Diffraction  Theory  of  Microscopic  Vision,  161 
from  the  lens.  At  d7  immediately  in  front  of  the  lens,  a 
piece  of  wire  gauze  having  about  30  wires  to  the  centimetre 
is  placed.  The  diffraction  pattern  produced  on  the  screen  e 
by  the  wire  gauze  is  shown  much  enlarged  in  fig.  5;  where  o 
1%.  5. 
"57"  s"  y~*,  ™7 
is  the  central  image,  ax  and  a2  are  the  spectra  due  to  the 
vertical  wires,  bx  b2,  those  due  to  the  horizontal  wires  ;  and 
Ci  c2,  dx  d2  are  spectra  due  to  the  combined  effects  of  both  sets 
of  wires.  By  cutting  small  holes  in  the  screen  £,  so  as  to 
allow  but  part  o£  the  diffraction  pattern  to  pass  into  the  eye 
at  /,  the  changes  in  the  appearance  of  the  gauze  may  be 
conveniently  studied.  If  the  screen  e  is  pierced  by  a  hole 
only  large  enough  to  transmit  the  central  beam  o,.  the  wire 
gauze  is  quite  invisible.  I£  a  horizontal  slit  is  used  which 
transmits  only  the  central  beam  o  and  the  spectra  a{  a2,  the 
vertical  wires  alone  are  seen.  If  the  slit  is  turned  vertically  so 
as  to  transmit  o  and  b1  fr2,  the  horizontal  wires  alone  are  visible. 
If  the  slit  is  turned  at  an  angle  of  45°  so  that  the  central 
beam  and  the  secondary  spectra  Ci  c2  pass  through  it,  neither 
the  vertical  nor  horizontal  wires  are  seen,  but  a  very  real 
looking  set  of  wires  appears  running  diagonally  in  the 
direction  d\  d2,  such  a  set  of  wires  as  would  in  fact  give  rise 
to  the  spectra  ex  c2  if  acting  alone.  If  the  card  is  pierced 
with  three  pinholes  which  transmit  the  central  beam  o  and 
the  two  second-order  spectra  a2  a2,  a  set  of  vertical  lines  is 
seen,  the  lines  being  half  as  far  apart  as  the  wires  in  the 
gauze. 
Phil  Mag.  S.  6.  Vol.  11.  No.  61.  Jan.  1006.  M 
