slowly  away  from  the  paper,  or  move  the  single  candle  toward  the
paper,  until  a  point  is  found  where  the  spot  nearly  disappears.  The
paper  now  receives  the  same  amount  of  energy  from  the  single  flame

as  from  the  four  flames,  but
it  will  be  found  that  the
row  of  flames  is  twice  as
far  from  the  paper  as  the
single  flame.
Thus,  by   doubling
Fig.  254.  the   distance,   the   in-

tensity of  illumination  is  diminished  fourfold.  In  a  similar
manner  it  may  be  shown  that  at  three  times  the  distance
it  takes  nine  flames  to  be  equivalent  to  one  flame.  Hence,
the  intensity  of  illumination  diminishes  as  the  square  of  the
distance  increases.  This  is  called  the  law  of  inverse  squares.
Experiment  232.  —  Introduce  the  paper  disk,  as  above,  between
a  candle  flame  and  a  kerosene  or  a  gas  flame,  and  so  regulate  the  dis-
tance that  the  central  spot  will  disappear ;  then  calculate  the  relative
intensities  of  the  flames  in  accordance  with  the  law  of  inverse  squares.
This  is  the  method  usually  employed  by  gas  inspectors
for  testing  the  intensity  of  light-waves.  Apparatus  ar-
ranged for  this  purpose  is  called  a  photometer.  "The
candle  power,  which  is  the  unit  of  intensity  generally  em-
ployed in  photometry,  is  the  intensity  of  the  flame  of  a
sperm  candle  weighing  one-sixth  of  a  pound,  and  burning
one  hundred  and  twenty  grains  an  hour."

The  relative  brightness  of  the  common  sources  of  light-
waves are  approximately  as  follows : l  —

Sun  at  its  surface 190,000  candle  power.
Most  powerful  electric  arc 55,900      "

Incandescent  calcium 1,300      "          "

Ordinary  gas-burner 12  to  16      "          "

Standard  candle 1      "

i  C.  A.  Young.
PHOTOMETRY,   VISUAL   ANGLE,   ETC.  291

271.    Visual  Angle.
