PHYSICS: a BARUS 
477 
M, for successions of 40 fringes replacing each other at the sodium lines, 
show the order of results: 10~^e = 1.55, 1.40, 1.60, 1,55 cm., so that 
per fringe 
be = Z9 X 10-6 
The computed value would be {(p, the prism angle) 
be = X/2cos(5/2) = 58.93X10-72 XO.81 = 36.4 XlO-^ cm., assum- 
ing b = 90° -(p. The difference is due both to the small fringes 
which are difficult to count and to the assumed value of b. The range 
of measurement is small (if M only moves), not exceeding 1.6 milli- 
meters for a moderately strong telescope. Usually but one half of this 
displacement is available as the fringes increase in size (usually with 
rotation) from fine vertical hair lines to a nearly horizontal maximum 
and then abruptly vanish. But one half of the complete cycle is thus 
available. 
If we regard the component beams, a, b, c, and a\ b\ c' as being of the 
width of the pencil diffracted by the slit of the collimator, it is clear that 
the maximum size of fringes will occur when c and c' are as near together 
as possible: furthermore, that as M moves toward P', c continually 
approaches c', until h drops off (as it were) from the right angled edge 
of the prism P' . To get the best conditions, i.e. the largest fringes, c 
must therefore also be moved up to the edge of P and very sharp angled 
prisms be used at both P and P' . The largest fringes (lines about 10 
times the BiDi distance) obtained with the right angled prism were 
often not very strong, though otherwise satisfactory. Much of the light 
of both spectra does not therefore interfere, being different in origin. 
Results very similar to the present were described long ago^ and 
found with two identical half gratings, coplanar and parallel as to rul- 
ings, etc., when one grating was displaced normal to its plane relative 
to the other. The edges of the two gratings must be close together, 
but even then the fringes remain small and the available paths also. 
Strong large fringes, but with small paths, were obtained by the later 
method^ of two identical transmitting gratings, superposed. 
If the prism P' is right angled, it may be rotated as in figure 2, so 
that the rays c and c' pass off towards the rear. They are then observed 
through an Ives' prism grating G and a telescope at T. This method 
admits of much easier adjustment. With the component beams, 
a, 6, a\ h\ coplanar, horizontal and of about equal length in the absence 
of the prism P' , the latter is now inserted with its edge vertical (rotation) 
and the white sHt images in T (without G) superposed, horizontally and 
vertically. G is then added and the micrometer at M or N manipu- 
