290 Professor A. M. Worthington [May 18, 



nation 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 

 satisfactorily seen of the splash, is not that the phenomenon is of 

 such short duratioD, 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 different stage is superposed upon it. Thus the resulting 

 impression is a confused assemblage of all the stages, as in the photo- 

 graph of a person who has not sat still while the camera was looking 

 at him. The problem to be solved experimentally was therefore 

 this : to let a drop of definite size fall from a definite height in 

 comparative darkness on to 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 subsequent to the one thus picked out. 

 The flash must 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 drawing of what is seen. If necessary the experiment 

 must be capable of repetition, with an exactly similar drop falling 

 from exactly the same height, and illuminated at exactly the same 

 stage. Then, when this stage has been sufficiently studied, we must 

 be able to arrange with another similar drop to illuminate it at a 

 rather later stage, say j^Vtf 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 

 ordinary lead pencil, and pivoted on a horizontal axle O. The rod 

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

 glass, whose surface 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 

 electromagnet C C. When the current of the electromagnet is cut 

 off the iron is released, and the end A' of the rod is tossed up by the 

 action of a piece 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 freo to fall from rest. 



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

 carrying 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 electromagnet the ends A' and B' of the two 

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

 and sphere begin to fall at the same moment. Before, however, the 

 drop reaches the surface on which it is to impinge, the timing sphere 

 strikes a plate D attached to one end of a third lever pivoted at Q, 

 and thus breaks the contact between a platinum wire bound to the 



