connected with the Polarization of Light. AAS 
feebler, but still quite sensible and polarized as before. On 
being viewed with the microscope, it was found that the number 
of furrows that penetrated entirely through the metal was much 
greater than before; but that the polarization of the dispersed 
light was still due solely to those striz which were merely super- 
ficial, and consequently were less than —1+—. millim. deep,—such 
being the thickness of the silver, as estimated by the coloured 
rings of iodide, which terminated in the middle of the first dark 
line. 
A general view of the results just described naturally leads 
one to anticipate that light may suffer changes of the same nature 
as that above described when it traverses extremely fine slits. 
The experiments, with a description of which I am about to close 
this memoir, prove that this is the case, and that under these 
circumstances also phenomena are produced closely related to 
the preceding. 
It is well known that, to reproduce certain experiments of 
interference and refraction, small instruments are constructed 
which are to be found in all physical cabinets: these are slits 
with narrow walls and straight parallel edges, which can be ap- 
proximated to each other from a distance of several millimetres 
to actual contact. If a pencil of light be suffered to pass through 
such a slit, the opening of which has been reduced to such a 
degree as to suffer but a trace of the light to pass, the emergent 
ray is invariably observed to be polarized in a direction perpen- 
dicular to the slit, the polarization being stronger in proportion 
as the slit is narrower. 
If a very bright light be employed, and if the observation be 
conducted with a microscope, openings still narrower may be ob- 
served; and by slightly inclining one edge to the other, a slit 
may be obtained in the field of the microscope which gradually 
decreases until the edges actually meet. Now in this case the 
light which passes near the point of contact is almost entirely 
polarized in a direction perpendicular to the slit. 
This phenomenon was at first attributed to the repeated 
reflexions of the light from one side of the slit to the other, 
reflexions which necessarily give rise to several phenomena of 
polarization; but we shall see directly that there are circum- 
stances hard to reconcile with this explanation. 
The first trials were made with a slit whose edges were of 
brass; steel, copper, and, lastly, silver were substituted for this 
metal: the phenomenon was but little modified, and that in a 
way which by no means agreed with the polarizing power of 
these different metals. Silver, for example, which by itself has 
but little polarizing power, polarizes almost completely the light 
which traverses a very narrow slit of which it forms the sides. 
