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



does arise enormously greater than that of all the rest put 

 together. 



We all know that this error varies with the individual and 

 the occasion. It is most constant in the experienced observer, 

 but even in his case it varies with the daily accidents of the 

 human organism, and even with him it is presumably constant 

 only for the particular observation to which the experience 

 applies. There is not even a presumption, I think, that the 

 personal equation belonging to an experienced transit observer 

 would apply to the same person's notation of the occultation 

 or emergence of a star, and still less, if possible, to any phenom- 

 enon outside his ordinary professional experience ; for we 

 must, of course, recognize that we carry this fallibility with us 

 in every act of life, and that it is just as present when we 

 attempt to determine the instant at which a race horse passes 

 the winning post, as when we seek to note the particular hun- 

 dredth of a second at which a star passes the wire. 



The very words " personal equation " imply that the errors 

 due to this fallibility can be ascertained and allowed for, and 

 may lead us to think (if we think carelessly) that there is a 

 personal equation always ascertainable ; whereas, as we in fact 

 know, it is only possible to apply the correction where long 

 habit has settled the amount of error to be expected with 

 regard to some one special phenomenon. 



The number of devices for obtaining and correcting the 

 personal equation, even in the special case of meridian observa- 

 tion, is, as those who have studied the subject know, surpris- 

 ingly great. I think I have myself examined more than fifty 

 such, and with hardly an exception they all exhibit variations 

 on one idea — the idea, that is, that the error must have been 

 committed first ; the committing of the error being assumed 

 to be an inevitable necessity for which subsequent correction is 

 to be made. 



I have thought, then, that it might be interesting if I were 

 to ask you to consider with me what may seem at first the 

 somewhat paradoxical suggestion, that means may be found by 

 which any individual, skilled or ignorant, may make, not only 

 meridian observations, but an observation of any sudden visible 

 event, of whatsoever nature, so accurately that we need apply 

 no correction, because the precision may be, if not absolute, at 

 least such that no correction will in ordinary practice be 

 needed. I may deceive myself in thinking that what I have 

 to suggest involves a novel idea, bat I am led to suppose so 

 from the fact that I have met no application of it in a some- 

 what extended reading on this point. 



