Some  Measurements  of  Wave-Lengths.  685 
Stefan's  law  for  the  black  body  could  also  not  be  considered 
conclusive.  This  state  of  affairs  has  been  changed  since  we 
know,  from  the  brilliant  experiments  of  Lebedew  and  of 
Nichols  and  Hull,  that  the  light-pressure  does  exist  and  does 
have,  very  approximately  at  all  events,  the  value  deduced 
a  priori  from  Maxwell's  theory.  It  still  remains,  however, 
highly  desirable  that  another  absolute  radiation  scale  should 
be  devised,  so  that  we  may  have  another  and  independent 
check  on  our  high  temperature  measurements.  Planck's 
formula  resls  on  too  many  assumptions  to  be  satisfactory 
theoretically  ;  and  WienV  displacement  law,  while  its 
theoretical  foundation  is  nearly  as  good  as  that  of  the  Stefan- 
Boltzmann  law,  does  not,  by  itself,  offer  a  good  method  from 
the  point  of  view  of  experimental  accuracy. 
U.S.  Department  of  Agriculture, 
Washington,  Jan.  17,  1906. 
LXIL  Some  Measurements  of  Wave-Lengths  ivith  a  Modified 
Apparatus,     By  Lord  Rayleigh,  O.M.,  Pres.  M.S.* 
AS  the  result  of  discussions  held  during  the  last  three  or 
four  years,  it  seems  to  be  pretty  generally  agreed  that 
the  use  of  the  diffraction-grating  in  fundamental  work 
must  be  limited  to  interpolation  between  standard  wave- 
lengths determined  by  other  means.  Even  under  the 
advantageous  conditions  rendered  possible  by  Rowland's 
invention  of  the  concave  grating,  allowing  collimators  and 
object-glasses  to  be  dispensed  with,  the  accuracy  attained  in 
comparisons  of  considerably  differing  wave-lengths  is  found 
to  fall  short  of  what  had  been  hoped.  I  think  that  this 
disappointment  is  partly  the  result  of  exaggerated  ex- 
pectations, against  which  in  1888  f  I  gave  what  was  intended 
to  be  a  warning.  Quite  recently,  Michelson  J  has  shown  in 
detail  how  particular  errors  of  ruling  may  interfere  with  results 
obtained  by  the  method  of  coincidences  ;  but  we  must  admit 
that  the  discrepancies  found  by  Kayser  §  in  experiments 
specially  designed  to  test  this  question,  are  greater  than 
would  have  been  anticipated. 
Under  these   circumstances,  attention  has  naturally  been 
directed   to  interference  methods,  and   especially  to  that  so 
*  Communicated  by  the  Author. 
t  Wave  Theory,  Enc.  Brit.;  Scientific  Papers,  iii.  p.  Ill,  footnote. 
+  Astro-physical  Journal,  xviii.  p.  278  (1903). 
§  Zeitschrift  fur  iviss.  Photographie,  Bd.  ii.  p.  49  (1904). 
