2/2 Bulletin of Laboratories of Denison University [Voi. xi 
by Lippmann in the paper cited. It consisted of a cylindrical 
bundle of about 200 capillary tubes, each 20 mm. long and i 
mm. in diameter, bound around a glass rod with fine platinum 
wire. This was allowed to rest upon a surface of mercury con- 
tained in a small glass vessel. The whole was immersed in 
dilute sulphuric acid in a larger vessel, with enough mercury to 
cover the bottom. The glass rod was attached to the short arm 
of a long lever supported on a knife edge fulcrum. The long 
arm was made very light, 120 cm. in length. It carries a glass 
scale divided to . i mm. at its end. When the two mercury sur- 
faces are brought to a different potential the bundle of capillaries 
rises or falls and with it the lever moves and the divided scale 
is observed with a low-power microscope. It showed a distinct 
movement with a potential difference of .0001 volt. With .1 
volt, the movement of the big lever was quite impressive. By re- 
placing the lever with a small mirror and reflecting sunlight 
across the room, the capillary movement might be shown to a 
large audience. 
Another instrument has a separate pressure apparatus with 
a flexible tube connected to a mercury reservoir capable of be- 
ing raised and lowered. This is used in connection with a 
capillary tip of Ostwald’s third form, very much as suggested 
by Burch in the London Electrician articles. 
At the suggestion of Prof. Stratton of the University of 
Chicago, I attempted to replace the lever arrangement of the 
capillary-bundle form by a pair of plane parallel plates, one 
being fixed and the other attached to the bundle of tubes, the 
movement of the latter to be estimated by the movement of 
interference bands across the plate when the arrangement was 
illuminated by sodium light. I got some results with this, but 
my mechanical arrangements were so imperfect that the requi- 
site degree of parallelism of the plates could not be long 
maintained. 
Several forms were made particularly for projection with 
the lantern, but Ostwald’s second form was found to be as good 
for projection as any of the special ones. It can be very satis- 
factorily shown on the screen. 
