304 



ME. N. BOHR ON THE DETERMINATION OF THE 



from right to left.) We see how the ends of the jet set free by the knives very rapidly 

 contract themselves into the corresponding jet-pieces, assuming a drop-like appearance. 



The plates were measured by examining them pressed together with a glass-rule 

 under a microscope, and reading on the scale the positions of the lines, perpendicular 

 to the jet, touching the drop-like free ends of the jet-pieces. Thereupon the mean of 

 the results found for the two ends of each cut was calculated, and the difference 

 between the means from two succeeding cuts, divided by the magnification of the plate, 

 was equated to the distance which the jet had moved while the rotation -apparatus 

 had made a revolution. The conditions of the correctness of this were partly that 

 the cuts moved independently of each other, partly that the ends of the jet contracted 

 themselves equally into the respective jet-pieces during the time which a cut took to 

 move from the one place where it was photographed to the other. That these 

 conditions were satisfied appears, first, from the fact that the part of the jet-pieces 

 placed midway between the cuts was completely undisturbed by the cutting of the 

 jet (see the photographs), and, secondly, from the symmetrical forms of the ends of 

 the jet facing each other. 



The magnification of the plate was found by taking a photograph of a glass-rule 

 placed directly under the jet. 



The interval of time between the cuts was determined as a mean from the number 

 of revolutions per second during the time of exposure ; the photographic plate also 

 giving a sort of mean of the single exposures, very great accuracy might be obtained 

 in this way. 



At each determination of the velocity of the jet photographs were taken for the 

 sake of the control with different times of revolution of the rotation-apparatus. 



The following table shows the result of an experiment by which four photographs 

 were taken : 



The values found show a very good agreement, the largest mutual deviation being 

 less than O'l per cent. 



Determination of the Wave-Length. 



In the experiments, jets were used with so small wave-amplitudes that the wave- 

 length could not be measured directly with sufficient accuracy either on the jet itself 

 or on a photograph of the same. 



