Am.  Jour.  Pharm.  \ 
November,  1894.  J 
A  New  Diaphragm. 
523 
A  NEW  DIAPHRAGM. 
By  Hans  M.  Wilder. 
When  we  wish  to  study  an  object  by  the  microscope,  we  can 
make  use  of  four  kinds  of  illumination  :  (1)  Ordinary  day-light  or 
lamp-light.  (2)  The  same  intensified  by  a  condenser.  (3)  Dark- 
ground  illumination.    (4)  Polarized  light. 
Hitherto,  in  changing  from  one  illumination  to  another,  it  has 
been  necessary  to  consume  not  a  little  time  in  exchanging  (respec- 
tively, inserting)  the  different  lenses  required,  and,  after  using  them, 
to  go  through  the  same  process  in  inverse  order.  This  is  tedious 
and  time-consuming,  not  to  mention  that  one  is  apt  to  forget  what 
he  noticed,  especially  with  the  other  kinds  of  illumination. 
My  diaphragm  obviates  all  this  trouble,  and  makes  any  necessary 
change  as  easy  as  winking  the  eye.  Instead  of  unscrewing  and 
again  screwing  on  the  various  lenses,  a  mere  quarter  of  a  revolution 
of  the  wheel  of  the  diaphragm  effects  the  change.  For  polarization 
an  additional,  but  quickly-made  motion  becomes  necessary.  The 
ordinary  analyzer  is  replaced  by  a  tourmalin,  which,  suitably 
mounted,  is  slipped  over  the  eye-piece  and  revolved.  It  has  the 
additional  advantage,  that  whilst  the  nicol  very  much  restricts  and 
circumscribes  the  field  of  view,  the  tourmalin  gives  an  unobstructed 
field.  The  modification  of  the  colors  by  that  of  the  tourmalin  is 
hardly  noticeable. 
The  diaphragm  is  the  usual  disk  (or  wheel)  pierced  with  three 
large  holes  and  one  smaller  one.  The  last  is  for  ordinary  light. 
Into  each  of  the  three  larger  holes  is  fitted  a  short "  sleeve,"  into  which 
again  fits,  a  little  tightly,  a  short  cylinder.  In  one  cylinder  is  fitted 
a  hemispherical  lens,  serving  as  condenser ;  into  the  second  is  fitted 
a  similar  lens,  having  on  its  plane  side  a  small  disk  of  black  paint, 
the  size  of  which  has  to  be  arrived  at  experimentally,  so  as  to  give 
a  good  black  field  (the  light  coming  around  the  circumference  gives 
dark-ground  illumination) ;  in  the  third  is  fitted  a  nicol  (polarizer) ; 
a  hemispherical  lens  on  top  of  the  nicol  will  increase  the  brightness 
of  the  colors. 
I  use  "hemispherical"  lenses  in  preference  to  the  usual  "piano- 
convex,"  because  the  focus  of  the  former  is  much  shorter.  The 
condenser  may  be  made  to  give  a  perfectly  blinding  light,  suitable 
for  the  most  difficult  bacteria,  and,  by  gradually  withdrawing  the 
tube,  the  light  can  be  softened  down  to  almost  any  extent,  using  in 
