AND TRANSMITTED BY CERTAIN KINDS OF GLASS. 
2G3 
Section II.—Amount of Light Refected at a nearly Perpendicular Incidence. 
The small percentage of light reflected by glass at a perpendicular incidence 
rendered its dh’ect determination difficult ; a fairly satisfactory method of measure¬ 
ment was, however, at length devised. The principle of the method was the obvious 
one of comparing the amount of light which reached the photometer when it came 
direct from the lamp with that which reached it after reflection from the glass. 
It was found necessary to use two lamps, as no single-lamp apparatus, such as had 
been used for the transmission experiments, could be employed. 
One of the lamps, a small Argand gas burner, was placed at the end of the photo¬ 
meter board (Plate 8, flg. 1), and a similar gas burner attached to the arm of a 
goniometer fixed at a short distance from the other end of the photometer board, the 
vertical axis of the goniometer being in the prolongation of the median line of the 
board. The glass of which the reflective power was to be determined could be placed 
with its surface vertical, and in the axis of the goniometer. 
The experiments were made by first comparing the illumination produced by the 
two lamps when the light from both fell directly on the “ photometer,” the arm of the 
goniometer carrying the lamp being in the prolongation of the line joining the fixed 
lamp and the axis of the goniometer; the glass plate was then attached to the gonio¬ 
meter with its surface vertical and nearly normal to the line joining the fixed lamp 
and the axis of the goniometer, the arm of the goniometer rotated until the light 
again fell on the photometer after reflection from the glass plate, and the position of 
equality determined. 
It was not necessary that the illuminating power of the two lamps should be equal: 
it was necessary that the ratio between their illuminating powers should remain as 
nearly constant as possible. The gas for the two burners came from the same supply 
pipe, and was passed through a bell-and-valve regulator; in spite of the regulator, it 
was found impossible to get satisfactory measurements, except in fauly still weather; 
the sliglit fllckerings in the flames which occurred whenever there was much wind 
prevented the position of equality of illumination being determined with any degree 
of accuracy. 
The measurements were always made in a dark room, a “ detector ” gas burner 
being used to read the position of the index on the scale ; and, in order to reduce the 
stray light as much as possible, cylindrical metal chimneys, 5'5 cm. in diameter and 
18 cm. in height, blackened externally and internally, were placed round the glass 
chimneys of the burners, a rectangular aperture being cut in each at the level of the 
flame. Two black wood screens with square openings, similar to those used in the 
transmission experiments (p. 248), were fixed at either end of the photometer board; 
there were black cloth screens behind and above the board, and the walls and ceilino- 
of the room were painted a dead black ; the metal clamp by which the glass plate was 
