O. N. Rood on a Photometer. 63. 
ored light show fully the degree of accuracy that can be attained 
in this direction with the photometer, the experiments, as far as 
they extend, being made with much care, though in such a way 
that each reading is entirely independent of the preceding, and 
has exercised no influence on that which follows. 
To measure the amount of light reflected from a polished sur- 
face, I have used a plan not unlike that employed by Potter ;? 
the slide, in the place of the lamp, carries the polished plate in 
an upright position and supported in such a way 
that it can be rotated on its axis and adjusted 
properly. Its axis corresponds with the centre of 
a divided quadrant by which the incidence of the 
3. 
e distance of the lamp’s flame 
from the mirror’s axis is always to be added to the reading ob- 
tained, By removing the shade I, and placing it so as to pro- 
“tect the mirror from light, the direct light of the lamp falls on 
the screen. As the lamp stands then a little obliquely, the 
slight error thus introduced must be allowed for, or, what is 
better, the lamp and mirror may be placed on either side of, 
and at equal distances from, the axis of the photometer. 
Prof. Silliman, Jr., pointed out to me some weeks ago, that 
small errors might be introduced by reflection from the walls of 
the room. This is guarded against by the use of several black- 
ened shades properly disposed about the instrument, or by ex- 
perimenting in a room with blackened walls, as is usual in de- 
termining the photometric power of illuminating gas. 
Determination of the amount of light transmitted by a plate 
of colorless, polished crown glass 73th of an inch thick: | 
Flame free. With plate. 
20-7 inches. 19-95 inches. 
20°85 =“ es 
05. etn 
Amount of light transmitted, 91-09 per cent. 
Determination of the amount of light transmitted by a plate 
of colorless glass, finely ground on one side, ;';th inch thick: — 
Flame free. With ground glass. ; 
20°2 inches. 13°1 inches. 
20s0. i,e. 
be vs 
la U5.. * 
Amount of light transmitted, 41°13 per cent. 
* Physical Optics, p. 112. 
