AND TRANSMITTED BY CERTAIN KINDS OE GLASS. 
289 
Explanation of Plate 8. 
Fig. 1. Double mirror photometer. 
A. Photometer board. 
B. Scale. 
C. Lamp. 
DD. Mirrors. 
E. Block carrying photometer. 
F. Photometer. 
G. Wood stops. 
H. Screens. 
I. Windows in screens. 
Fiof. 1a. Photometer. 
A. Base. 
B. Wooden prisms. 
CC. Pieces of white paper. 
DDD. Windows in casing. 
Fig. 2. Polarising photometer. 
AA. Nicols. 
B. Bight-angled prism. 
C. Lamp. 
DD. Pieces of white paper. 
E. Screen. 
Fig. 3. Prism photometer. 
A. Photometer board. 
B. Scale. 
C. Lamp. 
DD. Pieces of white paper. 
E. Block carrying photometric surface. 
F. Screen. 
G. Prisms. 
Fig. 3a. Photometer. 
A. Base. 
BB. Ptight-angled prisms. 
C. Screw. 
Fig. 4. Curves representing the percentage amount of liglit reflected by crown glass 
at various angles of incidence. 
1. Observed values with the glass in its original condition. 
2. Calculated values from observed index of refractioji. 
3. Observed values with repolished glass. 
MDCCCLXXXIX. -a. 2 P 
