Interferometer to the Horizontal Pendulum. 503 



the surrounding city. The slow normal variations were not 

 greater than 5 millimeters on a radius of 13 meters, correspond- 

 ing therefore to about 40 seconds of arc. The corresponding 

 change of inclination relative to the plumb line would be less 

 than 1/100 of this, depending on the period given to the hori- 

 zontal pendulum. 



The mass of the pendulum was 720 grams ; that of the grat- 

 ing holder originally 475 grams, and of the grating, etc., about 

 55 grams, making a total of 1250 grams ; but these masses are 

 to be much modified in the future. The center of gravity, at 

 G, with the grating in place, was originally about 80 centi- 

 meters from the axis AB. 



Fig. 3. 



The grating at g moves between the two opaque mirrors 

 (fig. 2) usually called M and N of the displacement interfero- 

 meter, in the way shown in my earlier work* on interferometry. 



But these mirrors M and N must in the present case be iden- 

 tically concave, silvered on their front faces, and at a distance 

 equal to their common radius of curvature from the center of 

 the ruled face of the grating. This center is illuminated by 

 the impinging beam of light from the collimator, and the 

 returned beams, reflected from M and iV, must pass through 

 the same area of illumination. In such a case the reflection at 

 JTand JV is always normal to those surfaces and the rotation 

 * See Carnegie Publications, No. 149. 



Am. Jour. Sci.— Fourth Series, Vol. XXXVIT, No. 222.— June, 1914. 

 35 



