connected with the Polarization of Light. 448 
This polarization of the regularly reflected ray may be ob- 
served with different metals without sensible modification,—the 
metals which have the least effect in producing ordinary polari- 
zation, such as gold and silver, giving results sensibly the same 
as those produced by the metals that polarize the most, such as 
platinum and zinc. This result seems to preclude us from attri- 
buting these effects to partial polarization by the sides of the 
furrows, which is the first idea which naturally occurs to us. 
Such reflexions appear at least not to be the principal cause of 
the phenomenon. 
In order to advance another step in the study of these singular 
phenomena, it was necessary to ascertain the exact dimensions 
of the small furrows which possess properties so singular. For 
this purpose recourse was had to exceedingly thin layers of 
silver deposited on glass from certain chemical solutions, and 
which may not only be used to replace the tin amalgam for sil- 
vering looking-glasses, but have even been employed with success 
by M. Foucault in the construction of a new species of telescope. 
The first glass plate, A, was covered with a very thin, though 
still perfectly opake layer of silver, and a straight band of ~ 
striz was produced on the metallic surface by means of a piece 
of cork covered with very fine emery. This band presented the 
phenomena above described, viz. polarization of the dispersed 
light parallel to its own direction, and polarization of the reflected 
light perpendicular to the same. On examining, by means of 
the microscope, the state of the striated layer by transmitted 
light, it was easy to perceive that the furrows had not in general 
penetrated through the metallic layer, some deeper than the rest 
forming exceptions. 
In order to ascertain the thickness of the metallic layer, a 
piece of iodine was placed on one point of the surface, and the 
coloured rings due to the film of iodide of silver produced by 
the evaporation of the iodine were suffered to spread from this 
point, which itself became completely transparent, owing to the 
change of the silver layer throughout its entire thickness into 
yellow iodide. From the part where the silver had not been in 
any degree affected by the emanations of iodine, to the transpa- 
rent spot of iodide, there was a series of coloured rings commen- 
cing with white, and which, on being observed through a red 
glass, appeared to be nine in number. The series terminated in 
the middle of the ninth brilliant rmg. The index of refraction 
of the iodide of silver being 2°246 (as deduced from the angle of 
polarization, which was 66°), the ninth bright ring gave for the 
iodide of silver a thickness of 34, millim. From the thickness, 
composition, and known density of the iodide, the thickness of 
the layer of silver was calculated to be 34455 millim. 
