Am.  Jour.  Pharm. ) 
February,  1914.  J 
Colloids  and  Crystals. 
79 
a  supersaturated  solution  of  the  same  substance,  of  discharging  the 
supersaturation  and  inducing  the  formation  of  crystals. 
Those  who  desire  to  follow  this  matter  further  should  read 
van  Weimarn's  little  book,  "  Grundziige  der  Dispersoidchemie," 
after  which  they  will  find  themselves  very  much  interested,  but 
somewhat  unconvinced.  Let  me  hasten  to  add  that  I  have  not  the 
least  desire  to  undervalue  the  brilliant  experimental  work  of  the  Rus- 
sian chemist.  It  is,  in  fact,  precisely  by  the  conception  of  more 
or  less  daring  hypotheses,  and  the  working  out  of  their  consequences, 
that  our  science  achieves  its  endless  victory  over  the  nescience 
about  us. 
VII. 
We  have  seen  that  the  wave-lengths  of  the  visible  radiations  are 
comprised  between  0.4  /x  and  0.7  With  objects  much  smaller,  the 
ordinary  microscopic  method  ceases  to  be  applicable.  Using  ultra- 
violet radiation  for  illumination,  quartz  lenses  in  the  microscope,  and 
receiving  the  image  with  the  photographic  plate  instead  of  the  eye, 
it  is  possible  to  advance  a  step  further  in  the  domain  of  the  infini- 
tesimal, but  only  a  step,  and  there  are  obvious  objections  to  the 
proceeding.  Since  some  of  the  particles  in  colloidal  solutions  are 
only  0.006  ^  in  diameter,  we  can  never  hope  to  see  them  as  little 
bodies  subtending  a  visual  angle.  The  ultra-microscope — -the  power- 
ful instrument  of  investigation  to'  which  most  of  our  knowledge  of 
colloid  systems  is  due — renounces  this  idea  and  makes  the  particles 
visible  merely  as  glittering  points  on  a  black  background.  The  sol 
is  placed  in  a  small  rectangular  glass  trough  and  a  horizontal  beam 
of  arc  light  or  sunlight  focussed  in  it.  The  microscope  is  placed 
vertically  above  the  trough.  It  will  at  once  be  seen  that  there  are  two 
fundamental  things  about  the  instrument :  to<  provide  intense  illumina- 
tion, and  to  make  sure  that  no  light  enters  the  microscope  except  the 
rays  which  emanate  from  the  particles.  The  principle  is  simple,  but 
the  system  of  diaphragms  and  lenses  needed  to  secure  the  second 
object  makes  the  ultra-microscope  an  elaborate  and  expensive  instru- 
ment in  practice. 
Cotton  and  Mouton  9  achieve  the  same  end  in  a  different  way. 
The  illumination  (arc  or  sunlight)  is  thrown  up  from  below  by  a 
paraboloid  reflector  so  ground  that  all  rays,  except  those  diffracted 
Compt.  Rendus,  vol.  136,  p.  1657  (1903). 
