OBSERVATION OF SUDDEN PHENOMENA. 43 
if not absolute, at least such that no correction will in ordi¬ 
nary practice be needed. I may deceive myself in thinking 
that what I have to suggest involves a novel idea, but I am 
led to suppose so from the fact that I have met no applica¬ 
tion of it in a somewhat extended reading on this point. 
Let me first remark that while such error as that in ques¬ 
tion doubtless belongs to all the senses in some degree, we 
are at this moment considering it in connection with the 
sense of sight only. 
When we see anything in motion (let us suppose for in¬ 
stance a passing train on the railroad) we have the well- 
known facts that— 
First. An instantaneous photograph is made by the optic 
lens upon the retina, there being a picture formed there, 
which is perfectly distinct, but which fades out upon the 
retinal plate in from one-tenth to one-quarter of a second, 
while the perception of this image is under ordinary cir¬ 
cumstances * sensibly instantaneous; (but)— 
Second. Nerves convey the distinct impression of every 
part of this picture to the brain, and it is here, if we have 
to act on this impression, that a certain time is lost, not 
only in the carrying of the message along one set of nerves 
and the bringing back the answer on the other, but in the 
decision that is being made by that unknown and inner 
self, which appears to us to exert here a more or less con¬ 
scious act of will. 
In the case of a sudden and startling event, the time 
elapsed may be almost indefinitely great; and in some cases, 
probably several entire seconds may pass without the con¬ 
sciousness of the observer. A very imperfectly appreciated 
interval must occur in all cases, for what we have just said 
applies to every event of our daily lives, and the profes¬ 
sional observation is only a particular instance of it. 
Now, I ask your attention to the practical instantaneity 
*The writer’s observations (Am. Jour. Sc., Nov., 1888) show that appre¬ 
ciable time is required for perception of the retinal impression, with certain 
excessively faint lights; but these are not here in question. 
