784 
Prof.  R.  W.  Wood  on 
A  nnmber  of  corrections  were  of  course  necessary,  for  the 
angles  in  air  are  greater  than  the  angles  within  the  glass,  on 
account  of  refraction.  Then,  too,  there  is  a  small  loss  due  to 
reflexion  within  the  prism  which  becomes  greater  as  the  angle 
is  increased. 
The   values   obtained    are   shown    graphically   in   fig.    2, 
Fig.  2. 
m 
m 
\ 
6fl 
' 
40 
\ 
■?o 
\ 
\ 
~~~®          Q 
8         G 
5' 
\5° 
10° 
25* 
30° 
intensities  being  plotted  as  ordinates,  and  angles  as  abscissae. 
The  observed  values,  which  have  been  corrected  for  the  small 
loss  due  to  internal  reflexion,  are  represented  by  circles,  while 
the  curve  drawn  through  them  is  the  theoretical  curve, 
calculated  on  the  assumption  that  the  intensity  of  the  radiation 
from  each  fluorescent  molecule  is  independent  of  the  direction 
within  the  glass.  Under  such  conditions  the  intensity  of  the 
illumination  of  the  surface  would  double  each  time  its 
apparent  area  was  halved  by  foreshortening. 
As  will  be  seen,  this  condition  appears  to  be  pretty  nearly 
fulfilled,  though  accurate  measurements  between  zero  and 
five  degrees  were  difficult.  The  fluorescent  radiations  obey 
therefore  within  the  medium  the  same  law  which  holds  in  the 
case  of  a  transparent  radiating  gas.  Outside  of  the  medium, 
that   is   in   air,   the   case   is  very   different.     The  intrinsic 
