Diffraction  Theory  of  Microscopic  Vision.  165 
incidence,  these  bands  could  be  brought  into  any  desired  part 
of  the  spectra.  The  double  grating  was  placed  upon  the 
stage  o£  a  microscope  supplied  with  an  iris  diaphragm  above 
the  objective,  and  the  instrument  was  sharply  focussed  on 
the  upper  grating.  The  field  of  view  was  then  illuminated 
by  monochromatic  light  obtained  by  focussing  an  arc  spectrum 
on  the  slit  of  a  collimator.  The  arrangement  of  the  apparatus 
is  shown  in  fig.  7,  where  a  is  the  arc,  S  is  a  spectroscope 
with  eyepiece  removed,  c  is  a  collimator,  ggf  are  the  two 
gratings,  and  m  is  the  microscope.  By  rotating  the  prism, 
the  gratings  could  be  illuminated  by  light  of  every  wave- 
length in  succession.  Using  yellow  light,  the  eyepiece  of 
the  microscope  was  removed  so  that  the  diffraction  spectra 
could  be  seen,  the  iris  was  closed  until  all  spectra  above  the 
first  order  were  cut  out,  and  the  angle  of  incidence  of  the 
light  on  the  gratings  was  then  varied  until  the  shadow-bands 
reached  the  yellow  part  of  the  spectra  and  caused  the 
disappearance  of  the  two  yellow  dots  of  light  which  repre- 
sented the  two  first -order  spectra.  The  eyepiece  of  the 
microscope  was  now  replaced,  and  the  prism  was  slowly  turned 
so  as  to  secure  illumination  with  different  colours  of  mono- 
chromatic light,  while  the  image  of  the  upper  grating  was 
observed.  As  had  been  anticipated,  the  image  was  normal 
and  the  lines  were  clearly  seen  when  the  field  of  view  was 
illuminated  with  any  colour  except  yellow,  but  when  yellow 
light  was  used  the  lines  were  completely  obliterated  from  the 
larger  part  of  the  field.  The  obliteration  of  the  lines  would 
doubtless  be  everywhere  complete  with  two  gratings  of 
perfectly  uniform  and  equal  brilliancy.  The  iris  was  now 
opened  so  as  to  admit  the  spectra  of  the  second  order.  Again 
the  image  of  the  lines  was  normal  except  when   the  field  of 
