REVERSIBLE AND IRREVERSIBLE SYSTEMS UNDER INFLUENCE OF LIGHT. 347 
from it. A description of the bath has also been given. A few alterations have been 
made in it. The quartz vessel was placed in an ebonite holder and kept in the 
position by the iron stand placed on the bath. The leads from the plates in the 
quartz vessel were passing in the air behind thick asbestos screens fixed to the bath, 
and were connected with two thick flexible copper leads, one being red, the other blue, 
to know the direction of the current. The copper leads were connected with the 
Nalder galvanometer Nl. 
On the left of the diagram are the arrangements for photographing the deflections 
of the Nalder Nl, caused by exposing one of the plates in the quartz vessel to light, 
as well as for calibrating these deflections in standard units, and for measuring the 
resistance of the liquid between the two plates in the quartz vessel. 
The incandescent Swan lamp (SwN2) was connected with another incandescent 
lamp enclosed in a box and serving as an additional resistance (SwR). The spot of 
light, made more brilliant by the use of an increased voltage, was thrown upon the 
galvanometer mirror (Nalder Nl) and reflected as a vertical line upon the horizontal 
slit of the chronograph (Ch) containing a revolving drum on which extra-sensitive 
bromide paper was fixed. The deflection due to the E.M.F. produced under the 
action of light upon the plates was photographed. 
The results obtained were calibrated in standard units A Clark N2 was 
connected with the key (E ;/ ), reversing key (rk^), the manganin resistances 
(83,600 ohms), and the galvanometer Nalder Nl (resistance 834'4 ohms) with a shunt 
(5 ohms). The deflection of the galvanometer was also measured on a wooden 
millimetre scale placed on the drum during the time of this measurement (the 
measurement of the E.M.F. in the dark). 
After the effect of light upon the plates was photographed and standardised, the 
resistance of the liquid between the plates in the quartz vessel, including the thick 
copper leads up to the terminals of the galvanometer, was measured by the method 
of Kohlrausch. Arrangements were made (E ///; E //;/ ) not to disturb during this 
measurement the connection at the terminals of the galvanometer. The blue lead 
could be disconnected from a small part of it connected with the galvanometer, and 
the red lead was connected at the terminal of the galvanometer with another lead of 
a negligible resistance which could be connected to one end of the bridge. The 
arrangements for these measurements can be clearly seen in the diagram. 
The Constant Source of Light (Acetylene and Arc). The Measurement and 
Adjustment of its Intensity. 
Acetylene. —-In his paper “ On Chemical Statics and Dynamics under the Action of 
Light,” ‘Phil. Trans.,’ A, 1902, vol. 199, pp. 351-363, the author gave a detailed 
account of how to arrange a constant acetylene light of 250 candles and more, and 
how to measure the intensity of light (by means of thermopile, galvanometer, &c., &c.) 
with an accuracy of 0T per cent, (see photographic curves, plate Nl ; lines Q-Cj, 
9 v 9 
lj JL Lj 
