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focus

## focus

```  4  definitions  found

From  Webster's  Revised  Unabridged  Dictionary  (1913)  [web1913]:

Focus  \Fo"cus\,  n.;  pl  E.  {Focuses},  L.  {Foci}.  [L.  focus
hearth,  fireplace;  perh.  akin  to  E.  bake.  Cf  {Curfew},
{Fuel},  {Fusil}  the  firearm.]
1.  (Opt.)  A  point  in  which  the  rays  of  light  meet  after
being  reflected  or  refrcted,  and  at  which  the  image  is
formed;  as  the  focus  of  a  lens  or  mirror.

2.  (Geom.)  A  point  so  related  to  a  conic  section  and  certain
straight  line  called  the  directrix  that  the  ratio  of  the
distace  between  any  point  of  the  curve  and  the  focus  to
the  distance  of  the  same  point  from  the  directrix  is
constant.

Note:  Thus  in  the  ellipse  FGHKLM  A  is  the  focus  and  CD  the
directrix,  when  the  ratios  FA:FE,  GA:GD,  MA:MC,  etc.,
are  all  equal.  So  in  the  hyperbola,  A  is  the  focus  and
CD  the  directrix  when  the  ratio  HA:HK  is  constant  for
all  points  of  the  curve;  and  in  the  parabola,  A  is  the
focus  and  CD  the  directrix  when  the  ratio  BA:BC  is
constant.  In  the  ellipse  this  ratio  is  less  than  unity,
in  the  parabola  equal  to  unity,  and  in  the  hyperbola
greater  than  unity.  The  ellipse  and  hyperbola  have  each
two  foci,  and  two  corresponding  directrixes,  and  the
parabola  has  one  focus  and  one  directrix.  In  the
ellipse  the  sum  of  the  two  lines  from  any  point  of  the
curve  to  the  two  foci  is  constant;  that  is:
AG+GB=AH+HB;  and  in  the  hyperbola  the  difference  of  the
corresponding  lines  is  constant.  The  diameter  which
passes  through  the  foci  of  the  ellipse  is  the  major
axis.  The  diameter  which  being  produced  passes  through
the  foci  of  the  hyperbola  is  the  transverse  axis.  The
middle  point  of  the  major  or  the  transverse  axis  is  the
center  of  the  curve.  Certain  other  curves,  as  the
lemniscate  and  the  Cartesian  ovals,  have  points  called
foci,  possessing  properties  similar  to  those  of  the
foci  of  conic  sections.  In  an  ellipse,  rays  of  light
coming  from  one  focus,  and  reflected  from  the  curve,
proceed  in  lines  directed  toward  the  other  in  an
hyperbola,  in  lines  directed  from  the  other  in  a
parabola,  rays  from  the  focus,  after  reflection  at  the
curve,  proceed  in  lines  parallel  to  the  axis.  Thus  rays
from  A  in  the  ellipse  are  reflected  to  B;  rays  from  A
in  the  hyperbola  are  reflected  toward  L  and  M  away  from
B.

3.  A  central  point;  a  point  of  concentration.

{Aplanatic  focus}.  (Opt.)  See  under  {Aplanatic}.

{Conjugate  focus}  (Opt.),  the  focus  for  rays  which  have  a
sensible  divergence,  as  from  a  near  object;  --  so  called
because  the  positions  of  the  object  and  its  image  are
interchangeable.

{Focus  tube}  (Phys.),  a  vacuum  tube  for  R[oe]ntgen  rays  in
which  the  cathode  rays  are  focused  upon  the  anticathode,
for  intensifying  the  effect.

{Principal,  or  Solar},  {focus}  (Opt.),  the  focus  for  parallel
rays.

From  Webster's  Revised  Unabridged  Dictionary  (1913)  [web1913]:

Focus  \Fo"cus\,  v.  t.  [imp.  &  p.  p.  {Focused};  p.  pr  &  vb  n.
{Focusing}.]
To  bring  to  a  focus;  to  focalize;  as  to  focus  a  camera.  --R.
Hunt.

From  WordNet  r  1.6  [wn]:

focus
n  1:  the  concentration  of  attention  or  energy  on  something  "the
focus  of  activity  shifted  to  molecular  biology"  [syn:  {focusing},
{focussing},  {centering}]
2:  maximum  clarity  or  distinctness  of  an  image  rendered  by  an
optical  system:  "in  focus";  "out  of  focus"
3:  maximum  clarity  or  distinctness  of  an  idea;  "the  controversy
brought  clearly  into  focus  an  important  difference  of
opinion"
4:  a  central  point  or  locus  of  bacterial  growth  in  an  organism;
"the  focus  of  infection"  [syn:  {nidus}]
5:  special  emphasis  attached  to  something  "the  stress  was  more
on  accuracy  than  on  speed"  [syn:  {stress}]
6:  a  point  of  convergence  of  light  (or  other  radiation)  or  a
point  from  which  it  diverges  [syn:  {focal  point}]
7:  a  fixed  reference  point  on  the  concave  side  of  a  conic
section
v  1:  focus  one's  attention  on  something  [syn:  {concentrate},  {center},
{centre},  {pore},  {rivet}]
2:  put  into  focus  [ant:  {blur}]
3:  bring  into  focus  or  alignment;  of  ideas  or  emotions  [syn:  {concenter}]
4:  become  focussed  or  come  into  focus;  "focus  an  image  on  a
screen"  [ant:  {blur}]
5:  put  into  focus;  as  of  an  image  on  a  screen;  "Please  focus
the  image;  we  cannot  enjoy  the  movie"  [syn:  {sharpen}]
[ant:  {blur}]

From  The  Free  On-line  Dictionary  of  Computing  (13  Mar  01)  [foldoc]:

FOCUS

A  hierarchical  {database}  language  from
{Information  Builders,  Inc.}

(1994-12-21)

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