SURFACE-TENSION OF WATER BY THE METHOD OF JET VIBRATION. 305 



The method used to determine the wave-length consisted in finding out the 

 summits of the jet (the points where the tangent-planes were parallel to the axis of 

 the jet), using the jet as an optical image-forming system. 



Fig. 3 represents a horizontal jet-piece S (placed so that the two perpendicular 

 planes of symmetry of the jet are respectively horizontal and vertical), a telescope T, 



M 



;B, 



L 



If 



Fig. 3. 



and a Nernst lamp L (the filament being vertical) fastened to the telescope 'in a 

 position vertically over its axis, seen from above and from the side. OC^ is a 

 horizontal line, perpendicular to the jet, through a summit. 



Seen from above, the jet acts as a lens with large focus- width, the front-surface of 

 which will form a virtual image at A, and whose back-surface will form a real image, 

 modified through the refraction during the double passage through the front-surface 

 at B. Seen from the side, all the light reflected can, on account of the small diameter 



VOL. ccix. A. 2 B 



