Surfaces, Clean and Contaminated. 391 



counting, and estimation of tenths. Usually the discrepancy 

 between Mr. Gordon's estimation and my own did not ex- 

 ceed a single tenth, and in a large proportion of cases there 

 was no difference. Probably the mean of our readings would 

 rarely be wrong by more than ^o °f a wave-length, when the 

 pattern was well seen. In the experiments specially directed 

 to the determination of the tension of a clean surface, it was 

 found advisable to work with an unknown distance ; other- 

 wise the recollection of previous results interfered with the 

 independence of the estimates. 



It is probable that somewhat greater accuracy in single 

 measurements might have been attained had the distance been 

 adjustable by a smooth motion within reach of the observer. 

 Each measuring edge might then have been set to the most 

 favourable position, that is, to the centre of a bright band. 

 The frequent removal of the apparatus for comparison with a 

 scale would, however, be rather objectionable ; and it was 

 thought doubtful whether any final gain would accrue in the 

 mean of several observations. 



Some trouble was experienced from the communication of 

 vibrations through unintended channels. In order to prevent 

 the direct influence of the interrupter fork upon the liquid 

 surface, it was found advantageous to isolate it from the floor 

 by supporting it upon a shelf carried upon the walls across a 

 corner of the laboratory. On one occasion it was noticed that 

 the waves were visible without the aid of the arrangement for 

 making the light intermittent. This was traced to a tremor 

 of one of the mirrors, supported upon the same shelf as the 

 interrupter fork. Such a method of rendering the waves 

 visible is objectionable, since it destroys the definition of the 

 measuring points. The tremor was eliminated by the intro- 

 duction of rubber tubing under the stand of the interrupter. 



During the experiments on greasy surfaces one pair of 

 forks only was employed. The frequency of the interrupter 

 was about 42 per second, so that the intermittent current 

 could be used to excite a fork of about 126. The beats be- 

 tween this and a standard Koenig fork of 128 were counted 

 at intervals, and found to be sufficiently constant. The pitch 

 of the standard has been verified by myself*, and at the 

 temperature of the laboratory may be taken with sufficient 

 accuracy to be 128. If we take the number of beats per 

 minute at 98, we have for the frequency of the interrupter 



/=i(l28-g)=42-12. 

 * Phil. Trans. 1883, p. 316. 



