AND TRANSMITTED BY CERTAIN KINDS OF GLASS. 
257 
Ordinary Plate Glass. 6 mm. thick. 
A. 
Readings. 
1 
1 
Half 
differences. 
a — J7. 
b. 
Percentage of 
light transmitted. 
1 
Percentage of j 
light transmitted.' 
Mean of A and B. 
1 cm. 
cm. 
94-9-I08-4 
6-7 
195T 
208-7 
87-39 
94-9-I08'4 
6-7 
195 I 
208-7 
87-39 
1 94'9-108‘6 
68 
195-0 
2U8-8 
87-22 
1 
i 
87-33 
B. 
1 94-4-I07-3 
6'4 
195-4 
208-4 
87-91 
94-4-I07-0 
6’3 
195-7 
208-3 
87-91 
1 94-1-106-9 
i 
6’4 
195-4 
208-4 
88-27 
88-02 
87-68 
These determinations were carefully made, and the results are fairly concordant; as, 
however, this agreement was not inconsistent with the existence of a constant source 
of error, it appeared desirable to repeat some of them by a different method. 
A polarising photometer was, therefore, set up (Plate 8, fig. 2), consisting of two 
Nicols and a right-angled prism ; the Nicols, which were furnished wdth divided circles 
and verniers reading to 1°, were placed in the same straight line, and about 30 cm. 
apart, the right-angled prism being between them, and so placed that the field of 
view of the analysing Nicol was bisected vertically by the edge of the prism. 
Two pieces of white paper were fixed in vertical planes at right angles to one 
another, both being illuminated by a small Argand gas burner; one was seen directly 
through the two Nicols, and the other, through the analyser only, by reflection in the 
prism. A blackened diaphragm was fixed between the prism and the second piece of 
paper, the aperture being of the same apparent size as the circular diaphragm of the 
polarising Nicol. 
On looking through the analysing Nicol a circular white field was seen, bisected 
vertically, the two halves being usually of unequal brightness. On rotating the 
analyser one half of the field (that due to the reflection of the second paper in the 
prism), remained unchanged, whilst the other varied in brightness, being quite dark in 
two positions (when the Nicols were “ crossed ”). There were, of course, four positions 
of the analyser in which the two halves of the field appeared equally illuminated. 
MDCCCLXXXIX. —A. 2 L 
