348 DR. MEYER WILDERMAN ON THE CHEMICAL STATICS AND DYNAMICS OF 
C 2 -C 2 , giving the maximum deflection in light, are perfectly straight and parallel to 
a 1 -b 1 , a 2 -b 2 , which give zero deflection in the dark). 
The Arc .—We had to investigate some reactions in which more actinic light than 
acetylene was urgently required and the use of the arc became there unavoidable. Of 
several arcs tried, that of Korting and Mathiesen seemed to be the best, but even 
it gave 15 to 20 per cent, variation, as may be seen from its photographed curves of 
the same plate Nl. 
The arc which I found most useful, and with which I succeeded in getting better 
results, is a Dubose lent me by the Royal Institution. It has the disadvantage of 
requiring to be wound up every 5 or 10 minutes, in order that the same part of the 
spring may be used, but it served well our special purpose, where short exposures were 
required. It allows a very sensitive continuous adjustment for very small variations 
in the distance between the carbons. The variations on the amperemeter show then 
continuous rapid oscillations of the amperes, within the narrow limits of 1 millim. or 
more, exceptionally 2 millims., when the total value equals 31 millims. (6'2 amperes); 
while the volts almost remain stationary, their variations amount to about ^ a 
division out of the total of 65 to 67. Those continuous oscillations of the amperes 
about one mean value counteract one another, so that the integral intensity of light 
remains the same within very narrow limits, and this is just what we require, because 
a constant light is in reality only a light whose oscillations about the mean value are 
rapid and very small; the great regularity of the numerous curves given in this 
communication and the straight lines obtained for the maximum E.M.F. in light give 
solid evidence that I succeeded in getting a constant light with the Dubose. 
The conditions found necessary to obtain the above result are :—(1) The voltage at 
the terminals of the arc should be 42, at the terminals of the accumulators 65 to 67. 
Idle amperes in the circuit should be between 6’0 and 6'2. The Fleming resistances 
(3 ohms) inserted into the circuit should never be changed. Unless the voltage and 
amperes at the given resistance of circuit were exactly those given here, constant 
results could not be obtained with the arc. (2) The carbons were never allowed to 
burn down more than 1 to 2 centims., the incandescent surfaces must be kept as 
uniform as possible, and must be trimmed from time to time with a file. (3) There is 
a special distance between the carbons (between 3 and 4 millims.) which gives the 
best result with the arc. It is difficult to give the exact distance, which has to be 
determined experimentally, using a Rubens thermopile and following the indications 
of the amperemeter. It is got by changing the resistance (FI) till the desired effect 
is got, when the resistance (FI) is always kept constant. (4) There is a special 
distance of the armature from the electromagnet, and a special tension of the spring 
of the arc, at which alone it gives the best result. (5) Great care must be taken in 
the choice of the carbons. This is a difficult point. The hard, short “ Imperial 
Crown carbons ” of the London Electric Company proved the best and most steady of 
all, but the author had also good results with a somewhat softer kind. The “ Imperial 
