518 



Mr. E. S. Johonnott on the Thickness of 



the thickness for each observation, the intensity-curves were 

 plotted for a series of curves for different thicknesses of film, 

 but alike in other respects. The observed intensities were 

 then plotted on the same plate. The broken lines on figs. 6 

 and 7 represent the calculated intensity-curves for light 

 polarized respectively in and perpendicular to the plane of 

 incidence. The constants used in formula (2) for the calcu- 

 lations were 



^ = 1-333, 



\ = 550 /jl/jl. 



The intensity-curves for D equal to 5, 6, 7, 8, 9, and 12 jx^jl 

 are plotted in fig. 6, and 5, 7, and 9 in fig. 7, for angles 

 of incidence varying from 0° to 70°. The abscissas represent 

 the angles of incidence, while the ordinates represent the 

 fractional part of the light incident on the second nicol N 

 (with the collimator in the zero position), that is reflected from 

 the film. 



Statement of Results. — The results of some observations at 

 an angle of incidence of 45° are first given in Tables V. 

 and VI. Two gas-jets were used as sources in Table V., while 

 in all other observations by this method the two sources, S 

 and S' (fig. 4), were two Welsbach burners. The number of 

 the observation is given in the firbt column and the intensity 

 in the second, calculated from the values of 6 in the third. In 

 the last column the thickness is given both in terms of the 

 wave-length and in absolute measure, the wave-length being 

 assumed to be 550/Lt/z. 



Table V. 



Polarized at 45° to plane of incidence ; angle of 

 incidence 45°. 



Obs. 



Intensity. 



»-§■ 



D. 



I. , 



II 



Ill 



IV 



•00106 

 •00073 

 •00098 

 ■00075 



l°-86 

 1-57 

 1-80 

 1-56 



X 



73 

 X 



88 

 X 



77 

 X 



88 



Mean 



10-6 mms. 



7-5 



6-2 

 71 



62 



6-7 mi 



