This  same  spectrum  is  produced  when  rays  of  light  are  made  to  pass  through
narrow  interstices,  or  edges  of  an  opening,  or  through  glass  ruled  closely  with
parallel  lines.  The  phenomenon  is  called  diffraction ^  and  the  spectrum,  diffr active
spectrum.
If  a  beam  of  white  light,  s,  be  allowed  to  fall  upon  a  prism,  p  (Fig.
45),  it  will  undergo  dispersion  into  its  component  wave-frequencies  (of
which  it  is  composed),  in  the  order  of  their  respective  refrangibilities,
the  violet  being  deviated  the  widest,  followed  in  order  by  indigo,  blue,
green,  yellow,  orange,  and  red,  which  is  deviated  the  least.  The  result  is
that,  instead  of  a  spot  of  white  light  appearing  on  the  floor,  e,  a  band  of
seven  colors  appears  upon  the  wall  or  a  screen  placed  at  a  height  above.
While  the  colors  of  the  solar  spectrum  merge  imperceptibly  into  one
mother,  so  that  manv  tints  might  be  said  to  exist,  the  colors  cannot  be
decomposed  further,  for  when  the  red  ravs  or  any  of  the  others  are  taken
singly  and  passed  through  a  second  prism  they  are  uniformly  refracted
and  give  a  spot  of  red  or  other  color.  These  colors  are,  therefore,  homo-
geneous, and  light  composed  of  them  is  known  as  monochromatic  light.
If  white  light  consists  of  a  mixture  of  these  variously  colored  rays
(wave-frequencies),  then  if  they  be  recombined,  white  light  must  result.
Thus,  if  the  spectrum  formed  by  one  prism  be  allowed  to  fall  upon  a  sec-


78


ELEMENTARY   PHYSICS.
