46 
Polarimeter  and  its  Use. 
A.m.  Jour.  Pharm 
Jan.,  1880. 
the  Other  half  to  the  left,  the  rotation  being  sufficiently  great  (90°)  to  decompose 
the  ray  and  to  produce  a  rose-violet  tint  uniformly  over  the  whole  field.  This  is 
known  as  the  sensitive  or  transition  tint,  also  the  tint  of  passage.  Secondly,  the 
analyzer  is  fixed  with  its  axis  corresponding  to  that  of  the  polarizer,  the  amount  of 
rotation  produced  being  measured  by  compensation,  effected  by  a  plate  of  quartz 
divided  into  two  wedges,  and  fitted  with  rack  and  pinion  motion,  by  which  they  are 
moved  over  each  other  so  as  to  increase  or  diminish  the  thickness 5  they  are  also 
attached  to  a  vernier  and  scale.  When  the  compensator  is  at  zero  the  whole  of  the 
disc  is  rose-violet,  but  the  Intioductlon  of  an  active  fluid  causes  one-half  to  become 
red.  The  compensator  Is  then  moved  through  a  sufficient  number  of  degrees  to 
restore  uniformity,  and  the  amount  of  rotation  is  thus  ascertained.  Actual  degrees 
are  not  marked  in  the  scale,  but  the  rotation  produced  by  a  plate  of  quartz  1  mm. 
thick,  equal  to  that  given  by  200  mm.  of  solution  of  sucrose  (i6*i9  grams  in  100  cc. 
of  water),  being  marked  on  the  scale  and  divided  into  100  equal  parts. 
The  Instrument  was  specially  constructed  with  a  view  to  its  use  for  sugar  solutions 
and  is  best  known  as  Soleil's  saccharimeter,  of  which  there  are  several  modifications, 
«uch  as  the  Soleil-Ventzke,  Soleil-Scheibler,  etc. 
Accurate  as  were  the  results  obtained  by  this  means,  there  were  some  difficulties, 
«uch  as  the  interference  of  colored  solutions  with  the  sensitive  tint,  the  shortness  of 
the  scale,  etc.,  which  have  caused  it  to  be  superseded  by  more  siinple  forms  in  which, 
as  in  Blot's  Instrument,  the  analyzer  is  made  to  rotate,  and  these  forms  have  been 
adopted  by  two  opticians  Dubo^cq  and  Laurent,  who  may  be  regarded  as  the  suc- 
cessors of  Soleil. 
In  i860.  Professor  JtUett,  of  Dublin,  described  to  the  British  Association  at 
Oxford,  a  new  analyzing  prism,  which  he  had  invented,  by  which  greater  accuracy 
could  be  obtained  than  by  any  previous  arrangement.  The  report  Is  as  follows  : 
^*  Professor  Jellett  described  to  the  section  a  new  anaij  zing  prism,  by  which  the 
plane  of  polarization  of  polarized  light  may  be  determined  with  great  precision. 
This  instrument  consists  of  a  large  prism  of  calc-spar,  which  is  reduced  to  the 
form  of  a  right  prism  by  grinding  off  its  ends,  and  sliced  lengthwise  by  a  plane, 
nearly,  but  not  quite  perpendicular  to  its  principal  plane.  The  parts  into  which  the 
prism  is  thus  divided  are  joined  in  reversed  positions  and  a  diaphragm  with  a  circu- 
Jar  opening  is  placed  at  each  end.  The  light  which  passes  through  both  diaphragms 
produces  a  circular  field,  divided  by  a  diametrical  slit  into  two  parts,  In  which  the 
planes  of  polarization  are  slightly  inclined  to  each  other.  If  then  light,  which  has  been 
previously  polarized,  be  transmitted,  It  will  be  extinguished  in  the  two  parts  of  the 
field  of  view  in  positions  which  he  close  together,  and  the  light  will  become  uniform 
an  a  position  midway  between  these.  This  position  determines  the  plane  in  which 
the  incident  light  was  polarized  with  a  precision  much  greater  than  has  been 
otherwise  attained.  Professor  Jellett  stated  that  the  different  observations  did  not 
differ  from  one  another  by  an  angle  greater  than  a  minute,  and  that  the  instrument 
was  equally  applicable  to  the  case  of  homogeneous  light." 
The  first  practical  application  of  this  invention  was  in  the  construction  of  a 
polarimeter  for  the  Professor  by  Bryson  in  that  year,  and  the  manufacture  is  con- 
linued  by  the  same  optician  at  the  present  time  }  It  is  the  most  simple  form  with 
