BLACKWELL. — DISPERSION IN ELECTRIC DOUBLE REFRACTION. 665 



which was mounted four feet from the negative (120 cm.). A fixed 

 telescope with micrometer eyepiece is being mounted, and will elimi- 

 nate this uncertainty in future. 



The difference between the positions of the voltage and zero bands 

 gives the apparent shift, — that part of the total amount not taken up 

 by changing the setting of the compensator. The reading of the com- 

 pensator was reduced to terms of the apparent shift by photographing 

 the zero bands for two settings of the compensator differing by from 

 five to ten turns of the screw, the range being limited by the field of 

 view. Thus it was found that for thirty turns, 2.73 cm. must be added 

 to the apparent shift to get the total shift, x. The values of x obtained 

 from the eight voltage bands thus far observed are recorded in Table V. 



TABLE V. 



Total Shift, x k , at 8990 Volts. 

 Summary of Tables III, IV, and similar ones, each for one experiment. 



Treatment of Data. 



• Before the true significance of these measurements can be brought out, 

 allowance must be made for dispersion in the natural double refraction 

 of the quartz compensator. When plane-polarized light passes through 

 quartz, the extraordinary wave is retarded over the ordinary by 8 cm., 

 an amount equal to the thickness, e cm., of quartz traversed, times the 



