198 THE SPLASH OF A DROP AND ALLIED PHENOMENA. 



motion of tlie drop must be largely conjectural, and it occurred to me 

 about eiglitecn years ago to endeavor by means of the illumination of 

 a suitably timed electric spark to watch a drop through its various 

 changes on impact. 



The reason that with ordinary continuous light nothing can be satis- 

 factorily seen of the splash is not that the phenomenon is of such short 

 duration, but because the changes are so rapid that before the image of 

 one stage has faded from the eye the image of a later and quite differ- 

 ent stage is superposed upon it. Thus the resulting impression is a con- 

 fused assemblage of all the stages, as in the photograph of a person 

 who has not sat still while the camera was looking at him. The prob- 

 lem to be solved experimentally was therefore this: To let a drop of 

 definite size fall from a definite height in comparative darkness onto a 

 surface, and to illuminate it by a flash of exceedingly short duration at 

 any desired stage, so as to exclude all the stages previous and sub- 

 sequent to the one thus picked out. The fiash nuist be bright enough 

 for the image of what is seen to remain long enough on the eye for 

 the observer to be able to attend to it, even to shift his attention from 

 one part to another, and thus to make a dra\Ying of what is seen. If 

 necessary the experiment must be cai)al)le of repetition, with an 

 exactly similar drop Mling from exactly the same height, and illumi- 

 nated at exactly the same stage. Then, when this stage has been 

 sulhciently studied, we must be able to arrange with another similar 

 drop to iliuminate it at a rather later stage, say one one-thousandth 

 second later, and in this way to follow step by step the course of the 

 whole phenomenon. 



The apparatus by which- this has been accomplished is on the table 

 before you. Time will not suffice to explain how it grew out of earlier 

 arrangements very different in appearance, but its action is very simple 

 and easy to follow by reference to the diagram (fig. 1). 



A A' is a light wooden rod rather longer and thicker than an ordi- 

 nary lead pencil, and pivoted on a horizontal axle, O. Tlie rod bears 

 at the end A a small deep watch glass, or segment of a watch glass, 

 whose surf\ice has been smoked, so that a drop even of water will lie 

 on it without adhesion. The end A' carries a small strip of tinned iron, 

 which can be pressed against and held down by an electro-magnet, C C. 

 When the current of the electro-nuignet is cut off the iron is released, 

 and the end A' of the rod is tossed up by the action of apiece of india 

 rubber stretched catapult-wise across two pegs at E, and by this means 

 the drop resting on the watch glass is left in mid-air free to fall from 



rest. 



B B' is a precisely similar rod worked in just the same way, but car- 

 rying at B a small horizontal metal ring, on which an ivory timing 

 sphere of the size of a child's marble can be supported. On cutting off 

 the current of the electro-magnet the ends A' and B' of the two levers 

 are simultaneously tossed up by the catapults, and thus drop and sphere 



