84 



DISPLACEMENT INTERFEROMETRY BY 



(above) . The line / 1 ' of the horizontal pendulum can thus be given any incli- 

 nation to the vertical, while the rods p,p' which receive the screws 5,5' 

 may be moved mormally 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 inclosed 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 

 h,h' of the pendulum, adjustably, so 

 that it can be moved up or down, and 

 rotated slightly above a vertical and a 

 horizontal axis. 



The interferometer consists essen- 

 tially of the 4 plate-glass mirrors M,M', 

 N, N', all but M being half -silver, the 

 collimator (beyond L) and the tele- 

 scope at T or T r , t" being a telescope 

 support. The collimated white beam 

 L is thus separated into the component 

 rays LNmadt and LbN'mcT, to be 

 observed at either T or T'. M' is on a 

 micrometer slide with the screw normal 

 to the face of the mirror. All mir- 

 rors must 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, 

 as specified above (Chapter IV, 43). Otherwise the fringes will be weak 

 or invisible. 



The telescope T should be provided with an ocular micrometer (centimeter 

 divided in tenth millimeters) standardized by aid of the sliding micrometer 

 at M', since the main purpose here is the measurement of small angles. More- 

 over, the image of the wide slit of the collimator adapted to the use of the 

 achromatic fringes should be placed at right angles to them, with the ocular 

 micrometer so placed as to read from end to end of the slit-image. A very 

 fine wire beam across the slit gives the fiducial line relative to the ocular 

 micrometer. Figure 59 shows the general arrangement, 55' being the wide, 

 oblique slit-image, //' the achromatic fringes, w the image of the fiducial wire 

 across the slit, and ss r the ocular scale. Of course the fringes may be made 

 horizontal or vertical ; but this requires much adjustment or else compensa- 

 tion, and is therefore an unnecessary complication of the preliminary work. 

 With this fiducial mark at the collimator (which is permanently out of reach) , 

 if the telescope is accidentally shifted, or temporarily removed, it maybe 



