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LXXII.  Note  on  the  Focometry  of  Concave  Lenses  and  Convex 
Mirrors.     By  Prof.  A.  Anderson,  M.A.,  LL.D* 
TO  those  who  are  interested  in  neat  ways  o£  making- 
physical  measurements,  the  following  methods  of 
finding  the  focal  length  of  a  concave  lens  and  of  a  convex 
mirror  may  be  of  some  interest. 
They  have  not,  I  think,  been  noticed  before.  They  are 
pin-point  methods,  and  as  easy  in  practice  as  the  well-known 
pin-point  method  of  finding  the  focal  length  of  a  concave 
mirror.  For  the  measurement  of  the  focal  length  of  a 
concave  lens  a  concave  mirror  is  required,  and  for  that  of  the 
focal  length  of  a  convex  mirror  a  convex  lens  is  required. 
Fig.  1. 
The  figures  explain  the  methods.  The  pin-point  is  placed 
at  P,  and  the  adjustment  consists  in  making  it  coincide  with 
its  image,  in  which  case  the  rays  of  light,  after  passing  through 
the  lens,  diverge  from  (fig.  1),  or  converge  to  (fig.  2)  the 
centre,  O,  of  the  mirror.  In  both  cases  the  mirror,  when 
properly  placed,  causes  the  rays  to  retrace  their  paths  in  the 
opposite  direction. 
Note. — I  have  since  found  that  a  method,  which  is  practically 
identical  with  that  shown  in  fig.  2,  is  given  in  Edser's  'Optics,' 
p.  117. 
*  Communicated  by  the  Author. 
