PHYSIOLOGICAL ACTION OF LIGHT. 145 
1. The gradual death of the Retina and Nerve.—When the eyeball is placed 
on the cushions in the manner above described, and the key interposed between 
the electrodes and the galvanometer is opened, there is at once a large deflec- 
tion. But this deflection diminishes as the nervous apparatus dies. At first 
the nerve dies by fits and starts, afterwards more slowly, and with considerable 
uniformity. After a lapse of from 20 to 30 minutes, the rate of dying, as 
measured by diminution of electro-motive force, is almost arrested, and the 
vitality of the nervous apparatus remains nearly constant. These facts will be 
evident on considering the data of the following experiment :— 
Rapsir’s Eye.—Rate of Death as measured by diminution of Electro-motive Force. 
Zero, 164. Polarity, 156. Total of first deflection, 118. 
Readings Readings 
Time P.M. IER Difference. Time P.M. pa Difference. 
H. M. S. H. M. §. 
ies) 0 O74 0 IL 5) 250 4 
wae 0 282 j : 8 Ve © 246 4 
Ab <0 288 ; ; 6 I ey (0) 242, 4 
146 0 285 , 3 158 0 238 4 
i aye : : 282 3 I Hg) 234 4 
1.47 30 Zero 164, A) 6) 231 3 
148 O 278 4 4 I O 230 1 
459) 0) 274 j : 4 WP 0) 239 1 
0: 0 270 : : 4 a (0) 228 1 
It Sn 266 4 2° 4 0 228 O - 
erb2 0 j ‘ 262 4 eG) 228 0 
teo230 Zero 164. yO 228 0 
i 333 258 : A 4 De a0 228 0 
ioe, (0 254 j : 4 

Time occupied in above experiment, 24 minutes. 
Total fall in electro-motive force during that period 46°. 
The polarity remained constant throughout the experiment at 156°. \ 
2. Change of the Zero point of the Galvanometer from Magnetic Alterations. 
—tThis was greater at certain times than at others. On several occasions the 
variations were so sudden as to prevent any accurate experiments being made. 
3. Changes in the Polarity of the Electrodes.—This was one of the greatest 
difficulties in the inquiry, but we have employed the most approved methods 
of overcoming it—such as by the use of neutral sulphate of zinc, by moistening 
the clay with a ‘75 per cent. solution of pure chloride of sodium, and by con- 
necting the electrodes together by a thick copper wire for several hours before 
using them. With a sensitive galvanometer it is almost impossible to maintain 
either a constant zero or a constant amount of polarity. This fact would be a 
very serious one in an inquiry such as this, if the variations to be observed in 
the deflections occurred slowly and through minute distances, because, in these 
circumstances, it would be manifestly impossible to discriminate between 
changes due to variations in polarity and those due to the action of light. But 
VOL. XXVII. PART I. ae 
