156 MR JAMES DEWAR AND DR M‘KENDRICK ON THE 
Paper cap on No, 2, the deflection of which is to the left. No. 1 was in action. 
Eyes readjusted on the pads. 
Obs. 169 Before light, 210 Impact, 212 Continued, 205+» Removal of light, 205 
lo iy 190 me 198 as eal 0 " 191 
aA . 178 alia 535) nS 182 
hptin® a 175 ey SO roledigs - 178 
eel ‘ 168 = sn gilda ote 68 a 173 
VI.— EXPERIMENTS WITH VARYING LuMINOoUS INTENSITY. 
For this purpose the eye was carefully guarded from extraneous light, as 
previously detailed, and the source of light was allowed to act on the eye at 
different distances. The following experiments were made on an eye of a frog 
that had been removed from the body two hours previously, and had been 
during that time lying on the clay pads of the electrodes :— 
a. Obs. 174 to 178.—The gas flame was placed at a distance of one foot. Rose 
eight degrees ; fell eight degrees below original point during action of light ; 
and rose twelve degrees on removal of light. This was repeated five times with 
the same results. 
b. Obs. 179 to 184.—The gas flame was now placed at a distance of four feet. 
The result was that on impact, rose five degrees, during continuance of the light 
fell five degrees below the original point, and there remained stationary. On 
removal of light rose to the original point. This happened in six experiments. 
ce. Obs. 185 to 188.—The gas flame was then placed at a distance of ezght feet. 
Rose one degree ; fell during continued light five degrees below original point ; 
and rose five degrees on light being removed. This was repeated four times 
with same result. | 
From these observations it results that when the luminous intensity varies 
in the proportion of 64 to 1, the electrical variation is as 8 to 1, showing that it 
by no means varies directly as that of the luminous source, but much more 
slowly. At various times we have made observations on the diminution of 
electrical effect from a difference of luminous intensity in the proportion of 100 
to 1, and as a mean of these observations we have found the electrical variation 
in different eyes to lie between proportions of 3 tol and6to1. That is to 
say, if a luminous source, at a distance of ten feet, gives an initial impulse 
corresponding to one degree of our galvanometer scale, then at a distance of one 
foot the electrical effect would be from three to six degrees. In this com- 
parison, we take the alteration produced in the galvanometer on the impact of 
light as the measure of the electrical variation, because the eye in this case is 
always compared under similar conditions, having been previously for a certain 
time in the dark, so as to allow it to recover from the effect of a previous — 
experiment, whereas if we regarded the electrical effects produced on the with- 
drawal of light as comparable, we would have to deal with different degrees of 

