ON  IODO-SULPHATE  OF  QUININE. 
19 
Nichol's  prism,  (a  rhomb  of  carbonate  of  lime  cut  in  a  peculiar 
manner,)  has  the  power  of  splitting  the  incident  ray  into  two 
equal  portions,  one  of  which  it  transmits  polarized  in  one  plane, 
whilst  it  obstructs  the  other,  or  that  polarized  in  a  plane  at  right 
angles  to  the  transmitted  ray.  Upon  revolving  this  prism  on  its 
axis,  that  beam  which  was  before  transmitted  becomes  obstructed 
in  it3  turn,  and  thrown  out  of  the  crystal  by  reflection ;  whilst 
that  before  obstructed  is  transmitted. 
The  phenomena  exhibited  by  absorption  are  almost  always 
studied  by  the  action  of  the  tourmaline  on  a  beam  of  ordinary 
light.  This  mineral,  when  cut  into  plates  parallel  to  the  axis  of 
the  crystal,  has  the  remarkable  property  of  dividing  the  ray  of 
light  into  two  parts,  one  of  which  it  transmits  readily,  but  slightly 
altered  in  color ;  the  other  it  wholly  absorbs,  at  least,  it  disap- 
pears and  is  not  to  be  found  by  the  experimenter  until  the  plate 
is  revolved  90°,  when  the  ray  becomes  transmitted,  and  the  for- 
mer one  now  disappears. 
When  two  such  crystals  are  superimposed  at  right  angles  the 
whole  of  the  light  disappears  ;  the  first  leaf  is  absorbed  by  the 
first  or  "polarizing  plate"  of  tourmaline,  technically  called  the 
"  polarizer,"  and  the  other  half  (slightly  tinged)  passes  through 
polarized,  which,  as  soon  as  it  impinges  upon  the  second  crystal 
of  tourmaline,  is  also  absorbed.  There  is  now  no  light  transmitted, 
the  field  of  view  is  as  dark  as  midnight,  or  as  opaque  as  the  dark- 
est bottle  glass. 
When  the  tourmalines  are  in  this  position,  if  a  thin  film  of  se- 
lenite  or  sulphate  of  lime  be  introduced  between  them  at  a  cer- 
tain angle  of  rotation,  the  light  will  now  pass  the  second  tourma- 
line and  be  transmitted  to  the  eye.  A  certain  thickness  is  re- 
quired in  this  experiment  as  well  as  a  certain  angle  of  rotation. 
If  the  crystal  be  less  than  0.00016th  of  an  inch  in  thickness,  the 
light  is  not  transmitted;  if  it  be  above  the  0.01818th  of  an  inch, 
the  light  passes,  but  perfectly  white  ;  if  of  any  intermediate  thick- 
ness, from  0.00124th  of  an  inch  to  the  0.01818th  of  an  inch,  the 
most  splendid  colored  lights  will  be  exhibited  ;  either  blue,  yellow, 
red,  or  in  various  shades  of  these  primary  colors,  or  in  various 
mixtures  of  them,  so  that  we  may  have  blue,  orange,  yellow, 
green,  indigo,  or  violet  according  to  the  thickness ;  the  law  of 
