96 S. P. Langley — Observation of Sudden Phenomena. 



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 accident to 

 a passenger on the passing train is the phenomenon, 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 accident, if it be instantane- 

 ous, either through the first window or the second. If we had 

 been led to anticipate that we should be called upon to say 

 through which window we saw it, I think we may all admit 

 that there would be no discrepancy on this point between dif- 

 ferent 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 they saw it would all 

 agree on this point. 



Now a connection can here obviously be established between 

 the place and the time, from which to infer the latter, if we 

 are granted the knowledge of two facts : the time at 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 latter 

 from the former; 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 atten- 

 tion 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 proportional, 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 essential task of the time 

 observer is only to say where any visible event apparently oc- 

 curred, and then let some mechanism outside of himself say 

 when. 



That at least is the idea, and if it has, as 1 hope, been clearly 

 apprehended by you, I will now ask your attention to a work- 



