Objects, and on the Oscillating Electric Spark, 249 



to grow at the end of a pipe half an inch or so in diameter 

 dipped below the surface of paraffin. By this means the 

 effective weight of the drop is very much reduced, and the 

 surface tension is reduced, so that a large drop can be formed, 

 and the process is made comparatively slow. 1 have found 

 that the development of the drop can be made still more 

 gradual, and that still larger drops may be formed by adding 

 bisulphide of carbon to the paraffin so as further to reduce 

 the effective weight of the drop. 



While trying to devise some way of representing this expe- 

 riment on paper, I saw at the Eoyal Institution an apparatus 

 exhibited by Mr. Friese Greene, with which photographs at 

 the rate of ten a second could be taken upon a travelling film 

 moved on between and stopped during each exposure. 



Examination of the growing drop through a small window 

 in a card disk made to rotate ten times a second, showed that 

 even this rate applied to large and slowly forming drops was 

 not sufficient to give anything like a continuous representa- 

 tion of the forms assumed. However, I found no difficulty in 

 obtaining twenty photographs a second ; and, had I required 

 them, I could certainly have obtained fifty sharp and fully 

 exposed photographs in this time, by the following method : — 

 An electric arc was focused by a pair of seven-inch lenses 

 upon the lens of an ordinary half-plate camera. A box with 

 a glass front and back, containing the liquids and the growing 

 drop, was placed immediately in front of the large lenses, and 

 the drops were focused by the lens of the camera upon the 

 ground-glass screen at the end of a slide forty-two inches 

 long. This could be charged with four 10x4 plates (two 

 10x8 plates cut longitudinally), and then be drawn by hand 

 from one end to the other of a seven-foot slide which protected 

 the plates from light everywhere except at one place where a 

 window 4x1 inches had been cut. Through this window the 

 image of the falling drops was cast upon the moving plate. 

 Just in front of the camera-lens a disk of card with an 

 aperture near the edge was made to rotate at any desired 

 speed by one of Cuttriss's P. 1 motors. Everthing was then 

 started, and the speed of the motor and rate of dropping ad- 

 justed until the really intermittent but apparently continuous 

 view of the phenomenon upon the ground plate was as desired. 

 The slide was then rapidly drawn from end to end by hand, 

 the speed being such as to obtain rather more than one com- 

 plete cycle during the motion of the slide. In the photograph, 

 of a portion of which Plate VI. fig. 1 is an exact copy, the 

 exposures were at the rate of fourteen a second, and lasted 

 about one five hundred and sixtieth of a second each. The 



