616 
PHYSICS: C BARUS 
a remarkable result, showing that the displacement of the mirror M 
per fringe is independent of wave length and equal to half the grating 
space. An interferometer independent of X and available throughout 
relatively enormous ranges of displacement is thus at hand. It appears 
that it is also independent of the angle of incidence at G. 
In case of the given grating and sodium hght 6 = 19° 37'. Hence 
if oe is the displacement per fringe- 
e = X/2 sin 9 = 10~' X 88 cm. 
Actuating the micrometer at M directly by hand the following rough 
data were successively obtained from displacements corresponding 
to 10 fringes: 
W X 8e = 65 95 90 80 60 cm. 
Without special precaution the fine fringes can not be counted closer 
than this, so that the data are corroborative. 
The fore and aft motion of produces no separate shift of fringes 
while the fringe bearing strip is displaced as a whole in mean wave 
length. Figure 1 shows at once that if G' were displaced to G\, the 
X rays b h' would lose their coincidence in T, while that property would 
now be possessed by the X' rays d d' , But the same path difference 
is added to both d and d' . The ratio of corresponding displacements 
at M and G' is tan ^ : 1. Equation (1) is of interest in interferometry, 
in view of the very long ranges of displacements available. For such 
purposes gratings of lower dispersion (preferably ruled gratings or 
else prisms) may be used to obtain greater luminous intensity in the 
spectrum. Path difference may also be introduced by compensators. 
If a thin sheet of mica is moved in either h or h' there is a lively skirmish 
of fringes, but they do not change size appreciably. A plate 2.8 milli- 
meters thick with strong fringes horizontal in the yellow, if placed in 
the h' pencil produces hair lines inclined toward the left in the red; 
if placed in the h pencil, hair lines inclined to the left in the green; etc. 
Plates v/ere tested up to 2 cm. To fully exhibit their effect it is neces- 
sary to produce the elliptic fringes, presently to be referred to. The 
shift from red to green, if produced without compensators by the dis- 
placement of M, shows scarcely any variation of fringes, either as to 
size or inclination. 
To change the size of fringes it is easiest to rotate the grating G' 
(relatively to G) on a horizontal axis normal to itself. They then both 
rotate and grow coarser, usually attaining the maximum of size when 
the fringes are vertical. Fringes quite large and black may be obtained 
