On  an  Instrument  for  Compounding  Vibrations.         127 
I  must  confess  that  I  have  never  fully  understood  Stokes's 
position  in  this  matter.  A  medium  is  non- re  tractive  and 
nearly  transparent  for  the  pulses  constituting  Rontgen  rays. 
What  reception  would  it  give  to  simple  waves  of  half 
wave-length  equal  to  the  thickness  of  the  pulses  ?  I  should 
suppose  that  it  would  be  non-refractive  and  transparent  for 
these  also,  but  Stokes's  argument  seems  to  imply  the 
contrary.  The  paradox  would  then  have  to  be  met  that 
the  medium  treats  simple  waves  less  simply  than  compound 
ones. 
Nov.  16, 1905. 
IX.  On  an  Instrument  for  Compounding  Vibrations,  with 
application  to  the  drawing  of  Curves  suck  as  might  represent 
White  Light.    By  Lord  Rayleigh,  0  j\L,FM.S* 
[Plate  III.] 
IN  discussions  respecting  the  character  of  the  curve  by 
which  the  vibrations  of  white  light  may  be  expressed, 
I  have  often  felt  the  want  of  some  ready,  even  if  rough, 
method  of  compounding  several  prescribed  simple  harmonic 
motions.  Any  number  of  points  on  the  resultant  can  of 
course  always  be  calculated  and  laid  down  as  ordinates  ;  but 
the  labour  involved  in  this  process  is  considerable.  The 
arrangement  about  to  be  described  was  exhibited  early  in  the 
year  during  lectures  at  the  Royal  Institution.  As  it  is 
inexpensive  to  construct  and  easily  visible  to  an  audience,  I 
have  thought  that  such  a  description  might  be  useful,  accom- 
panied with  a  few  specimens  of  curves  actually  drawn  with 
its  aid. 
A  wooden  batten  say  1  inch  square  and  5  feet  long  is  so 
mounted  horizontally  as  to  be  capable  of  movement  only  along 
its  length.  For  this  purpose  it  suffices  to  connect  two  points 
near  the  two  ends,  each  by  means  of  two  thin  metallic  wires, 
with  four  points  symmetrically  situated  in  the  roof  overhead. 
This  mounting,  involving  four  constraints  only,  allows  also 
of  a  rotatory  or  rolling  motion,  which  could  be  excluded,  if 
necessary,  by  means  of  a  fifth  wire  attached  to  a  lateral  arm. 
In  practice^  however,  this  provision  was  not  used  or  needed. 
The  movement  of  the  batten  along  its  length  is  controlled  by 
a  piece  of  spring-steel  against  which  the  pointed  extremity 
of  the  batten  is  held  by  rubber  bands.     Any  force  acting  in 
*  Communicated  by  th-3  Author. 
