Ill 
of Edinburgh, Session 1872-73. 
representation of the variations of the electro-motive force occasioned 
by the impact and cessation of light. It is clear that to register 
minute galvanometrical alterations, the only plan that could be 
employed would be to photograph on a sensitive surface, covering 
a cylinder rapidly revolving on a horizontal axis, the alteration 
of position of the spot of light reflected from the mirror, just as 
continuous magnetic observations are registered. As the apparatus 
required to execute these observations is very complicated, and 
would require much preliminary practice, we have in the meantime 
adopted a simpler method of registration. This plan is to note the 
position of the galvanometer at equal intervals of time, before, 
during, and after, the impact of light on the eye. In these obser- 
vations we have used a seconds’ pendulum giving a loud beat. 
One observer reads aloud the galvanometer, the other marks every 
interval of two and a half seconds, registers the numbers obtained, 
and regulates the supply of light. A little practice in the method 
above described has enabled us to obtain very satisfactory results, 
agreeing very closely in different observations, and showing in a 
decided way the salient points of the variation curve. 
These curves show, that on the impact of light there is a sudden 
increase of the electro-motive force; during the continuance of 
light it falls to a minimum value; and on the withdrawal of light 
there is what we term an inductive effect , that is to say, a sudden 
increase of the electro-motive force which enables the nerve to 
acquire its normal energy. The falling-off of the electro-motive 
force by the continued action of light is the physical representa- 
tive of what, in physiological language, is called fatigue ; the in- 
ductive effect exhibiting the return of the structure to its normal 
state. 
Occasionally the impact of light is not followed by a rise in the 
electro-motive force, but by a diminution. This is probably to be 
explained by the fact, that the death of the retina and nerve is 
indicated by a gradual falling of the electro-motive force, and that 
this change frequently goes on so rapidly that the impact of light 
is unable to produce any rise. In these circumstances, the spot of 
light, which before the impact of light was slowly moving down- 
wards, is on the impact steadied for a moment, and then pursues 
its downward course more rapidly. 
VOL. VIII. 
