PHYSICS: C. BARUS 
17 
and they happen to coincide. Thus Ae is 323 times as large as AN and corre- 
spondingly easy to measure. The impossibility of setting the micrometer for 
AN accurately enough, since it is graduated to only 5 X 10~ 5 cm.is completely 
obviated in Ae. Moreover, as 2 AN cos i = X (i being the angle of incidence 
45°, and X the mean wave length), we now have 0.0061/Ae cos i = X; so that 
the fringe displacement Ae = 0.014 cm. measured on the ocular micrometer 
corresponds to the wave length of light in the interferometer measurements. 
This is more than one scale part. There is however no difficulty in making 
the fringes larger and obtaining a much more sensitive apparatus in propor- 
tion. The achromatic fringes, moreover, when properly produced, contain a 
distinctive central black line, compatible with the measurement of 0.1 scale 
part, as here given; i.e. measurement to a few million ths of a centimeter are 
thus easily feasible under proper surroundings. The apparatus will be used 
elsewhere. 
If A(p, the angular fringe breadth, is given, Ae/AN may be computed from 
the equation in the earlier paper, to be 
Ae/AN = 2 LAp cos i/\ 
or 
LA<p = X/(2 cos i.AN/Ae) 
as the radius is the length L = 19.5 cm. of the telescope. Hence the fringe 
breadth in centimeters is, if 
X = 6 X 10- 5 cm., i = 45°, and WAN/Ae = 3.1, 
LA<p = 0.014 cm., 
the value actually observed. Thus if A<p is given or measured, Ae/AN may 
be deduced. 
The question finally to be determined is thus the value and the meaning 
of the fringe breadth A<p. Since 
2 AN cos i = ^X 
