INVESTIGATED BY Till: METHOD OF JET VIBRATION. 



309 



considerably. The execution of a measurement also demands much time, and this is 

 probably the greatest drawback of the method. 



The other method is as follows : 



The rays from a horizontal linear incandescent lamp L, (about 23 cm. long, 

 25 candle-power, 110 volts) is reflected from the mirror S perpendicularly down on 

 the jet * (see fig. 13). Close beside the jet is arranged a vertical photographic 

 plate P, upon which an image is formed, the approximate form of which is 

 shown by the line m-n on fig. 14. The lamp LI is enclosed in a shield ABC. 



Fig. 14. 

 The part BC of this shield has the form shown in fig. 1 5, making the illumination 



Section a-b. 



Fig. 15. 



of the jet the same for its entire length. In series with the lamp LI another lamp, I ... 

 is inserted, the power of which can be regulated by the help of a rheostat placed 

 parallel to it. I ... is arranged at the same height as the jet, and the light from it 

 produces a homogeneous fog on the plate P which is only interrupted by the 

 shadows s t -s a of the jet and T,-T a of the wire T which is arranged horizontally in 

 front of the plate. 



The manner in which the image ni-n (fig. 14) is formed will be explained only very 

 briefly. In fig. 1 6 is shown the circular cross-section of a horizontal jet. L is a vertical 

 ray ; its direction is after two refractions and one reflection changed to 1^. The angle 

 between L and L, is denoted by y and the refractive index of the liquid by n^ With 

 the syinlxils of the figure 



y = 4/>-2t. 



VOL. CCVII. A. 3 B 



