350 
PHYSICS: C. BARUS 
ment of the micrometer. The inclination of the line of suspending pivots 
was here about 1° to the vertical. A smaller angle would have correspondingly 
increased the sensitiveness. 
The apparatus, however, required a space about two meters long between 
the extreme mirrors, for its installation. This is a disadvantage, since small 
changes of temperature in the brackets and supports as well as in the pier 
would interfere with the full realization of the precision of the method. The 
rectangular interferometer with an auxiliary mirror is thus to be preferred; 
for here all the necessary parts may easily be placed within a distance of one 
foot from the wall of the pier carrying the horizontal pendulum. If the 
achromatic fringes are used, these are straight and intense, so that photographic 
methods are available. 
2. Apparatus. — The old horizontal pendulum made of thin steel tubing 
formerly described (1. c.) was again used. 
Figure 1 gives a sectional plan of the pendulum installation, and figure 2 a 
front view of the pendulum HH alone, in a somewhat reduced scale. Its 
general shape is that of an isosceles triangle and the distance from the line 
of pivots //' to apex about 110 cm., while the distance between pivots was 97 
cm. These pivots S, S' are fine screws ending in hard steel points, which 
enter a glasshard steel socket (below) and a steel groove (above). The line 
U' of the horizontal pendulum can thus be given any inclination to the vertical, 
while the rods p, p' which receive the screws S, S' may be moved normally to 
the wall of the pier PP' , inward or outward, and clamped to secure parallelism 
between the pier PP' and pendulum HH'. The apex B of the pendulum is 
also provided with a clamp, holding vane D submerged in an oil vat, v, for 
damping. 
The whole pendulum is enclosed by a flat case, CC, of tin plate provided 
with a plate glass window at g, through which the auxiliary mirror m of the 
interferometer may be seen. This is attached to one or two vertical tubes 
hh' of the pendulum, adjustably, so that it can be moved up or down, and ro- 
tated slightly about a vertical and a horizontal axis. 
The interferometer consists essentially of the 4 plate glass mirrors, M , M' f 
N, N', all but M being half silvers, the collimator (beyond L) and the tele- 
scope at T or T', t" being a telescope support. The collimated white beam L 
is thus separated into the component rays L, e,m,e, a, d, T,and L, b,f, m, f, c, T, 
to be observed at either Tor T'. M' is on a micrometer slide (not shown) with 
the screw normal to the face of the mirror. All mirrors should be capable of 
slight rotation about horizontal and vertical axes and the silvered faces all 
lie towards m for compensation of glass paths. The rays leaving M' for T 
must not only be accurately parallel but locally (visible as spots of light) 
nearly coincident; otherwise the fringes will be weak or invisible. 
The telescope T is provided with an ocular micrometer (centimeter divided 
in tenth millimeters), standardized by aid of the sliding micrometer at M'. 
Moreover the image of the wide slit of the collimator adapted to the use of 
