1891.] on Some Applications of Photography. ii65 



an atmosphere of other water containing acid and sulphate of iron, 

 which exercises a decolourising effect on the permanganate, and so 

 retards the general clouding up of the whole mass by accumulation of 

 colour. [Experiment.] Mr. Gordon will release the clip, and we shall 

 get a jet of permanganate playing into the liquid. If everything were 

 perfectly steady, we might see a line of purple liquid extending to the 

 bottom of the trough ; but in this theatre it is almost impossible to get 

 anything steady. The instability to which the jet is subject now 

 manifests itself, and we get a breaking away into clouds something 

 like smoke from chimneys. A heavy tuning fork vibrating at ten to 

 the second acts upon it with great advantage, and regularises the 

 disruption. A little more pressure will increase the instability, and 

 the jet goes suddenly into confusion, although at first, near the nozzle, 

 it is pretty regular. 



It may now be asked " What is the jet doing ? " That is just the 

 question which the instantaneous method enables us to answer. For 

 this purpose the permanganate which we have used to make the jet 

 visible is not of much service. It is too transparent to the photo- 

 grap)hic rays, and so it was replaced by bichromate of potash. Here the 

 opposite difficulty arises ; for the bichromate is invisible by the yellow 

 light in which the adjustments have to be made. I was eventually 

 reduced to mixing the two materials together, the one serving to 

 render the jet visible to the eye and the other to the photographic 

 plate. Here is an instantaneous picture of such a jet as was 

 before you a moment ago, only under the action of a regular vibrator. 

 It is sinuous, turning first in one direction and then in the other. 

 The original cylinder, which is the natural form of the jet as it 

 issues from the nozzle, curves itself gently as it passes along through 

 the water. It thus becomes sinuous, and the amount of the sinuosity 

 increases, until in some cases the consecutive folds come into collision 

 with one another. [Several photographs of sinuous jets were shown, 

 two of which are reproduced in Figs. 5, 6, Plate II.] 



The comparison of the two classes of jets is of great interest. 

 There is an analogy as regards the instability, the vibrations caused 

 by disturbance gradually increasing as the distance from the 

 nozzle increases ; but there is a great difference as to the nature of 

 the deviation from the equilibrium condition, and as to the kind of 

 force best adapted to bring it about. The one gives way by becoming 

 varicose ; the other by becoming sinuous. The only forces capable 

 of producing varicosity are symmetrical forces, which act alike all 

 round. To produce sinuosity, we want exactly the reverse — a force 

 which acts upon the jet transversely and un symmetrically. 



I will now pass on to another subject for instantaneous photography, 

 namely, the soap film. Everybody knows that if you blow a soap 

 bubble it will break — generally before you wish. The process of 

 breaking is exceedingly rapid, and difficult to trace by the unaided 

 eye. If we can get a soap film on this ring, we will project it upon the 

 screen and then break it before your eyes, so as to enable you to form 



