516 
When the eight mean values of the different plates are combined, 
the result becomes: 
0,156 + 0.008 
The arithmetical mean of the thirty-one separate values differs very 
little from this: 
0,156 0.007 
The theoretical value is 0.166. 
8. We collect the results in the following table. 
a A Am 
4750 0.156 + 0.007 0.166 
0.156 + 0,008 
5380 0.148 + 0.006 0.143 
0.148 + 0.012 
6510 0.125 + 0 007 0.115 
0.123 + 0.014 
The observed displacement of the bands is indicated under A. 
The mean error has been calculated in two ways, as was discussed 
above. The second values are those derived from the average of 
the mean values of the individual plates. 
Under A, the theoretical value is given calculated by the aid of 
the data for the index of refraction for the ordinary ray in quartz, 
taken from Kontrauscn’s data. 
It is not to be denied that taking the particular difficulties of the 
experiments into consideration, the agreement between theory and 
observation is very satisfactory. 
The change of the effect with wave-length as well as the absolute 
value of the effect are represented very well. In the discussion of 
the experiments with glass, for which the dispersion is greater than 
for quartz, we shall have an opportunity to point out the very 
pronounced influence of the dispersion term. 
9. The formula for the optical effect. We consider two of the 
rays which bring about the interference phenomenon, and which 
have passed over opposite paths. We shall denote quantities which 
refer to the first ray, by one accent, and those belonging to the 
second ray by a double accent. Each of the paths traversed, consists 
of three parts: 1 a path 1 in the air, 2 a path 2 in the quartz 
column, 3 a path 3 in the air. 
