376 PEOFESSOE BUNSEI7 AXD DE. H. E. EOSCOE’S PHOTO-CKOnCAL EESEAECHES. 
In order more fully to test our instrument, we made, some time later, other determi- 
nations which are of interest in a different point of -view. It is weU known that sup- 
porters are still found for the idea that photo-chemical phenomena are not to be ascribed 
to the action of the common luminous rays of a particular reffactire index, but to a 
peculiar agent differing enthely from light, and possibly subject to other laws. In order 
to free our investigations from any objections which might be made from this point 
of view, we have endeavoured to avoid as much as possible applying the general laws of 
light to the chemically active rays without previous experimental proof. On this account 
we thought it advisable, whilst testing the reliability of the instrument, to prove that the 
action of the chemical rays diminishes inversely as the square of the distance from the 
point from which the light emanates. These experiments were conducted by deter- 
mining the amount of chemically active rays which fall upon the insolation-vessel from 
the standard flame placed at various known distances in the blackened box. The first 
v'ertical column of the following Table contains the distance between the standard flame 
and the middle point of the insolation-vessel ; the second column the reciprocals of the 
squares of these distances ; the thuxl the chemical action, AV, measured by the instru- 
ment ; and the fourth the amount of action, Wi, calculated according to the distance by 
means of the formula 
Series of Experiments XI. 
Experiment 1. 
Experiment 2. 
r. 
1 
w. 
Wi. 
DilFer- 
r. 
1 
W. 
VVi. 
Differ- 
ence. 
ence. 
1 
0*3900 
6*575 
3*98 
4*11 
-0*13 
1 
0*3900 
6*575 
4*43 
4*32 
4-0*11 
2 
0*3310 
9-127 
5*56 
5*70 
— 0*14 
2 
0*3315 
9*108 
6*07 
6*00 
+ 0*07 
3 
0*2495 
16*064 
10*33 
10*05 
+ 0*28 
3 
0*2150 
21*634 
14*07 
14*24 
-0*17 
In these last experiments the standard flame was adjusted at exactly the same height 
as was the case in Series X., when the distance from the flame to the insolation-vessel 
vv^as 0“‘216=r,. These series of determinations may therefore be compared with one 
another by means of the formula Wo =-^, from which the followmg values are obtained 
for the corresponding action at the distance 0“-2I6, as employed in Series X. 
(1) 14-44, 
(2) 14-28, 
(3) 14-11. 
The last number agrees very nearly with the mean action 13-91 of Series X., the difler- 
ence being scarcely larger than that found among the single determinations of Series X. 
