138 Lord Rayleigh. [Juiie 5, 



becomes sensible in Plateau's apparatus is about -^ of a micro- 

 millimetre, or about -S-^QQ of the wave-length of yellow light. 



A tolerably concordant result is obtained from a direct estimate of 

 the smaller quantity of oil, combined with the former results for 

 camphor, which were arrived at under more favour-able conditions. 

 The amount of oil in two drops of the solution is about O'OOl? milli- 

 gram. This is the quantity which suffices to produce a visible effect 

 upon the needle. On the large surface of water of the former experi- 

 ments the oil required to check camphor was about 1 milligram. In 

 order to allow for the difference in area, this must be reduced 

 64 times, or to O016 milligram. According to this estimate the ratio 

 of thicknesses for the two classes of effects is about as 10 : 1. 



Very similar results were obtained from experiments with an 

 ethereal solution of double strength, one drop of which, evaporated 

 as before, upon platinum, produced a distinct effect upon the time 

 occupied by the needle in traversing the arc from 90 to 30. 



I had expected to find a higher ratio than these observations bring 

 out between the thicknesses required for the two effects. The ratio 

 15:1 does not give any too much room for the surfaces of ordinary 

 tap water, such as were used in the bath observations upon camphor, 

 between the purified surfaces on the one side and those oiled surfaces 

 upon the other, which do not permit the camphor movements. 



It thus became of interest to inquire in what proportion the film 

 originally present upon the water in the bath experiments requires to 

 be concentrated in order to check the motion of camphor fragments. 

 This information may be obtained, somewhat roughly it is true, by 

 dusting over a patch of the water surface in the centre of the bath. 

 When a weighed drop of oil is deposited in the patch, it drives the 

 dust nearly to the edge, and the width of the annulus is a measure 

 of the original impurity of the surface. When the deposited oil is 

 about sufficient to check the camphor movements, we may infer that 

 the original film bears to the camphor standard a ratio equal to that 

 of the area of the annulus to the whole area of the bath. Observa- 

 tions of this kind indicated that a concentration of about six times 

 would convert the original film into one upon which camphor would 

 not freely rotate. 



Another method by which this problem may be attacked depends 

 upon the use of flexible solid boundary. This was made of thin 

 sheet brass, and is deposited upon the bath in its expanded condition, 

 so as to enclose^ a considerable area. Upon this surface camphor 

 rotates, but the movement may be stopped by the approximation of 

 the walls of the boundary. The results obtained by this method 

 were of the same order of magnitude. 



If these conclusions may be relied upon, it will follow that the 

 initial film upon the water in the bath experiments is not a large 



