414 
PHYSICS: C. BARUS 
is in the field of the spectroscope. It may be necessary to actuate the 
micrometer screw at d to complete the adjustment. 
When the elhpses are centered, the direct vision spectroscope g re- 
moved, and the slit widened or removed, the residual or achromatic 
fringes appear in sight and are ready for use. These are always strong. 
The spectrum fringes are apt to be less so, since the parts of the ray L 
pass through two half silvered surfaces H1H2 or HiHz in succession.^ 
The spectrum fringes are sharp when the slit is fine. If the white resid- 
ual fringes are too dazzling a single or two half silvers may be placed 
before the objective of the telescope with advantage. Two plates with 
their half silvered sides in contact and held so by a steel clip, are excellent 
for this purpose while they are at the same time protected from sulphur 
corrosion. This in fact is the best method of preserving silver mirrors 
(in pairs) when not in use. 
When the spectra are in coincidence and the fringes sharp, the mirror 
m may be rotated around a vertical axis at A into some position, m'. 
In such a case the two spectra will move through the field of the tele- 
scope at r, but their coincidence will not be destroyed. The D lines, 
for instance, will continue to be superposed throughout. Considerable 
path difference is however introduced in this way and hence the fringes 
will march through the spectrum at an enormously more rapid rate. The 
following data may be given, where a is the angle of rotation of the mir- 
ror m and N the reading of the micrometer at (screw in the normal dn) 
necessary to bring the center of ellipses back to the sodium lines. In 
both cases the centers were out of the field (above or below), so that 
horizontal fringes were made the criterion for adjustment. This method 
is somewhat rough, but adequate for the present purposes. 
1. Fine thin fringes. Relatively large differential glass path. Dis- 
tance ah, figure \,2R = 21 cm. Thickness of glass plates (half silvers) 
e = .70 cm. 
a = 0° .05° .26° .30° .40° .50° .60° 
X 103 = 23 30 128 162 215 258 299 cm. 
This is curve a in figure 2. From it the mean rate 
AN 
— = .47 cm /degree, or 27 cm /radian 
Aa 
may be found. 
2. Coarse large fringes. Smaller differential glass path. 
a= 0° .1° .2° .3° .4° .5° .6° .7° .8° .9° 1.0° 
i\r X 103 = _25 +29 84 134 176 217 265 323 365 420 467 cm. 
