218 
my 
2 fx (m) sin Aar Um dm = —fZ' (dl) + kf Zmar. « - (20) 
ma 
In this Zijar is the measured ordinate of the highest point of the 
Z-curve; the bottom point of this ordinate was the zero-point J, 
from which c/ must therefore be measured along the axis of abscissae 
of the Z-curve. When a line is drawn parallel this axis and at a 
distance 4 Znax from it, parts will remain of the ordinates the 
length of which will again be a function of el, which function we 
shall call I (cl). 
According to (19) and (20) we now have: 
2 
I (d)=27 (a) — § Limar, = — fx (m) cos 4a lmdm . . (21) 
fe 
When the same line is drawn through the 7 curve, the remain- 
ing parts of the ordinates will represent a function II (c/’) so that: 
ye 
TI (el) =Z (cl) — A Liman — — = | (m) sin4al’mdm. . (22) 
i En 
The zero point N' on the axis of abscissae of the Z' curve may 
be determined on the following consideration. The function II (cl) 
is zero for /'=0O, but has several zero points, so that it is not yet 
known which of them is to be taken as N'; it appears, however, 
from equation (22) that the function is uneven in /'. When, therefore, 
of the function I] we determine the zero point with respect to which 
the other zero points lie symmetrically, we have found the point 
cl =0, hence the zero point N' on the axis of abscissae. This is 
the zero point, from which c/' must be measured on the axis of 
the Z'-curve. Especially for multichromatic light this symmetry will 
be very apparent. The reasoning involves that observations must 
have been made also for negative values of /', ie. that we must 
start with putting the movable prism somewhat nearer than would 
correspond with /’—0O, hence with /=— d. When we, therefore, 
begin with placing the prism at a distance greater than d before 
the plane of reference 2, and moving it from there with uniform 
velocity, we begin with a negative value both of / and of /', so 
that the initial points of the bands have negative abscissae. 
We may cursorily remark that the quantity v= d can be accurately 
determined from the situation of the zero points NV and WN’. 
The photographie plate was namely every moment struck by two 
beams of light, the intensities of which depended on two parameters 
