﻿Telescopic 
  Vision. 
  

  

  327 
  

  

  Now, 
  under 
  the 
  circumstances 
  described, 
  and 
  at 
  the 
  centre 
  

  

  of 
  the 
  disk, 
  a 
  millimetre 
  on 
  the 
  eidolon 
  corresponds 
  to 
  nearly 
  

  

  10 
  of 
  the 
  geographical 
  degrees 
  annexed 
  to 
  the 
  maps 
  of 
  the 
  

  

  so-called 
  ' 
  canals 
  ' 
  upon 
  Mars. 
  To 
  appreciate 
  this, 
  get 
  a 
  

  

  small 
  marble 
  somewhat 
  less 
  than 
  12 
  mm. 
  in 
  diameter, 
  and 
  

  

  view 
  it 
  from 
  a 
  distance 
  of 
  ten 
  inches 
  ; 
  for 
  it 
  is 
  on 
  to 
  such 
  a 
  

  

  tinv 
  sphere 
  that 
  we 
  must 
  conceive 
  the 
  details 
  recorded 
  in 
  the 
  

  

  maps 
  of 
  Schiaparelli 
  and 
  Lowell 
  to 
  be 
  crowded, 
  in 
  order 
  to 
  

  

  have 
  a 
  just 
  idea 
  of 
  what 
  it 
  was 
  that 
  those 
  skilful 
  observers 
  

  

  succeeded 
  in 
  glimpsing 
  on 
  this 
  minute 
  scale, 
  occasionally, 
  at 
  

  

  favourable 
  moments, 
  and 
  with 
  difficulty. 
  Many 
  years 
  ago, 
  

  

  after 
  the 
  second 
  of 
  Schiaparellr's 
  maps 
  appeared, 
  the 
  writer 
  

  

  *of 
  the 
  present 
  paper 
  took 
  advantage 
  of 
  a 
  good 
  opposition 
  of 
  

  

  Mars 
  to 
  study 
  what 
  could 
  be 
  seen 
  when 
  the 
  state 
  of 
  the 
  

  

  atmosphere 
  was 
  favourable, 
  upon 
  the 
  eidolon 
  of 
  the 
  planet 
  

  

  which 
  was 
  presented 
  to 
  him 
  by 
  a 
  good 
  reflecting 
  telescope 
  of 
  

  

  12-inch 
  aperture, 
  exceptionally 
  well 
  adjusted 
  *. 
  The 
  author 
  

  

  also 
  derived 
  specially 
  useful 
  instruction 
  from 
  making 
  careful 
  

  

  control 
  observations 
  upon 
  the 
  Moon 
  when 
  nearly 
  full 
  under 
  

  

  equi 
  cedent 
  optical 
  conditions 
  to 
  those 
  which 
  he 
  employed 
  upon 
  

  

  Mars. 
  These 
  will 
  be 
  described 
  in 
  the 
  last 
  Chapter 
  of 
  this 
  

  

  paper, 
  and 
  he 
  can 
  recommend 
  a 
  frequent 
  recourse 
  to 
  control 
  

  

  observations 
  upon 
  the 
  nearly 
  full 
  Moon 
  to 
  all 
  astronomers 
  

  

  who 
  have 
  occasion 
  to 
  make 
  it 
  their 
  aim 
  to 
  become 
  serious 
  

  

  -t 
  a 
  dents 
  of 
  Mars. 
  

  

  But 
  before 
  describing 
  these 
  comparative 
  observations, 
  we 
  

   have 
  first 
  to 
  inquire 
  under 
  what 
  conditions 
  certain 
  typical 
  

  

  * 
  The 
  author's 
  telescope 
  was 
  adjusted 
  by 
  the 
  special 
  Collimator 
  for 
  

   completing 
  the 
  Adjustments 
  of 
  Reflecting- 
  Telescopes, 
  described 
  at 
  p. 
  30 
  

   of 
  the 
  Report 
  of 
  the 
  Cheltenham 
  Meeting 
  of 
  the 
  British 
  Association 
  in 
  

   1856, 
  with 
  the 
  addition 
  that 
  the 
  line 
  of 
  colliination 
  of 
  the 
  tube 
  which 
  

   carries 
  the 
  eyepieces 
  (into 
  which 
  the 
  collimator 
  is 
  to 
  be 
  placed) 
  was 
  

   made 
  movable 
  through 
  a 
  small 
  range 
  by 
  three 
  screws 
  acting 
  against 
  

   springs 
  ; 
  and 
  that 
  the 
  adjustment 
  was 
  completed, 
  not 
  by 
  disturbing 
  the 
  

   adjustments 
  of 
  either 
  of 
  the 
  mirrors, 
  but 
  by 
  a 
  slight 
  motion 
  of 
  this 
  tube, 
  

   thus 
  altering 
  the 
  line 
  of 
  collimation 
  of 
  the 
  eyepieces. 
  This 
  was 
  found 
  

   to 
  be 
  optically 
  sufficient 
  and 
  rendered 
  it 
  extremely 
  easy 
  to 
  make 
  the 
  

   adjustment 
  — 
  so 
  easy 
  that 
  it 
  could 
  be 
  made 
  if 
  necessary 
  before 
  each 
  

   observation. 
  An 
  electric 
  spark, 
  made 
  by 
  that 
  kind 
  of 
  replenisher 
  sold 
  

   for 
  lighting 
  gas, 
  furnished 
  the 
  light 
  at 
  the 
  focus 
  of 
  the 
  collimator. 
  

  

  The 
  instrument 
  was 
  made 
  for 
  the 
  author 
  by 
  Sir 
  Howard 
  Grubb, 
  F.R.S., 
  

   and 
  when 
  employed 
  upon 
  a 
  sufficiently 
  good 
  telescope 
  was 
  found 
  to 
  add 
  

   materially 
  to 
  the 
  excellence 
  of 
  the 
  image 
  that 
  could 
  be 
  secured 
  by 
  the 
  

   nsual 
  methods 
  of 
  adjusting 
  the 
  two 
  mirrors. 
  

  

  The 
  adjustments 
  of 
  the 
  mirrors 
  were 
  in 
  the 
  first 
  instance 
  made 
  with 
  

   care 
  in 
  the 
  usual 
  way. 
  This 
  was 
  sufficient 
  to 
  bring 
  the 
  image 
  of 
  the 
  

   spark 
  well 
  within 
  the 
  field 
  of 
  view 
  of 
  the 
  collimator, 
  and 
  the 
  image 
  of 
  

   the 
  spark 
  was 
  then 
  brought 
  into 
  coincidence 
  with 
  the 
  sparks 
  by 
  the 
  

   three 
  adjusting 
  screws 
  mentioned 
  above. 
  

  

  