ON  THE  PEAK  OE  TENEEIFEE. 
471 
place,  determined  with  the  greatest  attainable  precision  for  homogeneous  light  of  some 
definite  refrangibility,  as  well  as  the  ratio  of  polarized  to  unpolarized  light. 
“ 7th.  A very  interesting  series  of  astronomical  observations  on  refraction  might  be 
made,  should  time  be  allowed,  if  a good  altitude  and  azimuth  instrument  could  be 
furnished,  and  erected  at  or  near  the  summit ; consisting  in  determining  the  form  of  the 
apparent  diurnal  orbit  of  a star  passing  through  the  zenith,  and  oc,  Andromedse  would  be 
an  excellent  star  for  this  purpose.  It  passes  almost  exactly  through  the  zenith  of  the 
Peak ; and  supposing  the  observer  to  be  there  in  August,  it  rises  about  three  hours  after 
sunset,  and  reaches  the  zenith  about  as  much  before  sunrise,  affording  the  most  favour- 
able conditions  for  regularity  in  the  disposal  of  the  atmospheric  strata. 
“ 8th.  The  obseiwer’s  attention  should  be  duected  to  any  instance  of  lateral  refrac- 
tion, like  that  remarkable  case  described  by  Humboldt  as  having  occurred  to  him  at 
Orotava. 
“ 9th.  The  temperature  of  the  sea-water,  taken  up  from  about  10  feet  below  the  sur- 
face, should  be  determined  daily,  or  several  times  in  the  day  and  night,  on  and  near  the 
tropic,  if  possible,  to  the  hundredth  of  a degree. 
“ 10th.  It  would  be  most  desnable  also  to  procure  thermographic  representations  of 
the  solar  spectrum  (as  described  in  my  paper  in  the  Philosophical  Transactions,  1842), 
and  to  examine  the  ‘ fixed  lines  ’ of  the  luminous  spectrum,  with  a Hew  to  ascertaining 
whether  they,  or  any  of  them,  originated  in  absorption  of  the  earth’s  atmosphere. 
“ I have  the  honour  to  be.  Sir, 
“ Your  very  obedient  Servant, 
(Signed)  “ J.  F.  W.  Herschel.” 
III. — Letter  from  Lieut. -Col.  James,  It.E.,  to  the  Secretary  of  the  Admiralty, 
dated  Ordnance  Map  Office,  Southampton,  bth  May,  1856. 
“ Sir, — 1st.  I beg  to  acknowledge  the  receipt  of  your  letter  of  the  2nd  inst.,  informing 
me  that  the  Lords  Commissioners  of  the  Admiralty  have  sanctioned  a scientific  expedition 
to  Tenerifie,  under  Professor  Piazzi  Smyth,  and  requesting  me  to  offer  any  suggestions 
which  may  occur  to  me  for  the  better  carrying  out  of  the  object  in  view. 
“ 2nd.  It  is  peculiarly  gratifying  to  me  to  leam  that  so  immediately  after  the  conclu- 
sion of  peace,  an  enlightened  Government  has  turned  its  attention  to  the  promotion  of 
science. 
“ 3rd.  The  special  object  which  the  scientific  world  have  in  view,  when  they  urge  the 
Government  to  avail  themselves  of  Professor  Smyth’s  voluntary  offer  of  his  services,  is  to 
obtain  that  closer  (so  to  speak)  and  more  perfect  Hew  of  the  heavenly  bodies  which  is 
to  be  obtained  at  a great  elevation  and  in  a purer  atmosphere  than  can  possibly  be 
obtained  in  this  or  perhaps  any  other  part  of  the  world. 
“ 4th.  From  Professor  Smyth’s  known  powers  as  an  accurate  observer,  and  from  the 
singular  facility  and  felicity  with  which  he  is  able  to  represent  and  describe  what  he 
MDCCCLVIII.  3 R 
