205. 
+1 
Fig. 2. Fig. 3. 
x, sin Ó 
interfere under a phase difference Sa 2x. Hence < A,OA, is 
2x, sin 0 SE 
— a, or if in the earlier notation we put this equal to c,, it 
appears that c, is = 222, sin 6, hence that it is a constant’) that 
2x, sin Ô 
a A ge Ay A 
depends exclusively on distances; << A,OA, = 
26 sin O 
a — x, when 6 is the width of the glass plates. The other 
fans are congruent with the first, the last ray of a fan always 
forming the same angle am with the first of the next; < am is 
namely the phase difference due to the difference of two paths of 
light, one going from a point of the righthand edge of a plate to 
Q, and the other from the lefthand edge of the following plate to 
the same point Q. When u and d are the index of refraction and 
the thickness of every plate, then ais = pd—d cos 6, hence again a 
constant, depending on measures of length and physical constants. 
This constant distinguishes itself from the first, because it 
depends on the wave length in consequence of the appearance of u. 
However u and hence a too, is an even function of m and for this 
reason we may consider « as a constant in the following discussion. 
Let us now return to the general case. 
When the instrument is struck by light of a twice as small 
wavelength, the fan will be drawn out twice as far, all the angles 
with the time-direction being proportional to m. It appears parti- 
cularly clearly with the echelon how in consequence of this the 
resulting vector, and together with this the intensity in the point 
of observation Q will change its value. The intensity is, therefore, 
a function of the number of waves m, and in order to prove that 
this is an even function of it, we shall reverse the sign of m. This 
1) i.e. independent of m. 
14* 
