306 MR. N. BOHR ON THE DETERMINATION OF THE 



of the jet, be considered as intersecting the jet-axis. If, now, the telescope is focussed 

 for the distance TA, a small vertical bright line a, and a less bright, but sharply 

 limited ellipse /3, with horizontal great axis will therefore be seen in a dark field. 



When the telescope is displaced parallel to the jet, the distance between the bright 

 line and the ellipse will vary, and the telescope being brought into a position quite 

 opposite a summit, the bright line will fall together with the minor axis of the ellipse. 

 Parallel to the jet was placed a fine glass-rule M, divided into -fa mm., which 

 divisions could be seen sharply in the field of the telescope, together with the bright 

 lines mentioned. 



The measurements were executed in such a manner that the telescope was partly 

 displaced parallel to the jet and partly turned around a vertical axis, until the vertical 

 spider-line fell together with the bright line at the same time as this halved the 

 ellipse. 



Fig. 4 shows the appearance of the telescope-field. Every time when such an 

 adjustment was obtained, the position of the spider-line was read on the glass-scale. 



The adjustment and reading could be done with an accuracy 

 of O'Ol mm. 



In the above we have supposed that the jet-axis was 

 horizontal. If, on the contrary, the jet formed an angle with 

 the horizontal plane and this must be the case at certain 

 places of the jet-piece examined on account of the curvature 

 of the jet the bright line and the minor axis of the ellipse 

 will form the same angle with the vertical spider-line. If, 

 however, care was taken in the arrangement that the centre 

 of the vertical line from the middle of the filament to the 

 telescope-axis was at the same horizontal height as the jet, the vertical plane through 

 the telescope-axis will here, too, as a closer examination shows, be perpendicular to 

 the vertical plane containing the jet and go through a summit when the vertical 

 spider-line goes through the middle of the bright line at the same time as this falls 

 together with the minor axis of the ellipse. 



The circumstance that the wave-amplitudes on account of the viscosity of the 

 liquid are decreasing in direction from the orifice has the effect that the distance OA 

 between the focus-lines and the jet is not the same everywhere. While this fact is 

 not of great importance when measuring the wave-lengths on a short jet-piece, it will, 

 wheii measuring on very long jet-pieces (as in the table, p. 310) have the effect that 

 the focussing of the telescope cannot be kept constant during the measuring, and the 

 readings of the single summits could therefore, in this case, not be executed with 

 quite as great accuracy as mentioned above. 



The differences between the readings indicate the distances between the projections 

 of the summits on a horizontal plane. Dividing the mentioned differences by cos a, 

 a being the slope of the jet at the place in question, we therefore get the distances 



