( 348 ) 
much better plan to analyse the light leaving the second Nicol with 
the aid of a spectroscope. The result becomes more accurate, if 
one combines a moderately thin plate of double refracting crystal 
with the mica sheet. The simple theory of the dark bands, now 
visible in the spectrum, is well known and needs not to be given 
here. Since the time of Fizeau and Foucault many physicists have 
used them for the measurement of phase differences. *) 
A plate of quartz, cut parallel to the optical axis, and of about 
2 m.m. thickness, gives 18 bands between the red and violet hydrogen 
lines. 
These bands are most distinct, if the principal sections of the 
Nicols and the mica be inclined at an angle of 45°. 
If the mica and the quartz plate be superposed in such a manner, 
that the principal sections correspond, a displacement of the bands 
in a certain sense, e.g. towards the red is observed. 
If then the mica be rotated through 90° the displacement is towards 
the violet. 
The amount of the total displacement observed, by the interchange of 
the two positions of the mica, is easily measured. The ratio of half 
this displacement and the distance of two succeeding bands gives the 
difference of phase produced by the mica for the region of the 
spectrum under consideration. 
K6nig 9 ) ajid Cornu ') suggested the use of a double quarter-wave 
plate with horizontal boundary line for the easier observation of the 
longitudinal magnetic effect. 
I myself often make use of mica plates of the same kind but 
divided into three fields, as indicated in the figure. The principal 
/ 
/ 
Fig. 1. 
sections of the fields are indicated by the arrows. 
l > See f.i. Cornu in the paper cited sub 3. 
2 ) W. Konig, Wied. Ann. 63 S. 268. 1897. 
3 ) A. Cornu. C. R. 126 p. 555. Octobre 1897, Eclairage Electrique 13 p. 24b * 
