258 
SIR J. CONROY ON THE AMOUNT OF LIGHT REFLECTED 
The light reflected by the right-angled prism was examined with a double-image 
prism and a plate of selenite, and was found to be completely unpolarised. 
The observations were made by first determining the position of the analyser in 
v/hich the field appeared equally bright throughout; then the plate of glass to be 
examined was placed between the right-angled prism and the white surface, and the 
new position of the analyser, in which there was equality of illumination, observed. 
As the intensity of the light which traversed the two Nicols varied as the square of 
the cosine of tlie angle between their principal sections, the percentage amount trans¬ 
mitted by the glass was given by 100 X cos^a'/cos^a, where a is the angle between 
the principal sections of the Nicol, when the field was uniformly bright without the 
glass, and ct ^^^hen it was interposed. 
The analysing Nicol was first rotated “ clockwise,” and readings made in each of 
the four quadrants of the position in which the two halves of the field appeared 
equally illuminated; the Nicol was then rotated “ counter-clockwise,” and four 
similar readings made, the mean of the eight readings being taken as the true 
position. The glass was then interposed between the diaphragm and the reflecting 
prism, and eight readings of the new position of the analyser, in which there was 
equality, made in the same way. 
To determine the light transmitted by each piece of glass, four sets of eight 
^observations were made without the glass, and four sets with it. Table III. giv^es 
the first set of each for the 6'5 mm. plate of crown glass ; the other sets were about 
as concordant. 
Table III. 
Without Glass. 
Eeaclings of analyser. 
Mean.* 
1 
O 
O 
0 
O 
42 
25 
42 
45 
42 
35 
47 
25 
137 
0 
139 
30 
138 
15 
48 
15 
222 
25 
223 
15 
222 
50 
47 
10 
317 
35 
318 
30 
318 
0 
48 
0 
Mean 
47 
42 
* As the intensity of the light varies as the squai’e of the cosine, the geometrical, and not the 
ai'ithmetical, is the true mean ; but the observations did not appear to be sufficiently concordant to make 
it worth while to employ the longer process. 
