S. P. Langley — Observation of Sadden Phenomena. 97 



ing plan for carrying it out. Numerous different devices have 

 been under my consideration. I will take one which is prim- 

 arily designed for the observation of any celestial 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 deter- 

 mine. 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, with an accuracy which ensures us that we 

 have not made an error so great as one-twentieth of a second. 



You see I hold in my hand a peculiar eye-piece, which has 

 been made to observe this or any other terrestrial or celestial 

 phenomenon of sudden occurrence. It can also be used for me- 

 ridian observation, but its special field seems to lie in noting an 

 event where no ordinary correction for personal equation is ap- 

 plicable. This event may be anything celestial or terrestrial, 

 from the entrance of Venus on the disc of the sun, to the ex- 

 plosion of a mine ; but, for the purpose of illustration merely, 

 let us take it to be the sudden appearance of the star. 



On looking into the telescope we see, in the first place, two 

 prominent wires crossing each other at right angles, dividing 

 the field of view into four quadrants. Now, by a simple 

 mechanism, which I shall shortly explain, any object that our 

 telescope is directed on — any fixed star for example — seems to 

 be revolving in the field, passing successively through the first, 

 second, third and fourth quadrants. If the star is hidden the 

 mechanism is working just the same, and when the star appears 

 it must evidently first be seen in some particular one of these 

 four quadrants, and experience shows that we shall have no 

 difficulty in telling in which one. The mechanism itself has 

 recorded for us by an electric contact the limiting instant 

 between which it is possible to see the beginning and the end 

 of the cycle during which revolution may be supposed to be 

 made. It is not necessary that this cycle should last just a 

 second ; but supposing it (still for illustration only) to be a 

 second, if it was seen in the first quadrant, it was seen in the 

 first quarter of the second ; if it was seen in the second quad- 

 rant ; some time in the second quarter of the second ; in the 

 third, in the third quarter ; in the fourth, in the final quarter. 

 All that we have to do in this case is to know in which 

 second it occurred ; for the quarter of a second we may say is 

 noted for us by the purely automatic action of the optic lens 

 and retina, since the first image on the retina must be that of 

 the star as seen in some particular one of the four quadrants. 



Going a little farther, we will now suppose each of the four 

 quadrants, which in turn correspond to quarter seconds, to be 



