The  number  of  small  planets  known
to  circulate  between  Mars  and  Jupiter
has  increased  to  more  than  four  hun-
dred. The  rapid  increase  of  the  past
four  years  in  the  rate  of  discovery  is
due  to  the  application  of  the  art  of
photography.  In  this  work  the  lead
has  been  taken  by  Charlois  of  Nice
and  Wolf  of  Heidelberg.  To  carry  on
the  search  a  sensitive  plate  is  exposed  in
the  focus  of  a  telescope  having  a  short
focal  length  (even  a  large  camera  might
suffice),  so  constructed  as  to  have  a  field
of  view  several  degrees  in  extent.  After
an  exposure  of  one  or  two  hours,  innu-
merable small  stars  will  be  photographed
on  the  plate.  If  a  small  planet  is  within
the  region  photographed,  it  may  be  de-
tected either  by  its  image  being  a  line  in-
stead of  a  point,  or  by  comparing  two
plates  taken  at  a  short  interval.  The
principle  on  which  the  discovery  rests  is,
that  while  the  stars  remain  perfectly  at
rest,  the  asteroids  are  in  motion.  The
number  of  asteroids  already  known
which  are  rediscovered  in  this  way  much
exceeds  the  number  of  new  ones.  We
conclude,  therefore,  that  the  new  ones
yet  to  be  discovered  are  extremely  small,
and  it  may  be  expected  that  before  many
years  a  limit  will  be  reached  beyond
which  it  is  not  worth  while  to  seek
for  more.  Even  now  it  is  not  possible
for  astronomers  to  keep  an  accurate
run  of  the  motions  of  the  four  hundred
known  ones.
in*  punet  Great   popular    interest

1*8T"  has  been  felt  in  certain

features  of  the  planets,  notably  Mars.
