108 THE INTERFEROMETRY OF 



The vertical columns of this tube are shown at C and C' (with accessory 

 mirrors removed) , and they are joined to the capillary tube p near the bottom 

 of C and C'. Details will be given in connection with figures 71 and 72. 



The ray HM strikes a mirror symmetrically at 45 to the vertical below C, 

 is thence reflected upward along the axis a, striking another mirror above, 

 also symmetrically at 45 and parallel to the former, whence it is reflected 

 to the opaque mirror M. The latter reflects the ray normally back, so that 

 it retraces its path as far as H, by which plate it is now transmitted to be 

 observed by the telescope at T. Similarly 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' satisfactorily, 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 im- 

 mediately in front of the objective and swiveled as described in figure 64, 

 Chapter IV, 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 71 a front sectional elevation of one of the shanks of the U-tube 

 is given with all appurtenances, and a similar sectional elevation at right 

 angles to the former is added in figure 72 for the top of the tube. In figure 7 1 

 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 72. 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 5 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 mirrors m, m' rotates 

 at pleasure in the vertical arms A, A' of square brass tube. A, A' in turn may 

 be slightly swiveled about the horizontal axis b, b', in a rigid lateral projection 

 of the rings r, r'. Thus m, m' are capable of rotation around three axes normal 

 to each other and adequately clamped in any position. 



The component ray HN may be adjusted to the center of the lower mirror 

 m' by placing the collimator L and then guided axially by m', m, N as described, 

 each being adjustable. The component ray HM may be similarly adjusted 

 to the center of the lower mirror m' (at 45) by slightly rotating the half- 

 silver plate H (on horizontal and vertical axes) and then guided axially by 

 m', m, M. As a whole the adjustment is difficult, though it need not be much 

 refined. Clear white slit images in the telescope T are an adequate criterion. 

 In the absence of a liquid in CC, figure 71, the fringes are easily found after 



