510 MR JAMES DEWAR AND DR M‘KENDRICK ON THE 
that purpose we placed over the apparatus a rectangular box, blackened without 
and within, having a draw-shutter as seen in the diagram, Plate X. After 
placing the eye on the cushions the key was opened, and the amount of deflec- 
tion observed. If not sufficiently great for experimental purposes, the eye was 
readjusted so as to secure accurate contact of the nerve with its corresponding 
cushion; if sufficiently great, the box was placed over it, and the eye was 
allowed for several minutes to remain in the dark. The impact of light took 
place on withdrawing the shutter. The change in the electro-motive force was 
noted at the instant of the impact. The subsequent change in the electro- 
motive force produced by continuance of light was not noted till the spot on 
the galvanometer scale became tolerably steady. When this. occurred the 
shutter was closed, and we noted the effect of the removal of light. In these 
experiments, therefore, time was not especially taken note of. 
SERIES I.—EveE or Froc.* 
The light from a magnesium lamp was passed through a glass vessel, con- 
taining a thickness of 14 inch of water, with the following result :— 




oa Before impact. On impact. During light. — light. val ar. el seen: se 
75 347 354 350 358,346 Rise of 7. Rise of 8. 
76 346 354 350 357,345 Rise of 8. Rise of 7. 
if Went off 
ne i | the scale. \ ue cP en n¥e 





Two hours afterwards the following experiments were made with the same 
eye :-— ; 
Obs. 78. Galvanometer at 215. The eye nearly in the dark. On removing 
the paper from the galvanometer lamp, which was used as a shade, the electro- 
motive force at once rose to 238. During the continuance of light the electro- 
motive force fell to 225. On lighting the gas in the room it rose to 288, and 
then slowly fell during the continuance of light. On shutting off all the light 
it rose immediately to 230, and then fell to 205. 
Obs. 79. Stationary at 205. Gas lit, rose to 224. In two minutes fell to 
217, and in one minute more to 213. On turning off the gas, fell from 212 to 
215, and then quickly to 197. 
Two hours still later the following experiments were madewith the same eye:— | 
Obs. 80. Stationary at 98. Gas lit in room, rose to 105, that is, a rise of 
seven degrees. During continuance of light, fell to 103. On removal of light, 
rose one degree. 
* In the following series of experiments the zero, polarity, and total first deflection, are not given. 
These points, however, were always attended to, and observations were always made on a deflection 
amounting to, at least, 300 or 400 degrees. 

