486 
THE  ADMIEALTT  ASTEOXOMICAL  EXPEEI3IEXT 
might  be  further  perfected,  but  from  the  qualities  of  the  atmosphere,  and  chiefly  from 
the  dust-haze  which  was  found  existing  in  dense  strata  far  above  the  clouds  of  the  north- 
east trade-wind.  The  most  likely  method  therefore  to  follow  out  on  a future  occasion 
would  be,  to  try  the  experiment  earlier  in  the  season,  when  there  is  less  of  the  haze 
existing ; or  to  try  some  mountain  higher  still  than  Tenerifle. 
(4.)  Solar  Photography  and  Polarization. 
All  our  observations  of  the  sun  laboured  under  the  inconvenience  of  being  performed 
in  the  open  air,  and  freely  exposed  to  the  direct  solar  rays : to  guard  against  these  the 
observer’s  head  and  the  eyepiece  were  enveloped  in  a black  bag.  TSTien  this  was  accom- 
plished, then  came  the  greater  difficulty  of  the  excessive  heating  power  of  the  sun,  con- 
nected with  its  powerful  radiation  on  a mountain.  The  eyepieces  became  so  hot  that 
they  could  not  be  touched,  and  the  black  bag  was  continually  getting  biunt,  and  with 
its  smoke,  irritating  the  observer’s  eye. 
Somewhat  depending  on  these  untoward  accompaniments,  which  might  be  much 
relieved  on  a future  occasion,  the  definition  of  the  sun  was  invariably  worse  than  that 
of  every  other  object  in  the  sky.  The  year  1856  was,  however,  so  near  the  minimum 
of  solar  spot-producing  disturbance,  that  little  of  importance  ever  appeared  on  the  disk. 
The  only  thing  that  I would  particularize,  is  the  “ silk  markiug  ” observed  on  Sep- 
tember 13th. 
This  was  a feature  almost  defying  any  attempts  to  delineate  by  hand  and  eye,  but 
should  yield  one  would  think  to  photography.  Accordingly,  Arfth  the  assistance  of  the 
yacht  carpenter,  I improvised  a photographic  box  to  fasten  on  the  end  of  the  telescope. 
Avith  a spring  trigger  to  make  an  instantaneous  opening  and  closing  of  the  aperture. 
But  success  was  small  by  reason  of, — 1st,  the  wooden  box  bemg  often  heated  to  smoking : 
2nd,  the  shaking  of  the  telescope ; and  3rd,  the  rapid  Aibrations  of  the  air  causing  bad 
definition. 
Making  the  best  of  these  circumstances,  definition  Avas  still  unprocurable,  until  by  a 
series  of  experiments,  the  very  unexpected  result  Avas  found,  that  the  chemical  focus  of 
the  telescope  was  -5  inch  longer  than  the  visual. 
This  circumstance  seems  to  settle  the  question,  that  the  rays  AA'hich  produce  the 
photographic  picture  are  by  no  means  the  luminous,  and  may,  or  may  not,  give  us  an 
idea  of  what  we  see  AAnth  the  eye.  The  black  photographic  effect  of  bright  yellow  is 
Avell  known ; and  a similar  diversity,  and  more  in  point  AAuth  regard  to  the  possibility  of 
obtaining  good  solar  photographs,  was  offered  to  me  frequently  in  Tenerifle,  chiefly  in 
the  lowlands,  when  taking  photographic  landscapes  on  collodion.  It  Avas  this : a distant 
mountain  range  was  seen  Avith  the  most  perfect  definition  of  innumerable  details  about 
its  flanks ; the  bushes,  the  cliffs,  the  dykes  in  these  Avere  distinct  and  even  prominent  to 
the  eye ; yet  in  the  photographs,  nothing  but  the  faint,  though  Avell-defiiied  outline  of 
the  mountain  appeared  against  the  sky ; as  if,  in  place  of  the  sun  shining  on  the  moun- 
tain, it  were  on  the  other  side  and  throwing  the  ridge  into  the  shade.  In  a word,  the 
