207- 
they only depend on distances; however cm need not be any longer 
the difference of phase of the interfering rays, this may have 
become cm + zr. 
After these extensive considerations, which as will appear later 
on, go to the core of the method, we proceed to the construction 
of the fan in the point Q in the case of two groups of points Pand P’. 
A point P, and a point P,’ give vectors which form respectively 
the angles c,m and c/‚m 2 with the time-direction. When we 
now reverse the sign of m, the first group of vectors will again be 
reflected in the time-direction; a vector from the second will, however, 
form the angle +2—c’,m instead of the angle a + c,’m with the time- 
direction, and would, therefore, be evidently reflected in the pro- 
duction of the time-direction. Ultimately al! the vectors will, therefore, 
again be reflected in the same line, through which the form of the 
fan will remain unchanged, hence the value of the intensity will 
remain the same, which latter is, therefore, again an even function 
of m. The thesis may, therefore, be extended as follows. 
In every instrument, in which the light can only undergo phase- 
shiftings of a whole number of times 2, the intensity is an even 
function of the number of waves when absolutely monochromatic 
light is used. 
The light fulfils these conditions naturally in all apparatus of 
refraction, also generally in those which are founded on interference. 
As will appear later on, through the suitable occurrence of phase 
shiftings among others in reflections against metal mirrors the 
intensity can also sometimes become a not-even function of the 
number of waves. 
Let us now consider the case of a beam continuously composed 
of some frequencies. Suppose it to be possible to draw up a second 
equation between 7 and s for a given parameter / by the aid of 
one of the current apparatus, where we shall suppose a favourable 
action of the phase shifting to be absent in the metal reflections, 
which is actually the case with most, if not with all instruments 
in general use. We can now easily show that the two quantities C 
and S must occur in the same combination as in equation (4). For 
+a 
if the new equation contains e.g. (, ie. f («) cos 4 ar le dx, it would 
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