330 Onthe Direction of the Vibrations of Polarized Light. 
| 40° | 50° | 60° | 70° | so° | 90° | 100° 
3 observed ......, 2:24| 30 | 4:54] @36| 742/96 | 123 
Calculated... 230 | 354] 537] 7-37 | 9:53 119-22 | 15-0 
where 6 is given by the equations 
COS : 
tan(45°—68) are Sey or sin +2, = sin 4 B, 
48 being the angle which the incident ray makes with the nor- 
mal to the erating, 3A, that made by the refracted ray with the 
same line, while Q, is the diffraction within the substance of the 
glass itself. 
It will be seen that experiment agrees very well with calcd 
tion, only that it gives results in all cases a little too small. 
Whatever, therefore, may be the cause of this difference, experi- 
ment most decidedly favours the hypothesis that the vibrations 
of polarized light are perpendicular to the plane of polarization, 
since in the opposite case 6 would be negative and of much 
greater magnitude. 
T next investigated the effect of diffraction by smoked metal 
gratings. When, however, these were of a perfect dull black, 
the diffraction images produced by them were far too feeble; 
they were therefore rendered smoother by passing a drop of oil 
of turpentine over them. Gratings of this description must, 
moreover, be rather fine and very accurately made. Some expe- 
riments made with a grating of 400 bars to the Paris imch (the 
thickness of the wire bars was 549th of a millim.), the grating 
being equally inclined to the incident light and the axis of the 
telescope, gave approximately the following results :— 
— |) ee ee eee eee eee eee 
The change of direction is positive, but much greater than it 
should be according to calculation. The polarization of the 
diffracted light was moreover slightly elliptical, from which it 
was evident that the reflexion from the metal surfaces of the 
wires was not entirely prevented by the smoke. On endeavour- 
ing to smoke the grating more perfectly, I partly destroyed its 
accuracy, and rendered it unfit for further experiments of this 
description. I have not succeeded in obtaining reliable results 
with other gratings of this description: there are peculiar diffi- 
culties in the way of checking the reflexion from the metal edges, 
and at the same time cana obte the diffraction image suffi. 
ciently large and distinct. 
