With  middle-aged  and  young  people,  however,  the  purpose  of  glasses  is  in
most  cases  to  prevent  eyestrain.     In  doing  this,  they  may  or  may  not
improve  the  distant  vision.
2  Small  refractive  errors  that  permit  of  clear  vision  are  even  more  likely
to  cause  eyestrain  than  the  large  ones  that  cause  indistinct  and  blurred
vision.     This  is  because  the  eyes  make  the  necessary  effort  to  counteract
the  small  defects,"  while  they  are  forced  to  accept  and  to  cease  trying  to
overcome  the  blur  from  the  large  ones.
266


PRINCIPLES   OF   HEALTH   CONTROL


though  a  frequent  cause  is  the  faulty  repairing  of  frames
after  accidents.  The  two  lenses  must  of  course  be  on  the
same  level  (Fig.  58).  While  it  is  the
duty  of  the  specialist  to  see  that  these
conditions  are  fulfilled  when  the  glasses
are  ordered,  it  devolves  upon  the  pa-
tient to  see  that  the  proper  adjust-
ments are  maintained.
Strain  from  Artificial  Illumination.
—  Artificial  light  of  all  kinds  is  more
or  less  irritating  to  the  eyes  and  much
strain  can  be  avoided  by  care  in  its
use.  Direct  glare  from  lamps  should
not  be  permitted  to  reach  the  eyes  as
so  frequently  happens  from  the  im-
proper lighting  of  public  halls  and
meeting  places.  If  one,  in  facing  the
speaker,  must  look  directly  into  a  light
(Fig.  59),  or  must  receive  the  direct
glare  from  the  side,  the  effect  is  especially  trying.  If  the
eyes  are  at  all  sensitive,  he  can  scarcely  subject  himself  to
such  conditions  without  strain.  The  improvements  that
are  now  becoming  quite  general  in  the  illumination  of
public  halls,  theaters,  and  churches  should  be  still  further
encouraged.  Indirect  and  semi-indirect  illumination  (Fig.
