1(50 
Mr.  A.  B.  Porter  on  the 
lines  to  the  centimetre  and,  since  no  second  order  spectrum  is 
given  by  rulings  finer  than  20,000  to  the  centimetre,  the 
images  of  rulings  between  these  limits  will  merely  show 
blurred  or  harmonically  shaded  lines  as  indicated  in  fig.  3. 
The  only  possible  method  of  increasing  the  resolving  power 
is  to  use  light  of  shorter  wave-length ;  this  may  be  done  by 
utilizing  the  violet  or  ultra-violet  rays,  or  by  using  an 
immersion-lens  and  filling  the  space  between  object  and  lens 
with  a  liquid  of  high  refractive  index  in  which  light-waves 
are  shortened.  In  the  photo-micrographic  outfit  designed  by 
Dr.  A.  Koehler  *  both  methods  of  securing  short  wave-lengths 
have  been  utilized  and  the  limit  of  resolution  has  been  raised 
to  about  90,000  lines  to  the  centimetre.  Although  the 
discussion  has  here  been  limited  to  the  visibility  of  periodic 
structures  such  as  gratings,  it  is  evident  from  the  general 
principle  involved  that  in  no  case  can  we  expect  to  secure  any 
advantage  by  increasing  the  magnifying  power  of  a  micro- 
scope much  beyond  the  point  at  which  the  half  wave-length 
of  the  light  used  becomes  an  easily  visible  magnitude. 
8.  It  is  sometimes  assumed  that  there  is  an  essential 
difference  between  microscopic  and  macroscopic  vision  and 
that  the  phenomena  of  diffraction  play  no  part  in  the  latter  f. 
Whatever  difference  there  may  be  between  the  two  cases 
arises,  however,  from  the  relative  size  of  the  objects  involved 
and  from  the  special  methods  of  illumination  employed  with  the 
microscope.  By  choosing  a  suitable  method  of  illumination, 
as  in  the  following  experiment,  it  can  be  readily  demonstrated 
that  the  images  of  periodic  structures  formed  by  the  naked 
eye  itself  are  due  to  diffracted  light.  The  experiment  is 
patterned  as  closely  as  possible  after  one  of  Abbe's  micro- 
scopical experiments,  but  with  the  microscope  left  out. 
Light  from  an  arc  lamp,  a  in  rig.  4,  passes  through  a  pinhole 
in  the  screen  b,  and  is  focussed  by  means  of  a  photographic 
lens  c  on  a  cardboard  screen  at  <?,  about  25  or  30  centimetres 
*  Zeitschrift  fur  wissenschaftlicfre  Mikroskojne,  xxi.  1904,  pp.  129-165 
anu  273-304. 
t  Carpenter-Dallinger,  '  The  Microscope,'  8th  ed.,  1901,  p.  62. 
