﻿122 
  On 
  Polishing 
  the 
  Specula 
  of 
  Reflecting 
  Telescopes. 
  [Dec. 
  17, 
  

  

  It 
  may 
  be 
  remembered 
  that 
  many 
  years 
  ago 
  Mr. 
  Lassell 
  invented, 
  and 
  

   described 
  in 
  the 
  eighteenth 
  volume 
  of 
  the 
  Memoirs 
  of 
  the 
  Hoyal 
  Astro- 
  

   nomical 
  Society, 
  a 
  machine 
  for 
  polishing 
  specula. 
  It 
  is 
  no 
  part 
  of 
  the 
  

   object 
  of 
  this 
  paper 
  to 
  disparage 
  or 
  supersede 
  that 
  machine, 
  as 
  with 
  it 
  

   he 
  has 
  polished 
  many 
  specula 
  sensibly 
  perfect, 
  some 
  of 
  which 
  are 
  now 
  in 
  

   his 
  possession, 
  in 
  whose 
  surfaces 
  he 
  can 
  find 
  no 
  imperfection 
  whatever, 
  

   and 
  which 
  he 
  should 
  vainly 
  attempt 
  to 
  improve 
  ; 
  but 
  it 
  possesses 
  scarcely 
  

   power 
  enough 
  for 
  polishing 
  a 
  two-foot 
  speculum, 
  though 
  the 
  specula 
  

   belonging 
  to 
  the 
  telescope 
  of 
  that 
  size 
  which 
  he 
  took 
  out 
  to 
  Malta 
  in 
  

   1852 
  were 
  polished 
  with 
  that 
  machine. 
  Indeed 
  the 
  first 
  surfaces 
  on 
  the 
  

   four-foot 
  specula 
  of 
  the 
  telescope 
  taken 
  out 
  to 
  Malta 
  on 
  his 
  last 
  expedi- 
  

   tion 
  were 
  obtained 
  in 
  the 
  same 
  way 
  ; 
  but 
  it 
  was 
  with 
  great 
  difficulty, 
  and 
  

   ultimately 
  the 
  machine 
  broke 
  down 
  hopelessly, 
  the 
  result 
  of 
  which 
  was 
  

   the 
  construction 
  of 
  the 
  present 
  one 
  described 
  in 
  the 
  paper 
  now 
  presented 
  

   to 
  the 
  Society. 
  But 
  reference 
  must 
  be 
  had 
  to 
  the 
  paper 
  itself, 
  and 
  to 
  the 
  

   drawings 
  which 
  accompany 
  it, 
  for 
  an 
  adequate 
  description. 
  

  

  In 
  this 
  machine 
  there 
  may 
  possibly 
  be 
  found 
  nothing 
  very 
  essentially 
  

   new 
  ; 
  it 
  contains 
  parts 
  adopted 
  from 
  others 
  and 
  modified, 
  the 
  principal 
  

   novelty 
  being 
  a 
  method 
  of 
  giving 
  a 
  regular 
  and 
  gently 
  controlled 
  axial 
  

   motion 
  to 
  the 
  polisher 
  while 
  it 
  is 
  undergoing 
  the 
  various 
  other 
  motions 
  

   proper 
  to 
  the 
  machine. 
  

  

  Mr. 
  Lassell 
  attempts 
  in 
  this 
  paper 
  to 
  describe 
  the 
  processes 
  with 
  suffi- 
  

   cient 
  precision 
  to 
  enable 
  persons 
  of 
  ordinary 
  intelligence 
  and 
  some 
  me- 
  

   chanical 
  aptitude 
  to 
  obtain 
  with 
  ease 
  and 
  certainty 
  surfaces 
  on 
  specula 
  

   (taking 
  a 
  two-foot 
  speculum 
  as 
  an 
  example) 
  which 
  shall 
  be 
  sensibly 
  per- 
  

   fect 
  in 
  figure 
  and 
  polish 
  ; 
  and 
  this, 
  as 
  his 
  words 
  imply, 
  without 
  the 
  tedi- 
  

   ous 
  trial-and-error 
  process, 
  which 
  amateurs 
  have 
  had 
  too 
  frequently 
  to 
  

   experience. 
  Another 
  object 
  he 
  has 
  had 
  also 
  especially 
  in 
  view 
  is 
  to 
  

   render 
  the 
  process 
  interesting 
  and 
  pleasurable 
  throughout, 
  by 
  devising 
  

   new 
  modes 
  of 
  performing 
  the 
  most 
  disagreeable 
  parts 
  of 
  the 
  operation, 
  

   such 
  as 
  the 
  formation 
  of 
  the 
  pitch-tool, 
  which 
  in 
  large 
  surfaces 
  is 
  apt 
  to 
  

   be 
  very 
  troublesome 
  and 
  annoying. 
  This 
  is 
  accomplished 
  by 
  simply 
  

   studying 
  the 
  properties 
  of 
  pitch 
  and 
  adapting 
  its 
  treatment, 
  so 
  to 
  speak, 
  

   to 
  its 
  peculiar 
  unaccommodating 
  humour. 
  A 
  further 
  aim 
  has 
  been 
  to 
  sim- 
  

   plify 
  to 
  the 
  utmost 
  the 
  mode 
  of 
  action 
  in 
  every 
  particular, 
  leaving 
  out 
  

   every 
  thing 
  which 
  long 
  'experience 
  has 
  shown 
  to 
  be 
  unnecessary 
  — 
  for 
  

   instance, 
  polishing 
  with 
  the 
  speculum 
  partly 
  immersed 
  in 
  water, 
  straining 
  

   pitch 
  through 
  muslin, 
  &c, 
  processes 
  tedious 
  and 
  vexatious 
  enough 
  with- 
  

   out 
  being 
  required. 
  . 
  

  

  Hules 
  are 
  given 
  in 
  the 
  paper 
  for 
  the 
  rates 
  of 
  motion 
  of 
  both 
  the 
  

   polisher 
  and 
  speculum, 
  as 
  well 
  as 
  for 
  the 
  lengths 
  of 
  the 
  strokes 
  of 
  the 
  

   crank-arms, 
  which 
  have 
  an 
  immediate 
  and 
  powerful 
  influence 
  in 
  the 
  pro- 
  

   duction 
  of 
  the 
  required 
  curve. 
  The 
  mode 
  of 
  making 
  the 
  polisher-base, 
  and 
  

   covering 
  it 
  with 
  pitch 
  in 
  squares 
  approximately 
  to 
  fit 
  the 
  speculum 
  at 
  

   once, 
  and 
  the 
  mode 
  of 
  keeping 
  the 
  polisher 
  for 
  a 
  considerable 
  time 
  and 
  

  

  