177 
OF GOLD {A^B OTHEE METAXS) TO LIGHT. 
racter, as should either reduce its effect to nothing, or render it so small as to cause its 
easy elimination. Either camphine or sulphide of carbon was found to answer the pur- 
pose with crown-glass ; but the latter, as it possesses no sensible power of rotation under 
ordinary circumstances, is to be preferred. Should a medium of higher optic force be 
required, it would probably be supphed by the use of that dangerous fluid, phosphorus 
dissolved in sulphide of carbon. A rectangular glass cell being provided, which did not 
itself affect the polarized ray, was placed in its course and filled to a certain height with 
sulphide of carbon. A plate of crown-glass was then introduced perpendicularly to the 
ray ; it did not affect it ; being inclined as before described, the effect on the ray was still 
insensible, the glass appearing to be, for all ordinary observations such as mine, quite as 
the medium about it. I could now introduce gold-leaf attached to glass into the course 
of the polarized ray, its condition as a flat film or plane being far finer than when 
stretched on a wire ring as before. It proved to be so far above the sulphide of car- 
bon, as to have powers of depolarization apparently as great as those it had in air, and 
being inclined, brought in the image at the analyser exceedingly well. It was indeed 
very striking to see, when the plate was moved parallel to itself, the darkness when 
mere glass intervened, and the hght which sprung up when the gold-leaf came into its 
place ; the opake metal and the transparent glass having apparently changed characters 
with each other. By care I was able to introduce a stretched piece of gold-leaf (without 
glass) into the sulphide of carbon ; its effects were the same with those just described. 
In all the experiments to be described, the plane of polarization and the plane 
of inclination had the same relation to each other ; the figure shows the position 
of the polarizing Nicol prism, as the eye looks through it at the hght, and a, h 
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 httle direct rotation of the analyser to 
regain the mmimum light ; that short of that red tints appeared, and beyond it blue or 
cold, these being necessarily 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 
thmnest, the results were interfered with, because the hght 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 oh, were found with the 
glass in such good annealed condition, as not to affect the ray ; but when they were 
moved, until the obhque colourless gold came into the course of the ray, it was depo- 
larized ; a red image appeared ; direct rotation of the analyser 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 difference between the heated 
and the unheated gold : probably the green tint of the latter, which would tend to extin- 
guish the red and produce a minimum, may be sufficient to account for the effect. Gold 
MDCCCLVII. 2 B 
