390 



Lord Rayleigh On the Tension of Water 



ambiguity in the wave pattern over the measured part of the 

 field. It would occasionally happen that in certain positions 

 of the eye a change of phase would occur in the middle of the 

 field, so that the bright bands in one part were the continua- 

 tion of the dark bands of another part. Near the transition 

 the bands would appear confused, a sufficient indication that 

 no measurement must be attempted. On the other hand, it is 

 not necessary that the contrast between the dark and bright 

 parts should be very great. Indeed the measuring marks 

 were better seen when no part of the field was very dark. 



Fig. 1 gives a general idea of the appearance of the field. 

 On the right is seen a paper with a notched edge, the use of 



Fig. 1. 



which was to facilitate the counting. The measuring arrange- 

 ment was something like a beam compass. Stout brass wires, 

 attached to a bar of iron, were shaped at their ends like brad- 

 awls, and the edges were placed parallel to the crests of the 

 waves. In order to avoid residual parallax, the rod was so sup- 

 ported that the edges were in close proximity to the water 

 surface. 



In many of the experiments the distance between the edges 

 was set beforehand, e.g. to 10 centim., and was not altered 

 when the wave-lengths varied with the deposition of grease. 

 The number of wave-lengths included was determined by 



