342 Profs. Remold and Riicker on the [June 21, 



that thickness so suddenly that the colours between the black and green 

 of the third order were quite invisible. 



The forms of the coloured portions of the films having been thus deter- 

 mined, the authors were now able to calculate their resistances. The 

 resistance of a ring of the cylinder of length Z, radius r, and of uniform 



thickness r, might be taken to be =-^-, where p is the specific re- 



2irrr 



sistance of the liquid, which was determined to be 222 ohms at the 

 temperature at which the experiments were conducted. 



The radii, both of the platinum ring and crucible, were determined by 

 means of the cathetometer. Both were found to be very nearly circular 

 and their mean diameters were 33*26 millims. and 33*77 millims. 

 respectively. The mean of these numbers, or 33*51 millims., was 

 taken to be the value of 2r. Since the thickness of each little ring was 

 not uniform, the value taken for r was the mean value of the thicknesses 

 of its upper and lower edges. It can easily be shown that the resistances 

 so calculated would be a little less than the true resistance, and a correc- 

 tion factor was introduced, the value of which is 



r i + r 2 



2(r x -r 2 ) 



log A 



where r 1 and r 2 are the numbers above referred to as those the mean of 

 which was taken to be the thickness of the film. The value of this 

 factor, which depends only on the ratio of r 1 to r 2 , was calculated for several 

 values of that ratio, and the numbers required were obtained by interpola- 

 r +r 



tion. The values of 1 — for the case considered are given in Column 



XI. of Table I., and Column XII. contains the values of the correction 

 factor. 



Hence, introducing the corrections for the oblique incidence of the 



light and for the refractive index, the resistance of the cylinder was 



given by the expression , 



1*395 x cos % 21k 

 222 x s- 



2irr t x + t 2 



The resistance so calculated was subtracted from the total observed 

 resistance of the film, and the number thus obtained was assumed to 

 give the resistance of the black portion. 



Table II. gives the results of the experiments. 



Column I. gives the time at which the observations of the electrical re- 

 sistance were made. 



Column II. the breadth of the band of black. 



Column III. the names and orders of the colours corresponding to the 

 thinnest and thickest portions of the coloured parts of the cylinder 

 when seen by light incident at 45°. 



