0:s  THE  PEAK  OF  TENEEIFFE. 
509 
to  the  lava  streams ; and  2ndly,  the  rocks  at  the  several  stations  were  found  to  be  slightly 
magnetic  in  hand  specimens ; the  most  powerful  of  them  was  one  from  Guajara,  consist- 
ing of  obsidian  and  trachyte  in  alternate  laminae : the  origin  of  the  specimen  was  a small 
crater  about  100  feet  below  the  station,  and  the  material  was  present  there  in  large 
masses. 
(7.)  Polarimeter. 
A polarimeter,  devised  by  Mr.  Airy,  constructed  and  kindly  lent  by  him  for  the 
Teneriffe  experiments,  was  employed  frequently ; but  not,  I find  now,  and  am  sorry  to 
say,  with  so  much  discrimination  as  would  have  been  advisable. 
The  instrument  consists  of  a principal  tube,  capable  of  being  turned  round  its  own 
axis,  and  of  being  pointed  in  any  dii’ection,  its  angular  distance  from  the  sun  being 
given  by  a shade-bar  on  a graduated  semicircle ; this  semicricle  being  mounted  on  a 
collar,  which  is  free  to  tuim,  or  can  be  clamped  on  the  tube.  The  light  which  passes 
down  from  the  sky  through  the  tube  falls  on  a bundle  of  reflecting  glass  plates,  sup- 
ported just  under  the  tube’s  lower  end,  on  a transverse  axis,  carried  by  two  arms  pro- 
jecting from  the  sides  of  the  tube.  This  axis  allows  the  glass  plates  to  have  their 
inclination  varied  at  pleasure,  with  reference  to  the  tube’s  incident  light,  and  has  con- 
nected with  it  a mechanism,  by  which  an  eyepiece,  armed  with  a Nicol’s  prism  and  plate 
of  calc-spar,  is  always  earned  in  the  direction  of  the  reflected  light ; graduated  arcs 
being  added  at  the  side,  for  reading  off  the  angles  of  incidence  and  reflexion. 
Now  with  this  apparatus,  when  its  eyepiece  is  at  the  angle  of  complete  polarization, 
the  polarization  produced  by  the  glass  plates  is  necessarily  shown,  and  the  corresponding 
coloured  rings  exhibited;  but  at  angles  of  such  imperfect  polarization,  that  their 
diminution  of  the  coefficient  of  etherial  vibrations  in  one  plane,  is  only  equal  to  the 
atmospheric  diminution  of  the  coefficient  of  vibrations  in  the  transverse  plane,  all  traces 
of  polarization  disappear.  Beyond  those  angles,  of  course,  the  complementary  rings 
due  to  the  polarization  of  the  atmosphere  or  blue  sky  are  seen. 
The  practical  method  of  obserring  was  therefore,  starting  from  a mean  angular 
position  of  the  eyepiece,  giring  great  intensity  of  glass-plate  polarization,  slowly  to 
decrease  the  angle  between  incidence  and  reflexion,  until  the  vanishing  point  of  the 
rings,  or  the  passage  of  the  one  set  into  the  other,  was  just  arrived  at:  the  readings 
being  then  noted,  the  angle  was  again  opened  out,  past  the  maximum  of  glass-plate 
polarization,  until  the  vanishing  point  on  the  other  side  was  arrived  at,  and  duly  read 
off  on  the  graduated  arcs ; when  half  the  angular  distance  between  the  two  vanishing 
points  was  set  down  as  “inverse  intensity  of  sky  polarization.” 
It  is  here  supposed  that  the  instrument  was  previously  adjusted,  by  turning  the  tube 
round  its  axis,  so  that  the  plane  of  reflexion  was  perpendicular  to  the  plane  of  sky 
polarization,  which  was  ordinarily  assumed  to  pass  through  the  sun ; but  the  instrument 
itself  is  competent  to  point  out  whether  this  adjustment  be  made,  since  otherwise  the 
rings  do  not  wholly  vanish  at  any  incidence. 
When  the  tube  was  pointed  to  within  20°  of  the  sun,  or  its  opposite  point,  the  sky 
polarization  was  so  weak,  that  the  rings  due  to  the  polarization  of  the  glass  plates  were 
