Am.  Jour.  Pharm. 
.  Jan.,  i£8o. 
Poiarimeter  and  its  Use. 
47 
•which  I  am  acquainted,  efficient  and  inexpensive  ;  it  is  the  instrument  now  before 
you,  the  one  with  which  my  observations  have  been  made,  and  which  I  have  com- 
pared with  those  of  Wild,  Laurent  and  Duboscq,  with  very  satisfactory  results. 
The  instruments  of  the  last  named  maker  still  retains  the  double  quartz  plate  of 
Soleil,  but  dispenses  with  the  compensator,  having  been  fitted  with  a  Jellett's  pri^^in 
as  analyzjsr  on  a  suggestion  made  by  him  in  1869. 
That  of  Laurent  has  as  its  special  feature  the  polarized  ray  passed  through  a  dia- 
phragm with  circular  opening,  one  half  of  which  is  covered  by  a  plate  of  quartz, 
the  division  of  the  field  by  this  means  giving  great  precision  to  the  readings  ;  the 
analyzer  is  an  ordinary  Nicoi's  prism.  By  this  means  the  optical  work  is  simplified 
as  compared  with  the  old  form  and  the  perfection  of  working  is  enhanced.  Wild's 
polaristrobometer,  manufactured  by  Hermann  and  Pfister,  of  Berne,  is  a  special 
form  of  the  instrument.  It  is  somewhat  elaborate  in  construction  ;  the  readings  are 
taken  at  the  disappearance  from  the  centre  of  the  field  of  certain  lines  or  bands 
which  cross  it  ard  which  are  produced  by  two  plates  of  calc  spar  crossed  at  right 
angles  to  their  principal  faces.  Those  who  work  with  this  instrument  speak  of  it  as 
giving  very  satisfactory  results.  In  187a,  Professor  Jellett,  in  a  paper  read  before 
the  Royal  Irish  Academy,  described  a  "  new  optical  saccharometer,"  an  ingenious 
arrangement  by  which  the  polarized  ray  is  made  to  traverse  a  fluid,  the  rotatory 
power  of  which  is  previously  determined  and  which  is  opposite  in  character  to  that 
of  the  fluid  to  be  examined.  In  general  terms  it  might  be  described  as  an  instrument 
by  means  of  which  the  relative  rotatory  power  of  any  transparent  fluid  to  that  of  a 
•standard  fluid  may  be  accurately  determined.  Although  delicate  in  its  results  it  is 
5omewhat]troublesome  in  working  and  does  not  appear  to  have  come  into  general  use. 
Originally,  ordinary  daylight,  or  that  from  an  Argand  lamp  was  used  ;  but  on 
discarding  the  more  complicated  instrument  of  Soleil,  with  its  compensator,  where- 
by the  decomposition  of  the  light  due  to  the  unequal  refrangibility  of  the  diffeient 
rays  was  overcome,  monochromatic  light  was  adopted.  Different  operators,  how- 
ever, used  different  colored  rays  with,  as  a  matter  of  course,  different  results  ;  hence 
it  became  necessary  when  stating  the  rotatory  power  of  a  body  to  indicate  by  what 
ray  the  reading  was  taken,  and  this  still  obtains  to  a  large  extent;  thus,  in  the 
"Agenda  du  Chemiste  "  last  year  there  are  four  tables  giving  the  rotatory  power  of 
76  bodies — 12  by  the  "  teinte  du  passago^''  7  by  the  red  ray,  10  by  the  yellow  and  20 
without  any  indication  as  to  the  ray,  and  the  remainder  indicated  by  letters  corres- 
ponding to  certain  Fraunhofer  lines,  as  used  by  the  authorities  from  whom  the  results 
are  quoted.  It  is  true,  we  have  a  factor,  '767,  by  which  to  multiply  the  values 
obtained  by  the  yellow  ray  to  convert  them  into  those  which  could  be  obtained  by 
the  red,  but  it  has  been  shown  that  this  is  not  constant  for  all  bodies.  Further,  one 
object  of  a  table  is  to  show  at  a  glance  without  calculation  the  relative  rotatory 
power  of  different  bodies;  now  this  clearly  cannot  be  the  case  with  such  tables  as 
those  referred  to.  This  chaotic  state  of  things  is  to  some  extent  in  process  of  recti- 
fication, and  modem  instruments  are  all  constructed  with  a  special  view  to  their  use 
with  the  yellow  flame,  corresponding  to  the  li;.e  D  of  the  spectrum,  or,  in  other 
words,  with  a  Bunsen  flame  containing  a  salt  of  sodium.    This  gives  a  grey  field 
