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field, i.e. of the image of the slit in the telescope, is a minimum. 
One of the planes of polarisation of the nicols is then about parallel 
to and the other normal to the principal plane of the wedge. 
3. The collimator-slit takes a part out of the spectrum, the wave- 
length of which varies a little in a direction normal to the slit. 
Thus different cylindrical beams of light issue from the collimator, 
which differ somewhat in wave-length, and the axes of which torm 
small angles with each other. The image of the slit in the telescope 
is, therefore, part of a spectrum. In this image there are seen some 
black lines, parallel to the longitudinal direction of the slit. In our 
case the number, depending on the width of the collimator-slit, was 
three. By experiment the following properties of these lines were found: 
a. The lines arise from the extinetion of definite colours. They 
become sharper as the purity of the spectrum increases and more 
numerous with greater width of the slit. 
b. The wedge-form of the quartz wedge is without influence, as 
a slit before the quartz wedge does not change the phenomenon. 
c. For one analyzer-position there are found two polarizer-positions, 
for which a system of black lines is observed. The lines of one 
system lie between those of the other. The angle between the two 
polarizer-positions is smaller as the principal plane of the wedge 
coincides more with the plane of polarisation of the polarizer or is 
normal to it, The middle-plane between the two polarizer-positions 
forms an angle of O° or 90° with the principal plane of the wedge. 
One of the two polarizer-positions is only normal to the analyzer, 
independent of the position of the principal plane of the wedge. 
d. When the plane of polarisation of the analyzer makes an angle 
of 0° or 90° with the principal plane of the wedge, the dark line 
in the polarisation-field of the two Nicols (§ 1) is visible in the slit- 
image. Supposing that this is about at right angles to the edges of 
the slit, the two systems of black lines are visible at the same time, 
one above, the other under the nicol line. This forms so to say, a 
transition between the two systems. The lines seem thickened there. 
e. On rotation of the polarizer from one to the other of the two 
positions, mentioned in c, the lines first become fainter, then they 
rapidly move through the field to those of the second system, in 
which the latter become very black. 
c suggests immediately a mode of procedure to place the wedge 
so, that the principal plane of the wedge coincides with the polari- 
sation plane of one of the nicols, and is normal to that of the other. 
This position is obtained with successive approximations. 
