connected with the Polarization of Light. 439 
its polarization, I have observed several unexpected and, as I 
believe, hitherto unnoticed phenomena, which appear worthy of 
the attention of some of the learned members of the Academy, 
to whom [ hasten to communicate them. 
Many observers, among whom I may mention Fraunhofer, 
Sir D. Brewster, Foreign Associate of the Academy, and more 
recently Messrs. Stokes, Holtzmann, and Lorenz, have remarked 
certain phenomena of polarization in the light emitted by regular 
gratings. These phenomena, however, seem to be entirely di- 
stinct from those I am about to communicate. 
On a plate of metal, say of silver, perfectly flat and polished, 
suppose a straight line to be drawn with a fine steel or dia- 
mond point, the surface of the metal being disturbed as little 
as possible, and the line so traced becoming finer and finer by 
degrees towards one end until it becomes imperceptible. If the 
plate on which the line has been so drawn be illuminated very 
obliquely, and in a direction perpendicular to the line, the latter 
will be visible in every position comprised in the common plane 
of incidence and reflexion, except its thin end, which, from its 
great tenuity, will be imperceptible. If the plate, illuminated 
in the same manner, be placed in the field of a microscope, a 
much larger portion of the thin end of the line will be visible, 
the amount depending on the power of the lenses employed. 
Under these circumstances, the plate being viewed perpendi- 
eularly to its surface, if a doubly-refracting analysing prism be 
placed between the eye and the eyepiece of the microscope, the 
different intensity of the ordinary and extraordinary images at 
once indicates that the lhght emitted by the brilliant line thus 
drawn is more or less polarized,—the amount of polarization 
being greatest towards the fine end of the line, and the plane of 
polarization being parallel to the direction of the line. Lines 
produced in various ways were observed, and, provided they were 
sufficiently fine, always with the same result. 
It was observed, moreover, that the plane of polarization, which 
was always parallel to the lines at their thinnest extremities, was 
perpendicular to them where they were a little broader, and 
became altogether imperceptible towards their thicker ends. 
On varying the angle of incidence of the light on the plate 
and the angle at which the line was viewed, which could be done 
within certain limits, depending on the shape of the microscope, 
by inclining the plate and shifting the position of the source of 
light, certain changes were observed in these phenomena, of 
which the following are the principal. 
The plate being still observed normally, if the line be illumi- 
nated less and less obliquely, the source of light being brought 
nearer to the eyepiece of the microscope, the amount of polari- 
zation rapidly decreases and soon becomes insensible. 
