AXD TRA^^SMITTEU BY CERTAIN KINDS OF GLASS. 
2G5 
on the photometer, the whole beam of light which passed through the aperture in the 
chimney fell solely on the screen at the end of the photometer board, and no light 
diffused from the lamp reached the photometer. When, however, the lamp was so 
placed that the light was reflected from the glass, part of the light fell on the wall of 
the room, and, although this was at a distance of about 2 metres and painted black, 
some light must have been diffused from it towards the photometer; the metal clamp 
also in which the glass was held, although made as small as possible and blackened, 
certainly reflected some light. In order to obtain some idea of the amount of light 
which reached the photometer from these sources, the clamp was fixed to the gonio¬ 
meter without any glass; the left half of the field was not absolutely dark, but the 
amount of illumination was far too slight to be measurable, in fact it was almost 
imperceptible when the light from the comparison lamp illuminated the other half of 
the field ; hence, the error from this cause can only be small. 
In order to eliminate as far as possible any error due to a change in the relative 
amount of illumination produced by the lamps, four readings were first made of the 
position of the photometer when the two halves of the field were ecjually bright, the 
light of both lamps falling directly on it; four readings of the position when the light 
was reflected from the glass v'^ere then made ; and then four more with the direct 
light : the mean of the eight readings being taken as the true position of the photo¬ 
meter when the light reached it directly. 
Calling the two sources of light m and n, the distance between them x, the two 
positions of the photometer in which there is equality of illumination x-^ and x^, and 
K the coefficient of reflection for the particular plate of glass, then 
therefore 
m 
n 
and 
Km 
n 
{X — iC])? 
^’2“ 
(« 
%)“ 
m = 
c> 
and 
Km = 
nx^ 
(x — x-^)^ 
(x 
- ^ 2 ? ' 
K== 
x^ {x — x^f 
or 
r *2 
(x - 
Xi^ (x — x^)^ 
(x - 
■ *2) I 
The lamps not being placed at the ends of the divided scale and the two translucent 
screens of the photometer being necessarily at some distance apart, in order to obtain 
x-^ and x^ the distance between the lan^p and the zero of the scale had to be added, 
and the distance between the translucent screen and the index subtracted from the 
scale reading; thus, the distance from the axis of the lamp carried by the goniometer 
to the zero of the scale being 45’6 cm., in the first series of experiments, and the 
index of the photometer being at a distance of h'7 cm. from the paper, the scale 
readings, plus 39'9, gave the value of x^ and x.^. Similarly, 16’9 cm. added to the 
difference between the readings and 200 gave the value of {x — x^ and {x — x.^. 
In the first set of experiments the distances from the lamps were measured from 
MDCCCLXXXIX.—A. 2 M 
