44 
LANGLEY. 
of the formation of this visual picture, which is known to be 
obtained where the duration of the phenomenon to be ob¬ 
served is much less than the one thousand-millionth of a 
second, and where we have every reason to believe that the 
actual formation of the image on the retina under known 
ordinary conditions requires a time of like order. 
We may say, then, that the casting of a picture on the 
retina is instantaneous. It is its fading out that requires 
time, and it is while this fading out takes place, and even 
long after it, that the work of perception, decision, and action 
is going on behind the retinal curtain in the chambers of the 
brain. Notice, then, that while to determine when a phe¬ 
nomenon occurs may require, under some circumstances, 
several seconds, and under all ordinary circumstances a 
notable fraction of a second, to determine where it occurs 
requires (sensibly) no time at all, for one single impression 
remains on the retina long enough to obtain full recognition 
and to be reproduced by processes of memory. 
I can make my meaning clearer, perhaps, by using the 
same specific instance as before. Let us suppose that an ac¬ 
cident to a passenger on the passing train is the phenome¬ 
non, the time of whose occurrence is to be noted, and that 
this accident is seen from a room in which there are two 
windows looking on the track. We must have seen the ac¬ 
cident, if it be instantaneous, either through the first win¬ 
dow or the second. If we had been led to anticipate that 
we should be called upon to say through wdiich window we 
saw it, I think w r e may all admit that there would be no 
discrepancy on this point between different observers, for in 
this case we are considering only the element of position, 
and the element of time does not directly enter at all, so that 
observers in the same position who had been bidden to note 
through which window 7 they saw it would all agree on this 
point. 
Now a connection can here obviously be established be¬ 
tween the place and the time, from which to infer the latter, 
if we are granted the knowledge of two facts: the time at 
