The  Black  Spot  in  Thin  Liquid  Films.  l¥l 
excellent  illuminant,  and  at  the  same  time  may  be  used  to 
heat  the  film. 
Fig.  1. 
Let  the  temperature  be  adjusted  with  the  flame  and  the 
pressure  with  the  mouth  until  the  first  black  film  is  about  to 
break  into  the  second.  Then  pinching  the  tube  with  the 
fingers  will  cause  the  second  black  to  appear  as  small  perfectly 
round  spots  in  the  first  black  film  ;  releasing  the  pressure, 
after  the  film  has  become  the  second  black,  will  cause  the  first 
black  to  appear,  in  a  similar  manner,  as  spots  in  the  second 
black. 
Thickness  of  the  Black  Film. — In  view  of  these  circum- 
stances, it  was  thought  advisable  to  repeat  the  measurements 
on  the  thickness  of  the  black  film.  The  work  was  begun  in 
the  Ryerson  Physical  Laboratory  of  the  University  of  Chicago, 
and  through  the  courtesy  of  the  officers  of  instruction,  in 
kindly  lending  the  necessary  apparatus,  was  carried  to  com- 
pletion elsewhere. 
In  order  to  vary  the  thickness,  by  changing  the  pressure, 
and  at  the  same  time  to  measure  the  thickness,  a  different 
form  of  the  interferometer  was  required.  In  fig.  2  is  shown 
a  familiar  form  of  the  Michelson  interferometer  that  was 
used.     The  object  that  was  to  be  gained  was  simplv  that  both 
3C2 
