The  Magnifying  Power  of  Glasses.
39
ting  the  Screen  at  farther  Distances,  you
may  magnify  your  Object  to  almoſt  what
Size  you  pleaſe.  But  I  would  advife  all
who  uſe  this  ſort  of  Microſcope,  to  regard
Distinctneſs  more  than  Bigness,  and  place
the  Screen  just  at  that  Distance  where  the
Object  is  seen  most  ſharp  and  clear.
CHAP.  IX.
Of  the  magnifying  Power  of  Glasses  in  the
Double  Microscope.
Should  now  ſhew  the  Method  of  computing ¬
  the  magnifying  Rower  of  our
double  Microscope,  as  I  have  done  of  the
single  ones:  but  a  Calculation  of  the  Powers
of  three  combined  Glasses,  would  appear  so
intricate  and  unintelligible  to'  People  unac
quainted  with  Optics  (for  whoſe  Service
chiefly  I  intend  this  Treatise)  that  I  believe
they  will  readily  excuse  my  perplexing  them
with  it:  and  as  for  the  learned  iin  that  Science, ¬
  they  probably  will  be  better  pleased
to  calculate  for  themselves.
The  double  reflecting  Microscope,  described ¬
  p.  16.  is  the  only  Instrument  at  present.
made  amongst  us  for  this  Purpose,  wherein
such  a  Combination  of  Glasses  is  emploved.
Here,  indeed,  the  magnifying  Power  of  the
Object-