves of light 
would appear obviously possible; and so long ago as 1868 Dr. 
Zenkert of Berlin explained the photographic reproduction of 
color, so far as it had then been accomplished, by reference to 
stationary light waves. But no definite proof of their produc- 
tion had been brought forward. A few years ago Hertz 
demonstrated objectively the electro-magnetic waves whose 
* Edmond Becquerel, Ann. de Chimie et de Physique, III, xxii, p. 451, 1848. 
+ Zenker’s Lehrbuch der Photochromie, Berlin, 1868. 
