140 THE INTERFEROMETRY OF 



The fringes in coincident spectra should first be sought. When found 

 they will usually be very small. They may then be centered by rotating 

 the mirror m about the horizontal axis, cautiously. They may be enlarged 

 by rotating m with the same caution around the axis parallel to its trace in 

 the diagram. The latter operation (or both) succeeds best with the achro- 

 matic fringes of a wide slit (no spectroscope). The horizontal axis is employed 

 for erecting them, the axis parallel to the diagram for enlarging them, use 

 being also made of the micrometer-screw at d on H s , figure 88, if the fringes 

 escape. In fact, the achromatic fringes are so sharp and luminous that in 

 a quivering system the fringes are distinctly seen at the two elongations, 

 doubled by the quiver. 



To adequately support the mirrors m and m' reasonably free from vibra- 

 tion, a rectangular wooden yoke F F, figure 97, is best, so long as the instal- 

 lation is not permanent. This yoke is made of inch boarding about 18 inches 

 high, 15 inches broad, and 3 inches deep. It is bolted below at d to the iron 

 carriage 5, and thus capable of rotation around a vertical axis and of slid- 

 ing normal to its face on the iron slides BE' (about 1.5 meters long) of the 

 base. 5 and B B' are arranged like a lath-bed. 



The mirror m', which is horizontal, is adjustably attached to the top of 

 the frame F F; three adjustment screws (two seen at a, a') and the strong 

 spring 5 (pulling upward) control the position of m' with the usual plane dot- 

 slot mechanism and axes of rotation parallel to and normal to the plane of 

 the frame. 



The board carrying the mirror m may be roughly set at 45 to the 

 vertical by rotation on the strong bolts bb' and clamped in position. The 

 mirror m is attached to it by three adjustment screws and a rearward-acting 

 spring, as shown in the case of m'. The axes of rotation are parallel to the 

 two edges of m, and all fine adjustment, together with the final rotation of 

 fringes and changes of their size, are made here. 



The columns of the U -tubes C and C' must be supported from an inde- 

 pendent bracket, suitably braced from the wall or from a separate pier, if 

 liquid surfaces are in question. A part of this free support, the table, also 

 of wood, appears at D, and holes are cut in it sufficiently large for the passage 

 of the two beams of light a, a', which retrace their paths between m and m', 

 passing through C and C'. The table D is provided with three leveling screws 

 (two shown at c and c'} on which a plate of thick glass G may be mounted and 

 made accurately horizontal by aid of a spirit-level. The two columns C and 

 C', joined by a flexible rubber pipe and provided at the bottom with glass 

 windows, are placed on G, so that the beams of light pass through their axes. 

 The magnetizing helix of each is shown at e and e'. 



It is obvious, therefore, that the two columns of liquid in C and C 1 are of 

 the same height and their ends parallel, other things being equal. Hence, if 

 the apparatus is adjusted for the achromatic fringes in the absence of the 

 U-tube C C', it will be nearly in adjustment when the columns are introduced; 

 and this proved to be the case. The rigid system is easily adjusted for strong 



