380 W. LeConte Stevens—Recent Progress in Optics. 
is desired to reproduce. The three gelatine films are soaked 
in aniline dyes of suitable tints, and superimposed between 
plates of glass. When viewed as a transparency such a print 
gives a faithful reproduction of the natural colors. 
The problem of color reproduction is thus solved, not indeed 
so simply, but more effectively, than by the method of inter- 
ference of light, or by those body color methods that have thus 
far been applied. To the imaginative enthusiasts who are 
fond of repeating the once novel information that ‘ electri- 
city is still in its infancy,” it may be a source of equal delight 
to believe that photography im colors, a yet more delicate 
infant, is soon to take the place of that photography in light 
and shade with which most of us have had to content ourselves 
thus far. But so long as an instrument is needed to help in 
viewing chromograms, the popular appreciation of these will be 
limited. We may take a lesson from the history of the stereo- 
scope. Yet itis gratifying to recognize the great impetus that 
this beautiful art has received within the last few years. We 
may quite reasonably expect that the best is yet to come, and 
that it will have an important place among the future applica- 
tions of optical science. 
The Infra-Red Spectrum. 
Among the splendid optical discoveries of this century pro- 
bably the most prominent are photography and spectrum 
analysis, each belonging jointly to optics and chemistry. 
Photography was at first supposed to be concerned only with 
the most refrangible rays of the spectrum, but Abney and 
Rowland have photographed considerably below the visible 
red. Beyond the range thus attained qualitative knowledge 
was secured by Herschel, Becquerel, Draper, Melloni, Miller, 
Tyndall, Lamansky and Mouton. But our quantitative know- 
_ledge of this region began with the invention and use of the 
bolometer by Langley,* whose solar energy curve has been 
familiar to all physicists during the last dozen years. During 
this interval the bolometer has been used with signal success 
by Angstrém, Rubens, Snow, and Paschen, who have made 
improvements not only in the instrument itself but in the 
delicacy of its necessary accompaniment, the galvanometer. 
The work of Snowt particularly, on the infra-red spectra of 
the voltaic are and of the alkalies, and that done by him in 
conjunction with Rubenst on refraction through rock salt, 
sylvite, and fluorite, exhibited the capacities of the bolometer 
* Langley, Selective Absorption of Solar Energy, this Journal, March, 1883, 
p. 169. | 
+ Physical Review, vol. i, pp. 28 and 95. 
t Astronomy and Astrophysics, March 1893, p. 231. 
