350 



SCIENCE. 



[N. S. Vol. II. No. 38. 



determinations clLfiFering from it about one 

 part in two millions. This achievement is 

 deemed a signal scientific triumph that 

 ranks with the brilliant work of Arago, 

 Fresnel and Regnault. In the conception, 

 mechanical design and execution it is 

 wholly and distinctively American; and it 

 tends to do away with the reproach, too 

 often deservedly cast against America, that 

 our people have little appreciation for scien- 

 tific work unless its value can be expressed 

 in dollars. 



The subject of luminescence was next 

 taken up, in connection with important work 

 done in Germany, by "Wiedemann and 

 Schmidt, and not yet fully published, with 

 a view to clearing up the uncertainties re- 

 garding the nature of this in its two chief 

 manifestations, phosphorescence and fluor- 

 escence. We have here, as in photography, 

 a transformation of radiant energy ; and it is 

 shown that in a large propoi'tion, if not all, 

 of the cases examined, at least a part of the 

 transformation is into chemical energy, to 

 which is superadded the retransformation 

 into energy of longer period; and this 

 either at the same time or long after the 

 action of the exciting rays. Many sub- 

 stances which manifest no luminescence at 

 ordinary temperatures after exposure, or 

 which do so for only a very short time, be- 

 come distinctly luminescent when warmed, 

 some even after the lapse of days or weeks. 

 This thermo-luminescence is thus analogous 

 to the chemical storage of electrical energy 

 in an accumulator cell. The capacity for 

 giving out colored light continues untU the 

 cessation of the chemical action thus 

 brought into play. The effect of great de- 

 pression of temperature is also considered, 

 some remarkable results having been at- 

 tained by Dewar on subjecting various lu- 

 minescent substances to the temperature of 

 liquid air. 



By proper selection of luminescent salts it 

 is possible to produce a selected series of 



tints during and after exposure to those 

 spectrum rays which are most effective in 

 photography ; but such colors cannot be 

 made fixed and permanent. The problem 

 of securing on the photographic plate a 

 faithful and lasting reproduction of the va- 

 rious hues of a spectrum thrown upon it has 

 long baffled most of those who grappled 

 with this subject. WhUe not yet com- 

 pletely solved, it has been handled with 

 much nearer approach to success during the 

 last five years than during an equal num- 

 ber of decades previously. Two quite 

 different methods are to be considered in 

 tracing this success. The first, originallj' 

 due to Becquerel, has been greatly im- 

 proved bj^ Lippmann in Paris. It depends 

 upon the production of stationarj^ waves of 

 light. The theoretical possibility of pro- 

 ducing these has long been apprehended, 

 but demonstrated success was attained for 

 the fii'st time a few years ago bj^ Otto 

 Wiener, in Strassburg, a physicist whose 

 admirable work in optics has received but 

 little attention in America. The conditions 

 requisite for success are here given, and 

 Wiener's method is explained; as is also 

 the application of his results to confirm the 

 views of Fresnel, in opposition to those of 

 Neumann and MacCullagh, in regard to the 

 relation between plane of polarization and 

 direction of vibration of polarized light, and 

 in regard to change of phase in the reflec- 

 tion of light at the boundary between two 

 media differing in density. 



The theory of Lippmann's method of 

 photographing in natural colors is now dis- 

 cussed, but the conclusion is expressed that 

 the method cannot long remain practically 

 important because, like the daguerreotj^pes 

 of fifty years ago, these colored photographs 

 are incapable of multiplication. Wiener 

 has lately published an elaborate research 

 upon this subject, in which he recognizes 

 the necessity for the employment, not of 

 interference colors, but rather of what he 



