862 
PEOFESSOE W. A. MILLEE ON THE MEASUEEMENT 
It was soon evident that the absorbent action of the bisulphide was far greater than I 
had imagined, and that in reality the spectrum which it transmitted was composed of 
rays which did not extend beyond one-tenth or one-twelfth of the enthe length of the 
spectrum obtained by the use of a quartz-train*. 
3. The dispersive power of rock-crystal is, however, comparatively low, and the diffi- 
culty of obtaining with it a spectrum free from the effects of double refraction through 
its entire length is great ; so that it appeared to be worth while, as a preliminary inquuy, 
to ascertain whether any singly refracting medium could be procui’ed, better adapted to 
researches of this nature by sufficient permeability to the chemical rays, and by tolerably 
high dispersive power. Although no material on the whole preferable to quartz has 
been found, the investigation gave results of considerable interest. 
4. Before proceeding to detail these results, it will, however, be convenient, as several 
distinct subjects will be discussed in this paper, to state the order in which I propose to 
arrange my remarks, and the heads to which they will be referred. 
I shall commence with 
(1) The ahsor^ption of chemical rays hy transmission through different media. 
a. By transmission through solids. 
h. By transmission through liquids. 
c. By transmission through gases and vapours, 
(2) The absorption of the chemical rays by reflexion from polished surfaces. 
(3) The photographic effects of the electric spectra of different metals taken in air, 
including 
a. Pure metals. 
b. Alloys, 
(4) Photographic effects of electric spectra of different metals produced by transmitting 
the sparks through gases other than atmospheric air. 
5. The general results of my experiments upon the absorption of the chemical rays 
are the following : — 
(1) Colourless bodies which possess equal powers of transmitting the luminous rays 
vary greatly in permeability to the chemical rays. 
(2) Piactinic solids (that is to say, solids which are permeable to the chemical rays) 
preserve their diactinic power both when liquefied and when converted into vapour. 
(3) Colourless solids which are transparent to light, but which exert a considerable 
* The absorptive power of the bisulphide for the chemical rays was, however, noticed by M. E. Becquebel 
as far back as 1843, as I find by again referring to his paper, Annales de Chimie, ser. 3. vol. ix. p. 301. 
In this paper M. Becqtjeeel describes the absorbent action of various solids and liquids upon the chemical 
rays, but, from having used solar light, he failed to remark the great difference between the absorptive' 
powers of quartz and glass. Although he used prisms of rock-salt, rock-crystal, and alum, his results do 
not indicate the real difference in their absorptive power ; and as in all his experiments on liquids he employed 
a vessel with flint-glass sides to hold them, his conclusions are vitiated by the same error which affected my 
own earlier inquiries on the subject. 
