Electrically  Prepared  Colloidal  Solutions. 
429 
the  critical  angle.     The  proper  adjustment  of  the  lens  L  and 
block  A  allows  the  beam  to  come  to  a  focus  at  C  just  at  the 
Fig.  2 
M 
Na/1 
drop  of  liquid.  As  the  particles  in  the  solution  come  into 
the  path  of  the  beam,  the  light  striking  them  is  now  no  longer 
totally  reflected,  but  is  scattered  into  the  microscope  M.  As 
the  field  of  view  is  otherwise  dim,  the  appearance  is  that  of 
a  sky  filled  with  moving  stars — the  Brownian  movement  of 
the  particles  being  at  once  recognized.  ]STo  means  is  afforded 
of  directly  comparing  the  size  or  shape  of  the  particles  as, 
in  general,  the  better  they  are  in  focus  the  smaller  they 
appear. 
An  adaptation  of  the  above  apparatus  was  used  to  count 
the  number  of  particles  in  a  given  volume  of  a  colloidal 
solution.     The  difficulty  of  determining  the  volume  of  liquid 
Fig-.  3. 
\                 ■     ■■- 
\*              ,  A     B 
1 
S     j 
© 
viewed  was  overcome  by  the  use  of  the  special  Zeiss  slide 
used  in  counting  the  number  of  blood  corpuscles  in  a 
unit    volume    of    diluted    blood.       Fig.    3    shows    the    con- 
