PHYSICS: C. BARUS 
335 
ilarly the transmitted component ray HN is guided by suitable reflectors 
at 45°, so as to take the path Ha'Na'HT, thus passing axially {a') through 
the tube C. 
It is necessary that the U-tube CpC be mounted independently of 
the block B on suitable bracket or arm attached to the pier. Otherwise 
any manipulation at N will disturb the surfaces of water in C and C 
Ordinary clamps admit of raising or lowering or rotating CC satisfac- 
torily, always providing that it shall not touch B. The telescope at T 
is also mounted apart from B on the table below. The direct vision 
prism grating g is placed immediately in front of the objective and 
swivelled (as described, loc. cit.) so that either the white slit images or 
their spectra may be seen in the field of view, according as g is rotated 
aside or is in place. 
In figure 2 a front sectional elevation of one of the shanks of the XT- 
tube is given with all appurtenances, and a similar sectional elevation 
at right angles to the former is added in figure 3 for the top of the tube. 
In figure 2 the mirrors m' and m are on horizontal axes and the component 
ray coming from behind the diagram strikes m! below, is reflected axially 
upward through CC, impinging on the mirror m (also on a horizontal 
axis) whence it is reflected horizontally toward the front of the diagram. 
The ray a and mirror m are given more clearly in figure 3. The lateral 
capillary tube appears at p and the tube C is closed below with a plate 
of glass e, cemented in place. 
To mount the mirrors m, m' , snugly fitting rings r and r' encircle 
the tube C near its top and bottom and can be fixed by the set screws 
s and s' . In virtue of these rings the mirrors m, m\ may be rotated at 
pleasure around the vertical axis a of CC. The horizontal axis of the 
