In the year 1863 the late Dr. W . A. Miller described his method of examining the diactinic properties of various saline solutions and organic substances. (Miller, “On the Photographic Transparency of Various Bodies, &c.,” Phil. Trans., 1863, I.) He found that the spectrum transmitted by lenses and prism s of rock-crystal was far superior in length as determined by sensitive photographic plates, to that obtained when lenses and prisms of glass and other materials were employed. He found particularly that the spectrum produced by passing electric sparks between silver points was very rich in photographic rays, and he ascertained how far various substances were transparent to these rays. Professor Stokes at the same time communicated results he had obtained when studying the long spectrum of the electric light by receiving the rays on a fluorescent screen. (Stokes, “On the Long Spectrum of Electric Light,” Phil. Trans., 1863, I.) He discovered the fact that certain solutions showed light and dark bands which were otherwise invisible. More recently, M. Soret has studied the absorption of the ultra-violet rays of the spectrum transmitted by various substances, using for the purpose a spectroscope of his invention, which has a fluorescent eye-piece. (Soret, “Becherches sur l’Absorption des Rayons Ultra-violets par diverses substances,” 'Archives des Sciences Physiques et Naturelles;’ Geneva, January, 1878.) In 1872 the apparatus of Dr. Miller was reconstructed by one of us, and certain improvements were introduced.
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Hartley et al. (1879) studied this question.