300 Professor A. M. Worthington [May 18, 



so as to display a particular stage, it will happen that in a suc- 

 cession of observations there is a certain variation in what is seen. 

 Thus the configuration viewed may be said to oscillate slightly 

 about the mean for which the apparatus is adjusted. Now this is 

 due both to small imperfections in the timing apparatus and to 

 the fact that the splashes themselves do actually vary within 

 certain limits. The reasons are not very far to seek. In the first 

 place the rate of demagnetisation of the electromagnets varies 

 slightly, being partly dependent on the varying resistance of the con- 

 tacts of crossed wires, partly on the temperature of the magnet, 

 which is affected by the length of time for which the current has 

 been running. But a much more important reason is the variation of 

 the slight adhesion of the drop to the smoked watch-glass that has 

 supported it, and consequently of the oscillations to which, as we shall 

 see, the drop is subjected as it descends. Thus the drop will sometimes 

 strike the surface in a flattened form, at others in an elongated form, 

 and there will be a difference, not only in the time of impact, but in 

 the nature of the ensuing splash ; consequently some judgment is 

 required in selecting a consecutive series of drawings. The only way 

 is to make a considerable number of drawings of each stage, and then 

 to pick out a consecutive series. Now, whenever judgment has to be 

 used, there is room for error of judgment, and moreover, it is impos- 

 sible to put together the drawings so as to tell a consecutive story, 

 without being guided by some theory, such as I have already sketched, 

 as to the nature of the motion and the conditions that govern it. 

 You will therefore be good enough to remember that this chronicle of 

 the events of a tenth of a second is presented by a fallible human 

 historian, whose account, like that of any other contemporary ob- 

 server, will be none the worse for independent confirmation. That 

 confirmation I am fortunately able in some measure to supply. When 

 I endeavoured eighteen years ago to photograph the splash of a drop of 

 mercury, I was unable to obtain plates sufficiently sensitive to re- 

 spond to the very short exposures that were required, and conse- 

 quently abandoned the attempt. But in recent years plates of 

 exquisite sensitiveness have been produced, and such photographs as 

 those taken by Mr. Boys of a flying rifle bullet, have shown that 

 difficulties on the score of sensitiveness have been practically over- 

 come. Within the last few weeks, with the valuable assistance of 

 my colleague at Devonport, Mr. R. S. Cole, I have succeeded in 

 obtaining photographs of various splashes. Following Prof. Boys' 

 suggestion, we employed Thomas's cyclist plates, or occasionally the 

 less sensitive " extra-rapid " plates of the same makers, and as a 

 developer, Eikonogen solution of triple strength, in which the plates 

 were kept for about 40 minutes, the development being conducted in 

 complete darkness. 



A few preliminary trials with the self-induction spark produced 

 at the surface of mercury by the apparatus that you have seen at work, 

 showed that the illumination, though ample for direct vision, was not 



