EEFLEXION OF VAEIOUS BODIES. 
875 
§ 2. THE ABSOEPTION OF THE CHEMICAL EAYS BY EEFLEXION FEOM POLISHED 
SHEFACES. 
22. In my earlier experiments I had much difficulty in obtaining a spectrum all the 
parts of which were even approximatively in focus in the same plane, and, with the view 
of remedying this defect, 1 tried the effect of substituting specular reflexion for the 
refracting action of a lens. This led me to compare the reflecting-power of different 
polished surfaces for the chemical rays. With this object in view, a small polished plate 
of the material under experiment was supported at an angle of 45°, as shown in fig. 3 
(par. 6), opposite the vertical slit of the apparatus, so that when the source of light e, was 
placed at right angles to the axis of the tube, the rays were reflected down the tube in the 
dhection of that axis. The arrangement of the prism, lens, and camera was the same as 
that already described (6). As, however, much less light was reflected upon the prism 
from the polished surface than that which fell upon the prism when the direct rays of 
the spark were employed, the exposure of the sensitive plate in the camera was prolonged 
from 5 minutes to 10. 
23. Among the metals and alloys thus submitted to trial were platinum, gold, silver, 
mercury, contained in a trough with quartz faces, lead, copper, tin, cadmium, zinc, 
aluminum, steel, brass, and speculum-metal. In addition to these, the reflecting-power 
of quartz, window-glass, and Iceland spar was also tried. 
No judgment of the perfection of the reflecting-power could be formed from the colour 
of the metal. Gold possesses the power of reflecting all the rays, even the most refran- 
gible, very equally, though somewhat feebly. Next to gold ranks burnished lead, some 
part of the spectrum of the electric spark reflected from lead being more intense than 
that from gold. The length of the spectrum obtained from the light of the electric spark 
between silver points, by reflexion from the surface of these two metals, extended from 
96'5 to 170'5, or over the full distance of that obtained by the direct light of the spark, viz. 
74 dirisions of the scale which I have adopted. With all the other metals the spectrum 
of the same reflected spark terminated at 159-5, covering only 63 divisions of the scale. 
The spectmm from a silver surface was remarkable. The impressed image was strong 
up to 112-5 ; then an abrupt cessation of the reflected rays occurred for a distance of 1-5 
division ; beyond this the reflexion gradually returned, and continued tolerably intense 
till it reached 159-5, covering 63 divisions of the scale. 
The reflexion from mercury was weak in the middle, but strong toAvards each extre- 
mity. Platinum, zinc, and aluminum resembled mercury in their effects, but the spec- 
trum was much less intense. The reflexion from cadmium was similar, but still weaker. 
The spectrum of the rays reflected from cojpper was deficient in strength for the last 
half of the more refrangible portion ; and that of brass was similar to it, but weaker. 
The reflexion from a surface of steel was more intense than that from any surface 
which I employed, but it ended abruptly at 159-5, or at the 63rd division of the scale. 
The spectrum reflected from tin was nearly as complete as that from steel. 
A small concave mhror of sjyeculum-metal gave an intense spectrum for the first half ; 
