One  of  his  sons  is  also  a  great  engineer;  he  is  the
Mr.  Parsons  who  invented  the  steam  turbine,  and
made  a  success  of  it  in  spite  of  enormous  discourage-
ments; and  he  told  us,  you  remember,  about  the
diamonds  in  meteorites.
Herschel's  four-foot  mirror  and  Lord  Rosse's  six-
foot  are  still  in  existence,  and  the  latter  is  actually  in
use  in  the  telescope,  though  the  former  has  been  dis-
mounted. But  the  surfaces  of  both  have  become
tarnished,  so  that  they  no  longer  show  things  clearly.
They  are  made  of  a  peculiar  kind  of  metal  called
speculum  metal,  which  remains  bright  for  some
time,  but  gradually  tarnishes  in  the  course  of  years
in  a  manner  difficult  to  remedy.  To  avoid  this
gradual  deterioration,  mirrors  have  been  made  re-
cently of  glass  which  is  then  silvered  with  a  thin  film
on  the  front  surface.  This  film  also  tarnishes  after

,


JOURNEYING   BY  TELESCOPE


97


a  time,  but  it  can  easily  be  removed  with  a  little  acid,
and  then  a  new  bright  film  can  be  put  on  so  that  the
mirror  is  as  good  as  new.  A  good  many  mirrors  two
or  three  feet  wide  have  been  made  in  this  way ;  but
the  first  really  big  one  was  made  by  Dr.  Common  at
Ealing.  It  was  five  feet  wide,  not  so  wide  as  Lord


The  great  Five-foot  Telescope  being  taken  up  Mount  Wilson.
Rosse's  (six  feet),  but  still  a  great  advance  on  any
previous  "  silver-on-glass  "  mirror.  With  it  he  took
some  wonderful  pictures  of  nebulae  and  other  objects
of  the  heavens,  but  he  did  not  live  long  after  he  had
made  it,  and  the  telescope  was  sold  at  his  death  to
the  Harvard  Observatory  in  the  United  States.
