of Methods for Observing Small Rotations. 93 



been stated that the former has a considerable advantage in 

 requiring only two portions, B and D, at the ends of a 



Fi<r. 4. 



H-» 



k- 



■\ 





1/ ' ' 



B 





D 



v — > / 



diameter, instead of the full mirror. But this advantage 

 disappears if we simply cut away those portions of the mirror 

 outside of the rectangle enclosing B and D*. When this is 

 done the simple mirror becomes considerably more efficient 

 than is indicated by the preceding comparison. This is 

 because the effective aperture b of the mirror is the distance 

 between the extreme edges, while the aperture V of the other 



then becomes possible to estimate and measure angular movements cor- 

 responding to less than j^ of a fringe, or less than i the amount 

 indicated as measurable in the case of a freely suspended system. For 

 such purposes, also, the interferometer possesses other considerable ad- 

 vantages in the way of lightness of the moving parts and (at least for 

 very large apertures) cheapness. 



* The advantages of this form of mirror, as compared with the full 

 circular mirror, have also been pointed out by Boys, who used it in his 

 work on the determination of the Newtonian Constant. (See memoir 

 already referred to, p. 32.) 



