ACOUSTICS AND GRAVITATION. 



67 



If the velocity is expressed in miles and seconds, or if v= 1.87X10' and 

 2 A N cos i = X, G may be computed in miles per fringe. This gives G = 0.0045 

 miles or 48 feet per fringe as the sensitiveness for light- waves for the given 

 6= 10 cm. The experimental datum below is well within this. 



60. The inclined revolving mirror and interferometer. It is obvious that a 

 method totally different from the preceding will have to be resorted to. Lens 

 systems in the light-paths are treacherous complications, to be avoided. 

 Parallel rays, preferably sunlight, should be used directly, if possible without 

 the intervention of glass condensers of any kind, unless it be in front of the 

 interferometer before rays separate. The solar image in a telescope of mod- 

 erately high magnifying power (15 to 2 5) subtends a sufficiently large angle and 

 is intense enough to be used directly. If a large field is needed the image 

 may be enlarged by putting it out of focus, and interferences of almost equal 

 distinctness obtained, provided the pencils of the two washed images are 

 quite coincident. This method of using images out of focus is here of con- 

 siderable importance; for fringe fields may be obtained in this way from images 

 which (because of the long distance traversed) in sharp focus would be mere 

 points. Finally, if the revolving mirror is so inclined as to move virtually 



f " * h^- 



t 



^'. 



95 





on the surface of a cone instead of a cylinder, its advantages are retained, 

 while rays are returned, only in the position for measurement. Figures 94 

 (plan) and 95 (elevation) will make these points clearer. 



Here L is a bundle of white parallel rays, preferably direct sunlight, from a 

 sufficiently wide heliostat mirror, silvered in front. M,M',N,N f are the mirrors 

 of the quadratic interferometer, all but M' being half -silvered and M and A^ 

 being equally thick. All mirrors are adjustable on three leveling-s crews, as 

 usual, and M' is on a micrometer with the screw in the normal direction s. 

 C and C' are thick glass-plate compensators, capable of rotating on a horizontal 

 and a vertical axis. The rays from L thus fall in two component beams on the 

 revolving mirror R, inclined at 45 to the horizontal. The prism carrying R 

 is mounted on the table tt, about a foot in diameter, capable of rotating with 

 .great speed around the axis A, which in practice is the shaft of an electromotor. 

 After leaving the mirror R the two beams are reflected by the adjustable 

 stationary mirror m, also at 45 to the horizontal, along the long path (10 

 meters) 5 S f to the distant mirror m' normal to the rays. They are thus re- 

 turned along the paths SmRN'T and S'mRNT and enter the telescope at T. 

 When R is stationary and in the position of the figure, and when T magnifies 

 sufficiently, two coincident images of the sun, quite intense and filling nearly 

 lialf the field, may be obtained, capable of interference. 



