S. P. Langley — Observation of Sudden Phenomena. 95 



Let ine first remark that while such error as that in question 

 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 instance 

 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 percep- 

 tion of this image is under ordinary circumstances * sensibly 

 instantaneous ; but — 



Second. Nerves convey the distinct impression of every part 

 of the 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 conscious 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 consciousness 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 professional observation is 

 only a particular instance of it. 



Now, I ask your attention to the practical instantaneity of 

 the formation of this visual picture, which is known to be 

 obtained where the duration of the phenomenon to be observed 

 is much less than the one thousand-millionth of a second, while 

 where we have every reason to believe that the actual forma- 

 tion of the image on the retina under known ordinary condi- 

 tions 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 phenomenon 

 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 



* The writer's observations (this Journal, Nov., 1888) show that appreciable 

 time is required for perception of the retinal impression, with certain excessively 

 faint lights; but these are not here in question. 



