ON  THE  PEAK  OE  TENEEIFEE. 
493 
of  others  than  Mr.  JOi!^ES,  and  am  perhaps  not  less  qualified  to  do  so,  as  I have  long  since, 
both  in  writing  and  in  engraving,  endeavoured  to  illustrate  that  same  remarkable  shading 
off  of  the  fight ; and  in  doing  so,  gave  it  not  as  anything  new,  but  as  in  the  opinion  of 
every  astronomer,  a leading  feature  in  the  appearance  of  the  phenomenon,  and  a pro- 
bable explanation  of  much  discordance  between  different  observers. 
The  shading  off  is,  in  fact,  so  very  perfect  over  the  whole  extent,  and  the  fight,  even 
at  its  maximum  point  of  intensity,  low  down  in  its  axis,  so  faint  for  human  vision,  that 
t>;vo  different  eyes,  or  two  different  degrees  of  transparency  of  the  air  on  one  and  the 
same  generally  clear  night,  will  entirely  alter  the  apparent  boundaries  and  size  of  the 
fight.  According,  too,  as  the  background  of  the  sky  may  be  fit  up  by  moonlight  or 
otherwise,  from  the  smallest  appreciable  effect,  to  the  maximum  degree  of  the  zodiacal 
fight,  so  will  this  be  seen  to  shorten  from  perhaps  60°  to  5°  or  to  0°. 
There  can  in  fact  hardly  be  anything  more  difficult  to  apply  numerical  measure  to, 
than  definite  parts  of  the  zodiacal  fight ; it  is  like  trying  to  determine  the  place  of  a 
comet  from  observations  of  the  end  of  the  tail  only : and  Sfi  John  Heeschel’s  admirable 
illustrations  to  his  Cape  volume,  show  some  strildng  instances  of  the  apparent  alterations 
m size  of  Halley’s  comet,  according  to  the  amount  of  twilight  illumination  of  the  sky. 
W hen  such  are  the  natiual  difficulties  of  the  case,  many  an  observer  would  leave  it 
altogether,  and  go  to  something  to  which  measure  can  be  applied  rigidly,  as  the  double 
stars ; but  we  are  not  therefore  all  of  us  to  neglect  the  zodiacal  fight.  It  is  an  existence 
in  Nature,  and  if  our  usual  methods  of  mensuration  will  not  apply  to  it,  we  had  better 
improve  them. 
One  of  the  first  elements  that  seems  to  be  demanded,  is  some  approach  to  a proof  of 
the  elimination  of  distm’bing  effects.  It  is  not  enough,  for  instance,  for  a man  honestly 
to  declare  what  he  sees  before  him ; he  must  understand  the  weight  and  effect  of  all 
attendant  circumstances.  When  in  South  Africa,  I found  a whole  season’s  observations 
rendered  abortive  by  the  presence  of  the  planet  Venus  very  near  the  zodiacal  fight, 
and  rejected  them  accordingly:  I cannot  therefore  understand  Mr.  Jones’s  observations 
with  the  moon  in  the  same  position,  as  being  altogether  unexceptionable.  Generally, 
too,  in  my  humble  estimation,  he  hardly  attaches  sufficient  weight  to  the  circumstances 
that  affect  the  visual  and  apparent  phenomenon ; and  overlooks  that  two  of  the  habitudes 
which  he  has  discovered  in  the  fight,  and  which  form  the  basis  of  his  theory  of  a terres- 
trial ring,  may  be  explained  in  this  manner.  Thus,  that  very  striking  circumstance 
that  he  has  given  of  the  fight  being  somewhat  to  the  north  of  the  ecliptic  when  he 
was  in  north  latitude,  and  the  contrary  when  he  was  in  south  latitude,  and  which  is 
abundantly  borne  out  by  his  diagrams  as  evidenced  in  the  compressed  fines  of  the  cone 
on  one  side,  is  accompanied  also  by  this  circumstance,  that  the  side  so  compressed  is 
almost  invariably  the  acute  angle  with  the  horizon,  where  the  vapours  of  the  lower 
atmosphere  would  infallibly  curtail  the  feeble  exterior  breadth  of  that,  as  compared  with 
the  opposite,  side  of  the  fight. 
In  his  theoretical  considerations,  again,  page  xix,  Mr.  Jones  seems  to  overlook  the  con- 
