440 M. H. Fizeau on several Phenomena 
If the plate be illuminated perpendicularly, but observed very 
obliquely though perpendicularly to the line, the phenomena are 
the same as in the first case, the hight reflected from the finest 
extremity of the line being stall polarized in the direction of the 
line itself. 
If the plate be kept in the same position as in the last case, 
but the source of light be brought nearer to the eyepiece, so that 
the plate is both observed and illuminated obliquely and nearly 
in the same direction, the polarization of the bright line is much 
increased, not only the fine end polarizing the hght parallel to 
itself, but the same species of polarization being observed towards 
the thicker part of the hne where the light was formerly polarized 
in the opposite direction, and even where the light was previously 
unaffected. 
The distinct polarizing power of single lines traced on a silver 
surface having been thus demonstrated, it was easily foreseen that 
the same property would be possessed by the innumerable striz 
produced on metallic surfaces by rubbing them with a body 
covered with some hard substance reduced to a fine powder; and 
it was anticipated that the large number of these lines would 
compensate for the want of brilliancy in each individually, so 
that the phenomena would be visible without the need of a 
microscope. 
On a silver surface, a straight striated band about 2 
centims. broad was accordingly traced by means of a ruler and 
a piece of cork charged with emery*. ‘The striated band thus 
produced exhibits in the most striking manner, on account of 
the intensity of the light reflected, the phenomena of polariza- 
tion before observed in the case of single lines. 
If the striated band be placed under the microscope and illu- 
minated obliquely, innumerable lines are observed of different 
degrees of brightness and of various colours, due no doubt to 
accidental phenomena of diffraction and interference: nearly all 
these lines polarize the light in their own direction, some of the 
thicker ones, however, produce the opposite effect. 
If the striated band, mstead of being straight, is in the form 
of an arc of acircle of about 50 centims. radius, and if the 
surface of the plate be horizontal, and the source of light be 
placed vertically above the centre of the band, at such a distance 
from the plate that the incident light may make an angle of 
from 60 to 80 degrees with the normal, then, the eye being 
placed behind the source of light a little on one side, and being 
* Emery powder, No. 40 of the opticians, answers very well for these 
experiments, the mean diameter of its particles being about 5}5 millim. 
Emery powder No. 20, ordivary Tripoli, or English red may also be em- 
] 
ployed. 
