ON  THE  PEAK  OF  TENEEIFFE. 
507 
(5.)  Stokes’s  Spectrum. 
At  Guajara  many  experiments  were  made  in  the  dark  optical  room  with  a quartz  train, 
lent  by  Professor  Stokes,  but  they  are  all  unfortunately  faulty,  by  reason  of  the  speculum 
employed. 
Of  two  large  drawings  that  were  made  on  August  10  and  12,  near  noon,  one  ter- 
minates at  the  limiting  line  in  a drawmg  of  the  extended  spectrum  furnished  to  me  by 
Professor  Stokes,  the  other  contains  two  lines  more.  What  would  have  been  the  result 
had  the  direct  solar  light  been  employed,  may  be  gathered  from  the  two  subjoined  draw- 
ings of  the  violet  end  of  the  ordinary  spectrum,  as  seen  with  the  glass  apparatus  at  Alta 
Vista:  they  were  taken  on  the  same  day,  September  12,  and  within  a short  time  of  each 
other. 
nebulous.  nebulous. 
Eeflected. 
clearly  resolved. 
Direct. 
The  mean,  then,  of  the  observations  on  August  10  and  12  shows  that  a little  more  of 
Stokes’s  new  portion  was  seen  on  the  mountain  than  has  been  seen  near  the  sea-level, 
notwithstanding  the  drawback  of  the  speculum ; while  if  we  add  for  that,  what  we  are 
fauiy  entitled  to  do  from  the  two  above  views  of  H,  dhect  and  reflected — we  may  assume 
that  there  is  a much  greater  amount  of  the  more  refrangible  rays  in  the  sun’s  light  in 
the  upper,  than  in  the  lower  regions  of  the  atmosphere. 
This  result  is  confinned  by  another  observation.  The  spectrum  formed  by  the  quartz 
train  was  frequently  photographed  on  a collodion  plate,  and  the  image  so  formed  was 
found  identical  with  that  presented  to  the  eye  by  uranium  glass.  Taking  then  powerful 
photographic  effects  as  an  indication  of  the  abundance  of  rays  of  high  refrangibility,  it 
is  very  remarkable  to  And,  on  looking  over  my  landscape  photographs,  about  120  in 
number,  and  pretty  nearly  equally  divided  between  the  three  stations,  Orotava,  Guajara, 
and  Alta  Vista,  that  not  only  is  there  always  a greater  intensity,  but  that  the  distances 
came  out  invariably  much  better,  in  proportion  as  the  station  is  higher. 
At  the  height  of  10,702  feet,  the  eastern  wall  of  the  crater  of  elevation,  distant  some 
four  miles,  is  given  with  all  the  detail  that  the  eye  could  appreciate  at  the  time ; 
while  at  the  sea-level  on  the  finest  mornings,  and  when  the  cliffs  above  Eealejo,  not 
three  miles  distant,  were  vividly  illuminated  by  the  morning  sun,  and  casting  on  one 
side  dense  and  dark  shadows — yet  the  photographs  would  persist  in  giving  nothing  but 
the  mass  of  the  mountain  in  one  uniform  tint,  save  only  one  remarkably  white  stratum. 
’This  at  least  testifies  to  the  focus  having  been  exact ; while  the  detail  of  things  in  the 
foreground  is  represented  with  such  vigour,  as  to  prove  that  the  quality  of  the  photo- 
graphic material  was  by  no  means  deficient. 
In  place  of  showing  the  mountain  with  all  the  intense  detail  brought  out  to  the  eye 
by  the  sun  shining  strongly  on  its  rocky  slopes,  the  appearance  was  rather  as  if  that 
luminary  were  on  the  other  side,  and  we  only  saw  the  shaded  form  of  the  ridge.  Or 
