106 
There appear black lines, if X= Y =O, so that: 
fa 
AR " ze Oe, 
or 
Ee © : ’ 
4, (1 |=, A=— A, 
It thus appears, that 5 1 +AA, and 4 a—A, are the azimuths of 
the two polarizer-positions, for which black lines appear. 
The wave-lengths, for which minima appear at given polarizer- 
positions, follow from d/: dd, = 0. This yields A = CA, : (Ccosy + 
+ p sin p)‚, hence: 
— ae cot = st Qs. (: + ne : 
‚doe varying little with the wave-length, C is independent of it. 
The difference of phase in the quartz wedge for a thickness d, is 
d,(Mg—ny): A, in which n,—n» = 0,009, 2A=6 Xx 10+. Ford, =3mm., 
which value is certainly not exceeded, the difference of phase is 
45 X 2”. Further e=15', or in radians 1; 240, «,=«e:d, about 
1:720, hence for «=S8mm., ¢,d,c smaller than a. Thus C is 
always smaller than 1. 
For AA, the minima appear at y= 2ka, and / then being 
= 0, they are perfectly black. It appears from the foregoing, that 
k is about 45. 
When we start from perfectly black bands, A=A,, p= 2ka, 
1=0, and let p increase by Ay, so that C+ ptgy=0, then 
Ay = —C:2ka, and A has risen to a very great value, likewise 
I, which depends on A?. It appears from this, that the dark bands 
get very much fainter on increase of the azimuth $+ A, of the 
polarizer, but are hardly displaced at all. When now gis increased 
by a very small amount Ay = C':2ka, so that gig p= C, then 
A is =$A,, 1/=+A,’. Accordingly the dark bands are very little 
displaced, when the azimuth of the polarizer decreases from }2+A, 
to Eat}. If the bands have to shift to places for which 
p=), A must decrease to CA,:(2k+ 4)2, or about 
A, :,300, for C =d, += 45... In „order, therefore, -to. displace 
the. minima over a fourth of their distance, 4 must decrease to 
a very small fraction of A,. The bands, which at first seem to 
remain in their places, when the polarizer is rotated, afterwards 
move through the field with great rapidity. Theory thus explains the 
phenomena given in 3e. We have further examined, what happens 
to the bands at the places for which g = (24+ 1)” at A>—A,,. 
C=1 
