110 THE INTERFEROMETRY OF 



traced. A vertical wire 2 or 3 mm. thick, placed symmetrically in front of 

 the objective of the telescope, makes the interference relatively strong and 

 sharp, by blotting out the third spectrum partially; but it at the same time 

 diminishes the light available. A wide slit in front of the objective subserves 

 the same purposes better. 



If the distance apart of the mirrors M and N and the grating G is large, it 

 is best to dispense with the rail RR altogether, and to mount the mirrors and 

 grating directly on the pier or wall. This has the additional advantage of a 

 large free space between M and G or G and N, so that spacious apparatus like 

 a fog-chamber may be independently mounted there. This was the case in 

 the optic experiments on the thermal coefficients of the refraction of air, etc., 

 below, where the distance between MG and GN was nearly 2 meters. In such 

 a case, moreover, in addition to the usual three adjustment screws of the 

 mirror M at the micrometer, it is desirable to have two others bearing on the 

 rigid parts of the support, so that the final adjustment may be made elastically. 

 By devising a tetrahedral plan of bracing M, G, N, independent of each other, 

 using short rods and clamping all parts on relatively short stems, I eventually 

 obtained a mounting which was almost free from tremors, even amid the dis- 

 turbances of the surrounding laboratory. In figure 79 one of these mount- 

 ings is suggested: a and b are ^-inch gas-pipes (about a foot long), sunk into 

 the wall of the pier at their rear ends ; cd is a cross-rod of same size and material, 

 clamped in place, and supporting the grating (or a micrometer) G. The 

 screw h abutting against the wall gives the horizontal elastic adjustment. 

 The braces e and /, which may be adjusted by rotation (screw) abutting in g 

 at the wall, give the grating vertical elastic adjustment. Thus h, e, and /, 

 rotate G around vertical and horizontal axes, respectively. 



60. Michelson's interferences. If the collimator, SL, is removed and 

 replaced by a strong sodium flame provided with a condenser, Michelson's 

 interferences will appear at T when the instrument is in adjustment. It is 

 rather surprising that, even in case of a film grating adjusted as above, they 

 are well-defined circles covering the whole field of the telescope. If the col- 

 limator SL is retained and the sodium light introduced from the side by aid 

 of a reflecting mirror, placed between the grating G and the collimating lens 

 L, both interferences may be observed at the same time in corresponding 

 telescopes. The mirror introducing the homogeneous light should in such a 

 case be provided with a clear space (silver removed), through which the white 

 beam, SL, may pass without obstruction. In a vertical plane the interferences 

 have the same size and character at the sodium line. Horizontally the spec- 

 trum interferences vary with the dispersion. 



If an apparatus constructed of gas-pipe is employed, however, it is far too 

 frail for the practical use of the Michelson interferences. Vibrations within 

 the apparatus are excited on merely touching it. For the purpose of displace- 

 ment interferometry, however, such an apparatus is quite adequate; for the 

 measurements are taken when the tremors have vanished. 



