150 
PHYSICS: C. BARUS 
2. Apparatus. — To fix the ideas it will be necessary to give a diagram of the 
apparatus (fig. 1) employed. It is the selfadjusting interferometer, very 
serviceable here because of the large number of separate adjustments to be 
made, each of which might otherwise require long searching for fringes. 
White light L from a collimator takes the paths 12345^ and 16785r, N being 
a halfsilver. The telescope T is provided with the direct vision grating g, 
capable of rotating around the axis ST (angle i^ip). T and g are preferably 
rotated together, as a rigid system. The mirror MM' consists of two inde- 
pendent, nearly coplanar parts, as shown, one of which, M for instance, may 
be displaced parallel to itself by the micrometer screw along the normal 5 
(displacement A N). Path difference to the amount 2 A iV cos 45° is thus 
introduced more than sufficient to pass the spectrum fringes through 
their maximum sizes between extremes of hair lines. By rotating w on a 
horizontal axis and m and M' on vertical axes, fringes of all sizes and 
inchnations when at their maximum may be obtained. The character of the 
fringes due to inclination is shown by the achromatics and hence the adjust- 
ment is made with reference to them. They depart but little, relatively 
speaking, from their slope throughout the experiment. 
3. Observations. — For the present purposes, the case of achromatic fringes, 
horizontal, vertical, and at about 45°, respectively, will suffice. Moreover, 
relatively small fringes, requiring much larger displacements (A N) than very 
large fringes, will generally be preferable. 
Figure 2 gives an example of the results for horizontal achromatic or mono- 
chromatic fringes, the ordinates showing the displacement of micrometer A N 
(at M fig. 1) in 10~^ cm., and the abscissas the corresponding rotation of spec- 
tro-telescope {gT, fig. 1), needed to produce sharp fringes in the spectrum of 
an indefinitely wide slit. When the fringes are small, a few degrees of exces- 
sive rotation A(p, either way, will cause them to vanish completely, so that the 
orientation for sharp fringes is quite sensitive. The symbols H (horizontal) 
and V (vertical) refer to the orientation of the edge of the prism, or the lines 
of the grating. The plane of dispersion is thus normal to H and V. 
