202 
hence S(/). Evidently 42/ has been taken for «. Now equation 
(6) passes into: 
PAD 1 | C (U) cos Aar le dl + 4 js (einAnledl . . (7) 
0 “0 
Accordingly the function (we) will be known, when we know C 
and S as functions of £, hence when we can determine C and S 
experimentally for every value of /. 
Data to attain this may be derived from “the curve of visibility” 
Jota —J nin 
of MrcneLsoN, whose coordinates he defines as VV —=— (in 
nur: + met 
which Jar, and Juin. represent the intensities in the successive maxima 
and minima of the system of interference fringes). It appears from 
(4) that V is a function of C(l) and S(/); when we assume V(U) 
to be sufficiently accurately known from photometric observations, 
we have at our disposal a relation between C(/) and S(/), but without 
more data we cannot determine these quantities separately. Only 
in a few simple cases which contained a second condition concerning 
(U) or SU) has MrenersonN derived the form of g(x) from that of V(!). 
The aim of our investigation is to find a means through which 
it is possible to find a second relation between C(/) and S(/) in 
any given case, and which therefore enables us to solve ¢(2). 
In equation (4) we can think J(l) experimentally determined 
+a 
for a given value of J. Also | ple) de can be measured, this is viz. 
—a 
the intensity that one of the beams causes in the middle of the 
field of vision, when it is not brought to interference with the other. 
In order to determine this quantity we have, therefore, only to cover 
one of the mirrors. Hence equation (4) can be considered as an 
equation with two unknown quantities C and S, to be taken for 
that value of 7 for which / has been measured. 
We shall have to find a second equation between C' and S for 
the same value of / to be able to solve both quantities. This 
means will fail, however, when in the second equation C and S 
appear combined in the same way as in equation (4), accordingly 
+a 
when they are again derived from fu (x) cos 4 a l (m+ 2) de, 
en Cl 
in which a variable parameter is meant by /, which need not have 
the same physical meaning as just now. : 
