CHAPTER I. 



ELLIPTIC INTERFEROMETRY APPLIED TO THE HORIZONTAL PENDULUM. 

 PART I. DIRECT DIFFERENTIAL REFLECTION. 



1. Introduction. Method. Before using interferences, it seemed inter- 

 esting to apply the interferometer adjustment to the case of simple reflection, 

 the mutual displacement of the two direct images from the front and rear 

 face of the mirror on the pendulum being used for the measurement of the 

 angle of deviation of the pendulum. In such reflection from a glass plate 

 there is necessarily considerable loss of light; but at radii of 20 and 30 meters, 

 when the source of light is a slit closely in front of a Nernst filament, this 

 difficulty is not prohibitive. 

 It is necessary, however, that 

 the lens of the collimator, as 

 well as the plates of glass and 

 mirror used, be of high opti- 

 cal quality; otherwise it is 

 impossible to obtain sharp 

 condensation at a very dis- 

 tant focus. One may also 

 concentrate the slit images to 

 a point by a cylindrical lens, cr ' 

 placed with its linear element 

 at right angles to the direc- 

 tion of the slit. 



The interesting feature of 

 the method is that it is inde- 

 pendent of any zero-point, as 

 the distance apart of the two images on the far screen at once measures the 

 inclination of the pendulum axis, the normal position being that in which 

 the two images coincide. If the two opaque mirrors are rigidly fixed, the 

 direct or incident beam of light from the source and the subsidiary reflecting 

 mirrors may shift without modifying the datum for the inclination. Further- 

 more, the sensitiveness is twice that of the case of single reflection, other things 

 being equal. The method is thus particularly adapted for the measurement 

 of the inclination of the plumb-bob relatively to the eaith. 



The annexed diagram, fig. i, will make the method clear. Here s is the 

 fine slit in front of the Nernst filament /, and / the condensing doublet (about 

 60 cm. focal distance; for rough work an ordinary spectacle-glass answers 

 very well) of the collimator. It is necessaty that this lens be wide, weak, 

 and good in order that a sharp focus and as little loss of light as practicable 

 be obtained at the distant focus. 



1 



FIG. i. 



