Diffraction  Theory  of  Microscopic  Vision. 
163 
has  been  produced  by  improving  the  lens.  The  image  pro- 
duced by  a  lens  admitting  spectra  of  the  eighth  order  is 
shown  in  fig.  6-VIII.        Here  two  dark   streaks  show  in 
F1G.6.-VE 
FIG.6.-V1II 
FIG.6.-IV 
FIG.6-V- 
each  bright  line.  Fig.  6-XI  shows  the  presence  of  three 
dark  bands  in  each  bright  line  of  the  image  formed  by  a  lens 
admitting  spectra  up  to  the  eleventh  order. 
Using  a  steel  tool,  a  number  of  coarse  gratings  were  ruled 
on  smoked  glass,  and  one  was  finally  secured  which,  when 
examined  with  a  wide-angled  lens,  showed  sharp  black  lines 
approximately  twice  as  broad  as  the  transparent  spaces. 
This  grating  was  illuminated  with  monochromatic  light  and 
examined  by  means  of  a  low-power  objective  backed  by  an 
iris  diaphragm  to  secure  variable  aperture.  The  chief  details 
shown  in  figs.  6-1  to  6-XI  were  clearly  seen  in  the  image 
as  the  iris  was  slowly  opened.  With  the  iris  contracted  so 
that  the  first-order  spectra  alone  were  transmitted,  the  bright 
lines  were  broad  and  fuzzy  while  a  distinct  bright  streak 
showed  along  the  centre  of  each  black  line,  making  each  look 
like  a  highly  refracting  transparent  filament.  When  the  iris 
was  slowly  opened,  these  bright  streaks  disappeared,  the  bright 
lines  became  noticeably  sharper,  and   soon  a   dark  streak  * 
*  It  was  the  appearance  of  these  dark  streaks  which  suggested  a 
detailed  study  of  the  images  ;  when  first  observed  they  were  mistaken 
for  narrow  lines  of  lampblack  left  by  the  ruling1  point. 
M2 
