478 
THE  ADMIEAETT  ASTEONOlSnCAE  EXPEEBrE3T 
The  usual  test  of  definition,  the  separation  of  close  double  stars,  especiaUy  when  there 
,s  much  difference  in  then  size,  was  tried  at  the  AltaVista  station  wrth  the  Patmson 
telescope.  I cannot,  unfortunately,  state  comparatare  results  with  this  instrument,  for 
though  I visited  its  hospitable  owner  in  England  on  three  different  occasions,  at  mter- 
vals  of  several  months,  yet  the  sky  was  invariably  clouded.  V e have  therefore  only  o 
look  to  the  aperture  of  the  object-glass,  7-25  inches,  and  compare  its  performances  on 
the  mountain  with  those  of  instruments  of  the  same  size  elsewhere. 
s Arietis,  \ Cygni,  62  Arietis,  all  double  stars,  and  with  the  distance  of  then-  compo- 
nents given  in  the  “ Cycle”  at  under  1",  were  completely  separated.  B and  C of  y An- 
dromedas,  with  a distance  of  0"-5,  night  after  night,  and  on  one  occasion  a^t  so  Imge  an 
hour-angle  as  d*  30”,  were  seen  divided,  but  not  exactly  separated;  for  although  t ere 
certainly  was  a dark  line  between  them,  yet  the  disks  were  mutually  compressed  on  that 
side  to  a small  extent.  A diagonal  eyepiece  with  a transparent  reflector  separate 
the  two  disks  completely,  by  making  them  smaUer;  but  the  defimbon  was  so  muc 
iniured,  that  this  testimony  to  the  duplicity  of  the  star  and  the  exceUence  of  the  tele 
scope,  was  not  deemed  more  satisfactory  than  that  afforded  by  the  direct  eyepiece  with 
its  larger  and  brighter  stellar  images.  t i i 
The  excellence  of  definition  at  AltaVista  seemed  to  extend  over  the  whole  sky,  anc 
was  still  most  satisfactory  when,  toward  morning,  I examined  Saturn  heliacaUynsms 
in  the  east.  The  fine  division  of  its  outer  ring,  and  the  transparency  of  the  dark  rmg 
were  abundantly  manifest;  but  the  general  perfection  of  finish  as  it  were  of  the  bor  en 
of  both  ball  and  ring,  struck  me  as  the  most  noticeable  point;  for  even  with  a power  o 
500, 1 could  not  fancy  anything  more  clear  at  its  edges  than  they  were. 
These  observations  were  repeated  on  Saturn  during  several  mommgs,  and  are  all  th. 
more  noteworthy,  since  that  is  about  the  time  when  telescopic  definition  m most  obsei 
vatories  becomes  exceedingly  bad.  On  the  mountain  the  sun  was  seldom  well  defined 
and  his  excessive  radiation  seemed  to  disturb  the  air,  and  in  a manner  that  often  as  e. 
in  the  western  regions  long  after  he  had  set;  but  night  usually  succeeded  in  qiuetin 
the  commotion.  , i j j} 
(2.)  Agents  in  pi'oducing  good  dejimmon. 
Thus  far  the  facts  of  observation  have  been  stated ; and  the  mam  explanation  of  thei 
is  evidently  the  other  fact,  that  wlien  looking  at  the  stars  on  the  mountam,  we  wei 
not  looking  through  those  grosser  and  denser  strata  of  the  lower  regions  of  the  atmc 
sphere  through  which  ordinary  observers  must  look.  On  the  mountain  we  found  anotln 
circumstance  operating,  whose  existence  is  not  generally  suspected:  this  is  tie  pieii 
lence  of  excessive  drought.  Had  there  been  any  actual  formation  of  dew,  there  woid 
have  been  difficulty  in  keeping  the  glasses  free;  and  mth  the  smallest  portion  formm 
on  them,  adieu  to  all  delicacy  of  vision.  But  in  a region  where  the  average  depressici 
of  the  dew-point  was,  as  we  found  it,  the  unheard  of  quantity  of  40°,  and  where  it  wi 
not  unfrequently  above  50°,  the  formation  of  dew  was  physically  impossible.  ^ ^ 
Hence  some  of  our  best  obserHng  nights  were  on  occasions  that  m a moister  atm- 
