Royal  Society.  311 
the  Aberration  was  really  increased  by  7"'l.  It  does  not  appear 
that  this  observation  was  repeated. 
A  result  of  physical  character  so  important,  and  resting  on  the 
respectable  authority  of  Professor  Klinkerfues,  merited  and,  indeed, 
required  further  examination.  Having  carefully  considered  the  astro- 
nomical means  which  would  be  most  accurately  employed  for  the 
experiment,  I  decided  on  adopting  a  vertical  telescope,  the  subject  of 
observation  being  the  meridional  zenith  distance  of  y  Draconis,  the 
same  star  by  which  the  existence  and  laws  of  Aberration  were  first 
established.  The  position  of  this  star  is  at  present  somewhat  more 
favourable  than  it  was  in  the  time  of  Bradley,  its  mean  zenith- 
distance  north  at  the  Royal  Observatory  being  about  100"  and  still 
slowly  diminishing.  With  the  sanction  of  the  Government,  therefore, 
I  planned  an  instrument,  of  which  the  essential  part  is  that  the  whole 
tube,  from  the  lower  surface  of  the  object-glass  to  a  plane  glass  clo* 
sing  the  lower  end  of  the  tube,  is  filled  with  water,  the  length  of  the 
column  of  water  being  35*3  inches.  The  curvatures  of  the  surfaces 
of  the  two  lenses  constituting  the  object-glass,  adapted,  in  conjunction 
with  the  water,  to  correct  spherical  and  chromatic  aberration,  were 
investigated  by  myself  and  verified  by  my  friend  Mr.  Stone  (now 
Astronomer  at  the  Cape  Observatory).  The  micrometer  is  con- 
structed on  a  plan  arranged  by  myself,  by  which  the  double  ob- 
servation in  reversed  positions  of  the  instrument  can  be  made  with 
great  ease.  The  reference  to  the  vertical  is  given  by  two  spirit-levels, 
both  to  be  read  at  every  single  observation.  The  work  of  construc- 
tion was  intrusted  to  Mr.  James  Simms,  who  carried  it  out  with 
great  ability.  Distilled  water  was  supplied  by  H.TV.  Chisholm,  Esq., 
AVardeu  of  Standards. 
Had  the  result  of  the  observations  been  confined  to  the  determina- 
tion of  an  astronomical  constant,  or  the  variation  of  its  value  for 
different  telescopes,  I  should  not  have  thought  it  worthy  of  commu- 
nication to  the  Royal  Society.  But  it  is  really  a  result  of  great 
physical  importance,  not  only  affecting  the  computation  of  the  velo- 
city of  light,  but  also  influencing  the  whole  treatment  of  the  Undu- 
latory  Theory  of  Light.  In  this  view  I  have  thought  that  an  informal 
statement  of  the  conclusions  may  be  acceptable  to  the  Society,  reser- 
ving for  publication  in  one  of  the  annual  Greenwich  Volumes  the 
details  of  the  observations. 
The  instrument  was  mounted  in  a  small  Occasional  Observatory 
first  constructed  for  the  transit-instrument  of  Mr.  Struve  when  he 
was  engaged  in  determining  the  longitude  of  Altona,  and  now 
planted  on  the  "South  Ground"  of  the  Observatory.  The  seasons 
at  which  the  meridional  zenith-distance  of  y  Draconis  is  most  affected 
by  aberration  in  opposite  directions  are  the  Equinoxes. 
For  understanding  the  following  Table,  it  is  to  be  remarked  that 
an  apparent  value  of  the  Geographical  Latitude  of  the  Instrument  is 
formed  from  every  observation,  by  subtracting  the  Observed  Instru- 
mental Zenith-distance  North  of  the  Star  from  the  Tabular  Decli- 
nation of  the  Star  given  in  the  'Nautical  Almanac'  The  observed 
zenith-distance  is  affected  with  the  True  Aberration  as  seen  in  the 
