482 Mr Filon , On the variation with the wave-length 
homogeneous throughout, the scale crossed by a number of dark 
horizontal lines. One of these is central and corresponds to n = 0: 
the others are parallel to it on either side at distances 
2\AF 
C Mr ’ 0 Mr ’ 
If however the light be homogeneous only for each cross- 
section, but vary gradually in wave-length from cross-section to 
cross-section, then a continuous spectrum will appear in the field 
of view, crossed in the direction of its length by black bands, of 
which the central one is straight and horizontal, whereas the 
others are curves which close in upon the centre as we move from 
the red to the violet end. 
The ordinates y of these black fringes measured from the 
straight central one as origin are proportional at each point to the 
\/G corresponding to the line of the spectrum considered. 
They accordingly map out continuously the variations of X/G 
as we go along the spectrum ; and if X can be determined at each 
point, they give the variation of 0. 
4. In the actual case we can only approximate to an arrange- 
ment of this kind. First of all, in order to double the effect, two 
beams of glass were used, instead of one. These are not of exactly 
the same dimensions, nor are they exactly at the same vertical 
height ; in both of them, however, the axes of polarization are 
throughout horizontal and vertical. 
Fig. 3. 
In the next place the pencils, which converge on the scale to 
form a spectrum, are not exactly normal to the sides of the beam, 
but the extreme pencils are more oblique than the others. 
Finally the pencils are not indefinitely thin, but are cones 
containing a finite solid angle, namely that subtended at the scale 
s by the circular diaphragm in front of the analyser. 
Let us examine therefore how the appearances should be 
affected by these various departures from the ideal arrangement 
discussed in the preceding paragraph. 
Suppose that AB , Fig. 3, is the circular diaphragm (viewed 
sideways) in front of the analysing Nicol. Let CD, EF be the 
