in a new Form of Ref Tactometer. 241 



It can readily be demonstrated that the path of the rays in 

 the in strument is equivalent to that given in the figure, Avhere 

 Fio-. 4. 



one of the rays follows the path q n m h, and the other the path 

 vol h. Suppose the mirrors c d and ef parallel. Then, as 

 has been previously shown, the curves of interference are 

 concentric circles formed at an infinite distance. Therefore 

 the rays q n, r o, whose path is to be traced, are parallel, and 

 from the point h they coincide. Their difference of path is 



2nm — 21d—op ; 



and their difference of phase is 



, 2nm 2hl op 



whence 



_ 2n . t 

 Xcosi 



2t r 



2n.t 2t, , . , N . . 



■ — (? van i— tan r) sin i 



\cosr \ v y 



■ncosr] (7) 



Let it be proposed to find the value of n which renders 

 any particular ring achromatic. The condition of achroma- 

 tism, according to Cornu, is 



dX 



which gives 



j. i li,/ dn • dr dn\ r . 



•0, 



