RECENT PROGRESS IN OPTICS. 123 



low temperature and then warmed, it gives out light. At the tem- 

 perature of liquid air many substances manifest luminescence which 

 ordinarily seem almost incapable of it; such are gelatine, ivory, and 

 even pure water. A crystal of ammonium platinocyanide, on the other 

 hand, when immersed in liquid air and illuminated by the electric 

 light, shines faintly when this is withdrawn. If now the liquid air be 

 poured off so that the crystal rises rapidly in temperature, it glows 

 brightly. 



LUMINESCENCE AND PHOTOGRAPHY. 



Photography, like luminescence, is a manifestation of the transfor- 

 mation 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 can not be made fixed and perma- 

 nent. The problem of securing on the photographic 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 can not yet be 

 affirmed, but the last few years have brought a far 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 optical principles. The 

 first is originally due to Becquerel (Am. de Chimie et de Physique (3), 

 p. 451, 184S), but lately, in the hands of Lippmann, it has been improved 

 and brought much nearer to success than by its originator. It depends 

 upon the production of stationary waves of light. Every one is famil- 

 iar 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. 

 Zenker ("LehrbuchderPhotochromie," Berlin, 1868) 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 production has been brought forward. A few years ago 



