107 
The results, however, are not recorded, because the phenomena 
take place at such small values of A, that they are difficult to observe 
experimentally. 
7. Similar results, as have been described in § 3 for one wedge, 
are observed with both compensator-wedges. 
a. In the slit-image black bands can appear for suitable nicol- 
positions; only one position of the polarizer, however, corresponds 
to a definite position of the analyzer. Compare 3c. 
6. These positions of the nicols are confined to a region, which 
becomes smaller, as the angle between the principal planes of the 
wedges departs less from 90°. 
From this follows a new criterion for the adjustment of the true 
position of the wedges (angle 90° between the principal planes), 
viz. that the black bands disappear on the slightest displacement 
of the polarizer. 
We may refer for the theoretical explanation to a thesis for the 
doctorate by C. A. Reeser, Amsterdam, 1921. In this the phenomena 
corresponding to 3e, which Reeser termed fading-phenomena, i.e. in 
which the dark bands are fading away, were left out of considera- 
tion as exceedingly complicated. The theory is therefore restricted 
to the occurrence of perfectly black bands. 
