696 
Lord  Rayleigh  :  Some  Measurements  of 
Motion  ?  *  •"  From  the  overpowering  brilliancy  of  the  yellow 
line,  it  may  be  inferred  that  the  pressure  is  not  very  low. 
Mercury  too,  for  which  the  principal  wave-lengths  have  been 
determined  with  great  accuracy  by  Fabry  and  Perot,  is  con- 
venient as  requiring  only  a  very  moderate  heating  ;  and 
cadmium,  in  spite  of  the  higher  temperature  demanded,  is 
indispensable.  Not  only  is  the  cadmium  red  by  general 
consent  the  ultimate  standard,  but  a  comparison  of  the  red 
and  green  ring  systems,  even  without  a  prism,  gives  rapid 
information  as  to  the  condition  of  the  gauge,  slightly  variable 
from  time  to  time  on  account  of  temperature  and  of  necessary 
readjustments.  Thus,  in  most  of  my  observations,  the  red 
ring  under  measurement  was  in  very  approximate  coincidence 
with  a  green  ring.  If,  owing  to  rise  of  temperature,  this 
ring  had  so  far  expanded  as  to  make  it  advisable  to  substitute 
the  next  interior  one,  there  could  still  be  no  uncertainty  as 
to  the  order  (one  higher)  of  the  ring  actually  under 
observation. 
As  cadmium  tubes  appear  to  have  been  found  troublesome, 
it  may  be  well  to  describe  a  simple  construction  specially 
adapted  to  private  workers  whose  skill  in  glass-blowing  is 
limited.  It  was  thought  that  alloying  and  consequent 
expansion  of  platinum  sealings  was  a  likely  source  of  difficulty, 
and  these  were  accordingly  dispensed  with.  The  diagram 
exhibits  half  the  complete  tube.  The  working  capillary  A, 
the  enlargement  B  D,  and  the  lateral  tube  G  for  attachment 
to  the  pump  are  much  as  usual.  But  the  enlargement  is 
continued  by  a  second  capillary  D  E,  perhaps  1J  mm.  in 
diameter  and  15  cm.  long,  through  which  passes  with 
approximate  fit  a  straight  aluminium  wire,  serving  as  electrode. 
The  air-tight  joint  at  E  between  the  wire  and  the  glass  is 
made  with  sealing-wax.     The  length  D  E  must  be  sufficient 
*  Phil.  Mag.  vol.  iv.  p.  215  (1902). 
