of Gold and other Metals to Light. 585 
rently changed characters with each other. By care I was able to 
introduce a stretched piece of gold-leaf (without glass) into the sul- 
phideof carbon: its effects were the same with those just described. 
In all the experiments to be described, the plane of polariza- 
tion and the plane of inclination had the same relation 
to each other: the figure shows the position of the po- 
larizing Nicol prism, as the eye looks through it at the 
light, and a, b represents the vertical axis, about which 
the plates were inclined. Whether they were inclined 
in one direction or the other, or had the glass face or 
the metal face towards the eye, made no difference. In 
all cases with gold-leaf, it was found that the ray had 
been rotated; that it required a little direct rotation of 
the analyser to regain the minimum light; that short of that 
red tints appeared, and beyond it blue or cold, these being ne- 
cessarily affected in some degree by the green colour of the gold- 
leaf. Thinned gold-leaf produced the same results ; but as holes 
appeared in those that were thinnest, the results were interfered 
with, because the light passing through them was affected by the 
analyser in a different manner, and yet mingled its result with 
that of the light which had passed through the gold. 
The gold-leaf plates, deprived of green colour by heating in 
oil, were found with the glass in such good annealed condition, 
as not to affect the ray; but when they were moved, until the 
oblique colourless gold came into the course of the ray, it was 
depolarized; a red image appeared; direct rotation of the ana- 
lyser reduced this a little in intensity and then changed the 
colour to blue. The reduction was not much, and both in that 
and the first appearance of the red image there is a differ- 
ence between the heated and the unheated gold: probably the 
ereen tint of the latter, which would tend to extinguish the 
red and produce a minimum, may be sufficient to account for 
the effect. Gold which had been re-greened by agate pressure 
acted in like manner on the polarized ray, but the experiments 
were imperfect. 
A glass plate having gold-leaf on one part of it, had a second 
glass plate put over it and gummed at the edges. In the sul- 
phide of carbon, therefore, it represented in one part a plate of 
air, and in another a compound plate of air and gold ; both acted 
in the same direction, but the air and gold much more than the 
air. Gold on glass in this medium, or gold in air, or glass in 
air, all gave results in the same direction, 7. e. required direct 
rotation of the analyser to compensate for them. 
I proceeded to examine the other forms of gold; and first, the 
deposits on glass obtained by electric deflagration. These affected 
the ray of polarized light exactly in the manner of gold-leat, and 
