ON  THE  PEAK  OE  TENEEIPEE. 
487 
aerial  effect  was  intensely  exaggerated  in  the  chemical  medium,  with  every  increase  of 
distance  and  illumination.  Of  the  latter,  a good  instance  was  shown  in  the  facility  with 
which  at  twilight  a good  photographic  image  of  the  moon  was  obtained ; while  by  day, 
no  matter  how  clear  the  moon  shone  out  to  the  eye  from  the  deep  blue  vault  of  heaven, 
no  impression  whatever  could  be  procured*. 
After  meeting  with  these  indubitable  instances  of  the  distinction  between  actinic  and 
optic  images,  no  photograph  would  be  admitted  as  a decisive  evidence  on  a certain  point 
suggested  for  inquiry  by  the  Astronomical  Society,  viz.  the  brightness  of  the  centre  as 
compared  with  the  borders  of  the  sun.  But  whether  forming  the  sun’s  image  on  a 
screen,  or  looking  at  it  direct,  there  was  never  the  slightest  doubt  on  my  mind  or  my 
eye,  as  to  the  centre  being  very  much  brighter  than  the  border.  The  centre  was  also 
whiter,  the  border  being  yellowish ; and  it  was  not  at  all  from  this  cause  that  the 
borders  w^ere  thought  to  radiate  less  light,  for  the  difference  was  something  far  greater 
than  variation  of  colour  could  explain.  This  experiment  may  be  perhaps  taken  as  con- 
clusive : with  the  transparent-reflector  eyepiece  of  the  Pattinson  equatorial,  the  held  of 
\iew  having  a stop  with  a small  perforation  of  about  3'  in  diameter,  I found  that  I could 
bear  perfectly  w’ell  to  look  at  the  border  of  the  sun  without  any  coloured  glass ; but  I 
could  not  with  impunity  allow  the  stop  to  pass  over  the  central  regions  of  the  solar  orb, 
by  reason  of  their  sui’passing  brightness. 
This  resrdt  is  confirmatory  of  the  conclusions,  I believe,  of  every  observer  but  one, 
M.  Akago  ; and  he  arrived  at  his  unique  \iew  by  means  of  a photometer,  based  on  his 
polariscope ; an  instrument  which  I was  also  requested  by  the  Astronomical  Society  to 
use  on  the  mountain,  and  which  I did  not  find  very  satisfactory. 
The  polariscope  was  arranged  by  its  ingenious  inventor  to  be  apphed  to  the  eye  end 
of  a telescope,  and  doubling  the  image  of  any  luminous  object  in  the  field,  to  colour 
them  complementarily  if  they  contained  polarized  light  to  a sensible  degree ; and  this 
M.  Aeago  is  stated  to  have  ascertained  to  be  the  case  with  planets  and  comets,  but  not 
with  the  fixed  stars. 
By  Mr.  Aiky,  the  Eev.  Baden  Powell,  and  Mr.  Cooke  of  York,  I was  furnished  with 
various  polarizing  materials  capable  of  being  fitted  up  into  such  an  arrangement ; but  on 
neither  planets  nor  satellites,  "viewed  with  the  naked  eye,  the  finder,  or  the  Pattinson 
equatorial,  could  I get  the  smallest  indication  of  complementary  colours.  The  planets  in 
question  were  certainly  not  favourably  disposed  for  the  polarizing  angle  of  their  reflected 
sunlight ; and  before  the  moon  arrived  at  that  part  of  her  orbit,  the  summer  weather 
had  broken  up,  and  brought  all  our  observations  to  an  end. 
(5.)  Rising  and  Setting  of  the  Sun. 
Accurate  observations  of  the  time  of  rising  and  setting  of  the  sun  I had  intended  to 
make,  under  expectation  of  the  phenomenon  taking  place  behind  the  sea-horizon ; but 
this  line  was  never  once  visible  during  our  whole  stay  on  the  mountain.  In  place  of  it, 
* Further  still,  it  was  found  that  the  pliotographic  plate  feels  and  renders  at  once  all  those  additional 
rays,  which  in  Stokes’s  spectrum  the  eye  cannot  perceive  without  the  assistance  of  uranium  glass. 
MDCCCLVIII.  3 T 
