OBSERVATION OF SUDDEN PHENOMENA. 
45 
which the carriage could have first come into view from the 
first window, and the time at which it must have passed out 
of view behind the second; for if we suppose the speed of the 
train to have been uniform, we have the means of deciding 
the fraction of the time when we know the fraction of space. 
Here, then, as in the case of a common clock or chronograph, 
or any device where time and space are proportional, we 
can infer the former from the latter; only let it be observed 
that we here need no recording apparatus. What we use is 
the memory of where the event occurred; in other words, 
we recall the impression on the retinal screen and have no 
need to bring into use what we may call the time-perception 
apparatus of the brain which lies behind it; nor do we in 
fact need that the object of our observation shall be really in- 
motion , but only that it shall be made to appear to be so. 
This last point is all important, and what I ask your at¬ 
tention to is an experiment heretofore, I think, untried, and 
which is perhaps a novel application of the fundamental 
horological idea that time and space must be made propor¬ 
tional, for it seems to me it must be theoretically possible, 
not only in the case of the clock or the chronograph, but 
always, to so connect the former with the latter that the es¬ 
sential task of the observer is to say where any visible event 
apparently occurred, and then let some mechanism outside 
of himself say when. 
That at least is the idea, and if it has, as I hope, been 
clearly apprehended by you, I will now ask your attention 
to a working plan for carrying it out. Numerous different 
devices have been under my consideration. I will take one 
which is primarily designed for the observation of any ce¬ 
lestial phenomenon, though it could very well be adapted to 
terrestrial ones; and in order to fix our ideas I will suppose 
that we have an event which we know the approximate 
time of, but which may burst upon us at some fraction of a 
second which we want to determine. I will assume (merely 
to fix our thoughts) that we wish to note the time at which 
a star emerges from behind the dark body of the moon 
