Wave-Lengths  with  a  Modified  Apparatus.  693 
It  is  obvious  that  p  and  ir  must  not  be  nearly  equal.  I£  p 
be  the  larger  number  corresponding  to  the  greater  interval, 
7r  should  not  exceed  \  p.  On  the  other  hand,  too  great  a 
reduction  of  it  would  lead  to  difficulties  on  account  of  the 
increased  angular  diameter  of  the  rings.  Perhaps  it  was 
for  this  reason  that  Fabry  and  Perot  adopted  an  altered 
course.  In  my  experiments  the  longer  interval  was,  as 
already  mentioned,  about  5  mm.,  and  the  shorter  interval 
was  about  1  mm.,  so  that  the  angular  diameter  of  the  rings 
was  rather  more  than  doubled  in  the  latter  case. 
The  facility  with  which  angular  diameters  larger  than 
usual  could  be  observed  is  due,  in  part  at  any  rate,  to  the 
special  construction  of  my  apparatus.  MM.  Fabry  and  Perot 
employed  a  fixed  interference-gauge  and  a  fixed  telescope, 
measuring  the  diameters  of  the  rings  by  an  eyepiece  micro- 
meter. There  are,  I  think,  some  advantages  in  a  modified 
arrangement,  whereby  it  becomes  possible  to  refer  the  rings 
to  a  wire  fixed  in  the  optic  axis  of  the  telescope.  To  this 
end  the  wire  is  made  vertical,  and  the  rings  are  brought  to 
coincidence  with  it  by  a  rotation  oij  the  gauge,  which  is 
mounted  upon  a  turntable  giving  movement  round  a  vertical 
axis.  The  middle  plane  of  the  gauge  is  vertical  and  adjusted 
so  as  to  include  the  axis  of  rotation.  In  this  way  of  working 
the  reference  wire  is  backed  always  by  the  same  light,  whether 
opposite  sides  of  one  ring  or  of  different  rings  are  under 
observation.  It  is  perhaps  a  more  important  advantage  that 
the  same  part  of  the  object-glass  is  always  in  use,  and  to  a 
better  approximation  the  same  parts  of  the  plates  of  the 
gauge.  The  diaphragm  which  limits  the  latter  should  be  as 
close  to  the  plates  as  possible  (or  to  their  image  near  the  eye), 
but  when  the  multiple  reflexions  are  taken  into  account  it  is 
impossible  to  secure  that  exactly  the  same  part  should  always 
be  in  action. 
The  revolving  turntable  carried  with  it  a  thick  strip  of 
plate-glass  upon  which  was  scratched  a  radial  line.  The 
point  observed  described  a  circle  of  10  inches  radius,  and  the 
rotation  was  measured  by  means  of  a  travelling  microscope 
reading  to  *001  inch.  The  angles  involved  are  sufficiently 
small  to  allow  the  diameter  of  a  ring  to  be  taken  as  pro- 
portional to  the  difference  of  readings  at  the  microscope. 
As  regards  the  gauge  itself,  the  plates  are  by  Brashear. 
For  the  mounting  of  the  5  mm.  gauge,  which  is  of  brass,  I 
am  indebted  to  my  son  Mr.  P.  J.  Strutt.  The  1  mm.  gauge 
is  of  iron  and  was  made  by  my  assistant  Mr.  Enock.  They 
are  much  after  the  design  of  Fabry  and  Perot.  For  the  final 
adjustment  to  parallelism  the  eye  is  moved  in  various  directions 
