Double Suspension Mirror. 485 



So far we have been concerned with sensitiveness as 

 obtained by measuring deflexions ol the beam itself. If now 

 we consider a b to represent a mirror whose deflexions are 

 to be observed, a little further consideration is necessary. 

 By (2) the deflexions </> of the mirror and ifr of the beam 



have the relation — = 7 , and our new sensitiveness S. will 

 y a 



S 



be similarly related to S^ or ^~ = j. S^ then represents the 



original sensitiveness multiplied by the magnification - which 



becomes great when h is small. From this combined with (5) 

 we obtain 



y \ pi p 

 which is a rectangular hyperbola whose centre 1? fig. 7, is 



k 2 

 on O/i at distance — from 0. It lies vertically below 0' the 



(7) 



-Fig- 8. 



2 



O r 100° 

 h*o 



80° 



48-8 



seer 



340' 



centre of the hyperbola giving deflexions of the beam under 

 the same conditions of experiment. The two hyperbolas 



