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is only a mathematical artifice, to which no physical meaning must 
be assigned. Points P,, which in their movement were first c,m 
behind compared with P,, and whose vector lay, therefore, at an 
angle c‚m on the righthand side of the time direction, will now be 
as it were c,m in advance of /,, and give a vector which again 
forms an angle c,m with the time direction, but now lies on the 
lefthand side of it. The angle between every vector and the time- 
direction being proportional to m, all the vectors will reflect in this 
direction, and the form of the fan, and with it the resultant, hence 
the intensity, will remain unchanged. Hence for any instrument in 
which light propagates normally, the intensity will be an even 
function of the number of waves. In this absolutely monochromatic light 
is supposed, because for compound light the intensity is no function 
of the number of waves on account of the integration with respect 
to m between definite limits. 
3. 
Now we shall, however, also admit reflections against a denser 
medium in the instrument. In this the phase shifts suddenly zr; 
such an abrupt change of phase we shail call a phase shifting. Let 
us suppose that some points P’ of the instrument are illuminated by 
rays which have undergone such a reflection an uneven number of 
times. Their vectors now no longer make the angle c’m, but the 
angle c’m + a with the time-direction. Before P, gave a vector by 
the aid of whose direction the time-direction was defined. When, 
however, the point P, belongs also to the points ?’, its vector 
will also turn over an angle x, hence obtain the opposite direction 
from what it had before. As the other group of points of the 
apparatus continues to receive normally propagated light, their vectors 
continue to form the angles cm with P’s former vector. 
When we now define the time-direction anew, and do so as the 
direction of the vector originating from that point P, of the apparatus 
through which the shortest optical way passes from the source of 
light Z to the point of observation Q, quite apart from all possible 
phase shiftings, the time-direction will always be the constant direction 
in the following considerations, with respect to which we determine 
the position of the vectors. We shall again think it to be always 
directed vertically upward. A reasoning based on this time-direction 
will therefore hold both for the case that this point P, receives light 
that has changed its phase, and for the case that normally propagated 
light falls on it. We shall let the constants c keep their value, because 
