AND TRANSMITTED BY CERTAIN KINDS OF GLASS. 
261 
flirection of that reflected back to the prism was also nearly normal to the paper, so 
that the distance of the paper from the lamp could be altered without altering to any 
considerable extent the angle at which the light fell upon it, or the angle under which 
it was seen. 
The distance of the moveable surface from the lamp, when its reflection in 
the prism and that of the fixed surface appeared equally bright, was determined 
(1) without any glass being interposed, (2) with a plate of glass between the fixed 
surface and the prisms, and (3) between the moveable surface and the prisms, six 
readings being made of each of these positions, and then the prisms reversed so that 
the surface which had been seen by reflection in the one was seen by reflection in the 
other, and six more readings made. 
Calling the apparent brightness of the fixed surface C, the distance of the moveable 
surface from the lamp wfithout the glass x, with the glass between the fixed surface 
and the prisms x, and with the glass between the moveable surface and the prisms x", 
and the coefficient of transparency of the particular piece of glass h, then the two 
surfaces will appear equally bright when 
whence 
C = -„; 
Qk = 
/\2 ’ 
and C = 
{x'J 
or more simply 
The percentage amount of light transmitted by the 6‘5 mm. and the 24'3 mm. plates 
of crown glass was determined in this way. Table V. gives the results. 
Table V. 
6'5 mm. plate. 
Keadiugs. 
Percentage of 
light transmitted. 
x". 
x'. 
cm. 
cm. 
78-0 
84-4 
92 42 
77-8 
84-9 
91 64 
Mean 92'03 
i 
