370 MR, P. 0. PEDERSEN ON THE SURFACE-TENSION OF LIQUIDS 



It is easily shown that y is maximum when 



sn = 



In order to illustrate the positions of the emergent rays, fig. 17 is drawn. The 

 ray for which y is maximum cut the plate P in B, and at that point the intensity of 

 illumination will be maximum. All the points B, collectively, form the image m-n 

 (fig. 14).* 



Let the maximum value of y corresponding to zf be denoted by y,, then y,-y has the 

 same sign as b n cos kz. The wave form of the image m-n is produced in this manner. 



P 



Fig. 17. 



As the amplitude of the curve m-n is much greater than that of the jet, it is 

 much easier to determine the wave-length by measuring on the image m-n than on 

 the jet itself. 



The measuring of the wave-length takes place in the following manner (fig. 14) : 



"The"" -distance L between two homologous suitable points on the image m-n is 



determined. By dividing this length by the number v of waves between the points 



* For further information "about this question see J. M. PERNTIIER ' Meteorologische Optik' (Wien and 

 Leipzig, 1902), p. 482. 



t In r 



cos n<t>. cos kz . . . [(1), 1]. 



