390  Royal  Society  : — Dr.  Huggius  on  the 
b.  Comparison  of  B  and  C. 
Duration  of  Duration  of 
Periods.  ascent  (B).  Periods.  descent  (C). 
T.   1833-92  to  1836-98     3*06  years  1836*98  to  1843*75  677  years. 
II.  1843-75  to  1847*87     4-12     „  1847*87  to  1856-31  8-44     „ 
III.    1856-31  to  185969     3*38     „  1859-69  to  1867*12  7'43     „ 
«■/.-.  0 
■Katl°  b*  Difference  from  mean. 
I.. 2-212]  +0-061. 
II 2-044  I  Mean  2-151  -0-107. 
Ill 2*198  J  +0-047. 
The  agreement  of  these  ratios  with  each  other,  and  the  small  dif- 
ferences from  the  mean  of  the  single  ratios,  justify  us  in  the  mean 
time,  until  a  greater  number  of  periods  are  before  us,  to  state  the 
connexion  between  the  two  branches  of  the  periodic  curve  from  one 
minimum  to  another  in  the  following  more  precise  terms  : — 
1/T  be  the  time  of  duration  of  sun-spot  increase  from  a  minimum 
to  a  maximum,  then  2*15  X  T  (with  a  probable  error  of  less  than 
+  0*05)  will  be  the  duration  of  the  sun-spot  decrease  until  the  next 
minimum. 
This  law,  together  with  the  fact  which  we  have  previously  esta- 
blished, that  a  longer  period  shows  generally  a  depressed  curve, 
while  a  shorter  is  characterized  by  great  peaks,  points  strongly  to 
the  conclusion  that  the  energy  of  the  ultimate  causes  of  sun-spot 
production,  whether  these  causes  be  intrasolar  or  extrasolar,  is  for 
every  period  constant. 
"Note  on  the  Telescopic  Appearance  of  Encke's  Comet."  By 
William  Huggins,  D.C.L.,  LL.D.,  F.R.S. 
The  first  three  figures  which  accompany  this  note  represent  the 
comet  on  evenings  on  which  its  appearance  was  described  in  a  note 
on  the  spectrum  of  the  comet  which  I  had  the  honour  to  present 
to  the  Koyal  Society*.  A  continuance  of  bad  weather  prevented  me 
from  making  later  observations  of  the  comet,  with  the  exception  of 
one  evening,  December  5,  when  figure  4  was  obtained  under  unfa- 
vourable circumstances. 
Fig.  1.  November  7,  7.30  p.m. — From  Oct.  17,  when  the  comet 
consisted  of  a  nearly  round  nebulosity  without  condensation  in  any 
part,  to  Nov.  7  no  observations  could  be  obtained.  At  the  latter 
date,  the  remarkable  fan-form  which  distinguishes  this  appearance 
of  the  comet  was  already  distinctly  presented.  The  faint  light  by 
which  the  comet  was  surrounded  terminated  on  the  side  from  the 
sun,  that  from  which  the  tail  is  usually  projected,  in  a  straight  boun- 
dary at  right  angles  to  the  longer  axis  of  the  comet.  At  the  opposite 
side,  that  towards  the  sun,  the  faint  nebulosity  expanded  and  became 
fainter  until  it  could  be  no  longer  traced.    The  minute  stellar  nucleus 
*  Proc.  Roy.  Soc.  vol.  xx.  p.  45. 
