4.44, M. H. Fizeau on several Phenomena 
A second glass plate, B, was covered with silver-leaf, opake 
like the former, except for rays of more than a certain degree of 
intensity ; for on looking at the sun through this plate, the body 
of that luminary was visible, stripped of his rays, and of a rich 
blue colour. I am persuaded that this silver-leaf contained a 
small quantity of gold. 
The metallic surface so produced was found too unequal and 
too loosely adherent to the glass to be scratched with emery like 
the former. On being rubbed, however, with pure cotton, the 
polarization of the dispersed hght was very distinct. Under the 
microscope it appeared to be torn in every direction, so that 
nothing could be inferred from this experiment; nevertheless, 
for the sake of comparison, the thickness of the metal was deter- 
mined by the same means as before. In this case the rings were 
four in number, the layer becoming transparent in the middle 
of tke fourth, whence it was concluded that the thickness of the 
metal was not more than 5,4, millim. This extreme thinness 
of beaten silver-leaf agrees with what we know with respect to the 
gold leaves used in gilding; these leaves, which, as is well known, 
are transparent and of a green tinge, have a medium thickness 
of about +, millim., as I ascertamed by weighing measured 
quantities of them. ‘Three different specimens gave the values 
0:000108, 0:000095, and 0:000091 millim. 
Returning then to the employment of glass plates covered 
with a layer of silver chemically deposited, one was obtained 
(which we will call C) not so thick as the former, as was proved 
by its greater transparency. ‘The light transmitted by this plate 
was of a greyish blue, the reflected light of a yellowish blue, 
with this property, that towards the angle of maximum polariza- 
tion it became of a pure blue for light polarized normally to the 
plane of reflexion. The rings of iodide formed on this surface 
terminated in the middle of the second bright line, beyond which 
the layer was completely transparent, from which it was con- 
cluded that the thickness of the silver layer was —4,5 mullim. 
This metallic film, when scratched with the same emery as the 
former, polarized the dispersed light very distinctly. On being 
examined microscopically, a very large number of the furrows 
seemed to have penetrated through the metal; but the majority 
did not, and consequently must have been less than 5-45, 
millim. deep. It was ascertained also that the lines which, when 
illuminated obliquely, produced a distinct polarization in their 
own direction, were of the latter lind. 
Lastly, a fourth layer of silver, D, was prepared thinner than 
all the others; it was more transparent than the last, and the 
light reflected by it possessed the same properties to a greater 
degree. Scratched hke the former, the dispersed light was 
