ME. W. CEOOKES ON EEPTJLSION EESULTING- FEOM EADIATION. 
337 
138. A candle was placed 36 inches from the bar, and the deflections of the index 
were taken, after various screens were interposed in the path of the light (109): — 
Candle, naked flame 
. 180 millims. 
Do. 
shining throu^ 
ih yellow glass 
. 161 
„ 
Do. 
99 99 
blue glass 
. 102 
99 
Do. 
99 99 
green glass 
. 101 
99 
Do. 
99 99 
red glass 
. 128 
99 
Do. 
99 99 
40i|r millims. greenish glass . 
. 125 
99 
Do. 
91 99 
81 
. 99 
99 
Do. 
99 99 
3tt millims. of water in cell 
. 48 
„ 
Do. 
99 99 
7| 
. 47 
99 
Do. 
99 99 
alum plate 5 millims. thick . 
. 27 
99 
139. A candle was now placed on the left side of the bar, 48 inches off. The luminous 
index moved 95 millimetres. Another candle was placed on the right side of the bar, 
48 inches off. It drove the index back to zero, and, after a few oscillations, kept it 
stationary a few millimetres the other side of it. I moved the right-hand candle 
49 inches off, and the index soon stood steadily at zero. By shading off either of the 
candles the index ray instantly moved 95 millimetres one side or the other. This gives 
a ready means of balancing two sources of light one against the other. Thus, retaining 
the standard candle 48 inches off on the left of the bar (deflection of index=95 millims.), 
the index was brought to zero by 
2 candles, on the right 67 inches off. 
1 candle, behind solution of sulphate of copper 7 % millims. thick 6 „ 
1 „ „ alum plate 5 millims. thick 14 ,, 
A small gas-flame (bat’s- wing) 113 „ 
140. These experiments show how conveniently and accurately this instrument can 
be used as a photometer. By balancing a standard candle on one side against any 
source of light on the other the value of the latter, in terms of a candle, is readily shown ; 
thus, in the last experiment, the standard candle 48 inches off was balanced by a small 
gas-flame 113 inches off. The lights were therefore in the proportion of 48 2 to 113 2 ; or 
as 2304 : 12,769, or as 1 : 5*5. The gas-burner was therefore equal to 5-^ candles. 
By interposing screens of water or plates of alum, and so cutting off all the dark heat, 
the actual luminosity is measured. In addition to this, by interposing coloured glasses 
or solutions, any desired colours can be measured, either against the total radiation from 
a candle, its luminous rays, or any desired colour. One coloured ray can be balanced 
against another coloured ray by having differently coloured screens on either side. If 
one screen is a cell of iodine in disulphide of carbon, dark heat can be balanced on one 
side against light and colour on the other side (109, 110). 
