1857.] of Gold (and other Metals) to Light. 437 



medium of higher optic force be required, it would probably 

 be supplied 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 car- 

 bon. A plate of crown-glass was then introduced perpendicu- 

 larly 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 con- 

 dition 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 carbon, as to have powers of depolarization appa- 

 rently 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 light which 

 sprung up when the gold-leaf came into its place ; the opake 

 metal and the transparent glass having apparently changed cha- 

 racters 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 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 g 

 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 

 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 thinnest, the results were 

 interfered with, because the light passing through them was 

 affected by the analyser in a different manner, and yet mingled 



