1885.] On the Formation of Vortex Rings by Drops, 41 7 



Circumstances prevented the realisation of this scheme till quite 

 recently, when I put into Mr. Hilger's hands a grating presented to 

 me by Mr. Rutherfurd. 



The result is excellent. It is possible to observe C and F, for 

 instance, together quite conveniently, with either a normal or a tan- 

 gential slit. The only precautions necessary are to see that half of 

 the light passing through the object-glass falls on the half grating, 

 and that the rays which come to a focus on the slit plate are those 

 the wave-lengths of which are half way between the wave-lengths of 

 the two lines compared. 



V. " On the Formation of Vortex Rings by Drops falling into 

 Liquids, and some allied Phenomena." By J. J. THOMSON, 

 M.A., F.R.S., Fellow of Trinity College, Cavendish Pro- 

 fessor of Experimental Physics, Cambridge, and H. F. 

 Newall, M.A., Trinity College, Cambridge. Received 

 November 28, 1885. 



When a drop of ink falls into water from not too great a height, it 

 descends through the water as a ring, in which there is evidently 

 considerable rotation about the circular axis passing through the 

 centres of its cross sections ; as the ring travels down through the 

 water inequalities make their appearance : more ink seems to collect 

 in some parts of it than in others, and as these parts of the ring 

 descend more rapidly than the rest, it assumes some such appearance 

 as that shown in fig. 1. These aggregations as they descend develope 



Fig. 1. 



fresh rings in the same way as the original ring was developed from 

 the drop. The ring is thus split up into several rings, each of which 

 is connected with its neighbours by threads of ink affording a very 

 pretty illustration of the continuity of motion in a liquid (see fig. 2). 



As the secondary rings descend they develope other rings just as 

 they were developed from the original one, and the process of repro- 

 duction seems to go on indefinitely. 



It is not every liquid, however, which, when dropped into water, 

 gives rise to rings, for if we drop into water any liquid which does 

 not mix with it, such as chloroform, the drop in consequence of the 



