144 MR JAMES DEWAR AND DR M‘KENDRICK ON THE 
electricity circulating through the wires, magnetism in the coil, heat in the helix, 
and motion in the needles.” A consideration of this remarkable experiment led 
to the hope that we might be able, by a somewhat similar method, to observe 
the action of light on the electro-motive force of the retina and optic nerve. 
The apparatus adopted for obtaining the normal electro-motive force was 
that of Du Bors-Reymonp, usually employed in determining the electro-motive 
force of muscle or nerve. This consists of Du Bois-ReymMonp’s well-known non- 
polarisable electrodes, which are shallow troughs of zinc, carefully amalgamated, 
containing a solution of neutral sulphate of zinc, and having inserted into them 
cushions of Swedish filter-paper, on which to rest the preparation, which, in our 
experiments, was the eyeball or optic nerve. To protect the latter from the 
irritant action of the sulphate of zinc, a film or pad of sculptor’s clay, moistened 
with ‘75 per cent. solution of common salt, and moulded, if necessary, by the 
fingers, to a point, is placed on each cushion. These electrodes are connected 
with a galvanometer, a key being interposed by which the current from the 
electrodes may be shut off at pleasure. The eyeball, carefully freed from 
adherent particles of muscle, is now placed on a glass support between the two 
clay points, and these are now adjusted to it so that one touches the cornea, and 
the other the transverse section of the optic nerve. On now opening the key, 
the galvanometer at once indicates a strong current passing from the longitu- 
dinal to the transverse section of the nerve. (The arrangement of the apparatus 
will be understood by referring to the plate. Plate, No. X.) 
The first experiment we performed was on the eye of a rabbit. The gal- 
vanometer employed was a large multiplying instrument made by SAUERWALD 
of Berlin. The animal was killed by chloroform, and the eyeball removed as 
quickly as possible from the orbit. It was then placed between the troughs in 
the manner already described. We obtained a deflection of from 10 to 15 
degrees. On allowing the beam of light from a magnesium lamp to impinge on 
the cornea there was no effect observable. This experiment was repeated many 
times with the same negative results. It then became evident that a much 
more sensitive instrument was required, because the amount of deflection was 
so small, and the variation, if any, of this deflection produced by the action of 
light would probably be so minute as to escape notice. 
We then used a sensitive THomson’s galvanometer, which was kindly placed 
at our service by Professor Tarr. With this instrument we at once obtained 
a deflection of several hundred degrees on a millimetre scale placed at a dis- 
tance of 26 inches, so that we had now an opportunity of observing whether 
light produced any variation in the electro-motive force. 
After placing the galvanometer in position, secondary difficulties now pre- 
sented themselves. It will be necessary to allude to these difficulties, and the 
methods by which we avoided or overcame them. 

