AND TRANSMITTED BY CERTAIN KINDS OF GLASS. 
279 
The stage reading- gave the position in which the light was incident upon the surface 
at an angle of 45°. To verify the adjustment, the Nicol tube was clamped, first, at 
an angle differing by fi- 90°, and then at one differing by — 90°, from its original 
position. 
The reading of the Nicol, when light polarised in the plane of incidence was cut 
off, had been carefully determined some years previously, and was again verified. To 
reduce, as far as possible, the errors due to the Nicol (which was of the ordinary 
construction, with its terminal faces not perpendicular to its geometrical axis), eight 
readings were made with the prism in one position, and eight more after it had been 
turned round through 180°. 
The actual readings made with the 6'5 mm. plate of crown glass, which had been 
repolished by Mr. Hilger, were— 
O / O / 
107 
06 
107 
25 
107 
03 
107 
08 
107 
20 
107 
09 
107 
27 
107 
09 
o / o / 
106 
59 
106 
55 
107 
02 
107 
04 
107 
08 
106 
56 
107 
04 
106 
55, 
The readings made with the other glass surfaces were about as concordant. 
Table XV. gives the angles of polarisation as deduced from the means of these 
readings. 
Table XV. 
Angles of Polarisation. 
Crown glass-- ^ , ,, 
6‘5 mm. plate, repolislied by Mr. Hilger. 56 22 30 
11‘5 „ not repolished. 56 14 30 
11'5 ,, No. 2, repolished by Mr. Hilger and again with rouge 56 28 30 
II'5 ,, „ ,, with putty powder. 56 49 0 
15 0 „ ,, by Mr. Hilger. 56 42 30 
15’0 ,, ,," with putty powder. 56 49 30 
18'5 „ not repolished. 56 21 30 
I8'5 „ not repolished. 56 25 30 
24‘3 ,, repolished by Mr. Hilger. 56 37 30 
Wedge used for reflection experiments repolished with putty powder, 
and subsequently with rouge. 56 52 0 
Flint glass— 
Face of block—not repolished. 58 14 0 
,, „ repolished with putty powder and examined 
immediately. 58 48 0 
„ ,, repolished with putty powder and examined after 
two days. 58 44 0 
