128 RECENT PROGRESS IN OPTICS. 



Mr. Ives lias devised a special form of camera by which the three 

 elementary negatives are taken simultaneously, and also an instrument, 

 the photochromoscope, in which a system of mirrors and lenses "brings 

 to the eye a combination similar to that projected with the triple lan- 

 tern. A double instrument of this kind forms the most perfect type 

 of stereoscope, bringing out with great vividness from the prepared 

 stereographs the combined effect of color, form, and binocular perspec- 

 tive. It is only within the past year that these improvements have 

 been perfected. By further application of the same principles, Mr. 

 Ives has produced permanent colored prints on glass, which do not 

 require to be examined by the aid of any instrument. Each of these 

 negatives is made with a colored screen which transmits tints comple- 

 mentary to those which it is desired to reproduce. The three gelatine 

 films are soaked in aniline dyes of suitable tint 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 sim- 

 ply, but more effectively, than by the method of interference 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 " electricity is still in its infancy," it may be a source 

 of equal delight to believe that photography in colors, a yet more deli- 

 cate 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 stereoscope. Tet it is gratifying to recognize 

 the great impetus that this beautiful art has received during 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 appli- 

 cations of optical science. 



THE INFKA-RED SPECTRUM. 



Among the splendid optical discoveries of this century, probably 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, Miiller, Tyndall, La- 

 mansky, and Mouton. But our quantitative knowledge of this region 

 began with the invention and use of the bolometer by Langley 

 (" Selective absorption of solar energy," Am. Journal of Science, March, 

 1883, p. 169), 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 Angstrom, Rubens, Snow, and 



