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THE 

MICROSCOPE 

CONRAD 
BECK 


QH205 


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(UllF  E  B.  Bill  ffiibrarQ 

Nnrtlj  (Earnltna  g>tatF 
ImtirrHitg 


This  book  was  presented  by 

Theodore    G.    Rochow 

QH205 

B3  5 


^^^VAiVVj! 


NC     STATE    UNIVERSITY      D.H     HILL    LlBRAR 


S001 39483  S 


Collins  &  Gray 
Scientific  FJooks  &  Periodicals 
■iQA  Museum  Street, 
]      London,  W.C.I.        hol  '>l''n 


THIS  BOOK  IS  DUE  ON  THE  DATE 
INDICATED  BELOW  AND  IS  SUB- 
JECT TO  AN  OVERDUE  FINE  AS 
POSTED  AT  THE  CIRCULATION 
DESK. 


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THE  MICROSCOPE 


THE   MICROSCOPE 


A   SIMPLE    HANDBOOK 


BY 


CONRAD  BECK 


FIRST   EDITION 


TWO   SHILLINGS   AND  SIXPENCE  NET 


LONDON 

R.   &  J.   BECK,   LTD. 

68  GORNHILL,  E.G. 
1921 


PREFACE 

This  book  is  intended  as  a  guide  to  the  use  of  the  microscope. 
The  correct  use  of  the  instrument  follows  directly  from  a  know- 
ledge of  the  functions  of  the  different  parts.  It  has  therefore 
been  found  best  to  develop  the  method  of  manipulation  in  the 
course  of  the  descriptions  of  the  component  portions.  Such 
particulars  as  are  given  of  the  principles  of  its  optical  construc- 
tion are  of  the  simplest  character,  and  their  comprehension 
requires  no  optical  knowledge.  Certain  theoretical  matters 
are  stated  in  this  book,  but  are  not  explained.  Some  of  the 
descriptions  may  appear  to  be  extremely  elementary  to  experi- 
enced microscopists,  but  it  is  hoped  that  even  they  may  find  useful 
matter  which  is  not  available  in  the  ordinary  text-books. 

The  author  hopes  to  publish  a  further  volume  at  a  later  date, 
which  will  deal  with  the  optical  theory  and  the  use  of  the 
microscope  in  greater  detail. 


CONTENTS 


CHAPTER  I 

PAGE 

A   SIMPLE   DESCRIPTION   OF   THE   MICROSCOPE         .  .  .11 


CHAPTER  II 

ILLUMINATING  APPARATUS  AND  SOURCES  OP  ILLUMINATION  .  26 

CHAPTER  III 

APPARATUS   FOR   HOLDING   SPECIMENS  ....         50 

CHAPTER  IV 

SUNDRY   APPARATUS  ...  ....        67 

CHAPTER  V 

OBJECT  GLASSES  AND  EYEPIECES 76 

CHAPTER  VI 

THE   MICROSCOPE   STAND 92 

CHAPTER  VI  r 

THE   MICROSCOPE   AS   A   RECREATION 125 

INDEX    ..........       143 


DESCRIPTION   OF   FIG.    1 

A.  Base  to  support  instrument. 

B.  Pillar  to  support  instrument. 

C.  Joint  for  inclining  instrument. 

D.  Stage  for  reception  of  object  to  be  examined. 

E.  Hole  for  attaching  mechanical  stage. 

F.  Mirror  for  illuminating  transparent  objects. 

G.  Substage  for  carrying  illuminating  apparatus. 
H.  Substage  focussing  adjiistment. 

J.  Substage  condenser  for  regulating  the  illumination, 

K.  Iris  diaphragm  for  varying  the  light. 

L.  Limb  for  holding  the  body  and  stage. 

M.  Body  for  carrying  the  observing  lenses. 

N.  Drawtube  for  lengthening  body. 

Nl.  Drawtube  stop  to  prevent  reflections  from  tube  entering  eyepiece. 

O.  Eyepiece,  combination  of  lenses  nearest  observer's  eye. 

P.  Fine  adjustment  for  delicate  focussing  of  body. 

Q.  Coarse  adjustment  for  rapid  focussing  of  body. 

R.  Object  glass,  combination  of  lenses  nearest  the  object. 

S.  Nosepiece  with  universal  screw  for  carrying  object  glasses. 

T.     Eyepoint  position  where  all  emergent  light  passes  through  a  small 
area  and  where  observer's  eye  should  be  placed. 

U.    Position  where  the  primary  image  formed  by  the  object  glass  is 
produced. 

V.    Position  where  final  virtual  image  formed  by  eyepiece  is  produced. 


8 


Fig.  1. — Diagram  of  a  Microscope. 


THE  MICROSCOPE 

CHAPTER  I 

A  SIMPLE  DESCRIPTION  OF  THE  MICROSCOPE 

The  microscope  is  an  apparatus  for  producing  an  enlarged 
image  of  a  small  object.  In  its  complete  form  it  is  an  elaborate 
instrument,  but  to  understand  its  constrviction  it  may  be  looked 
upon  as  a  complex  form  of  magnifying  lens  with  the  addition  of 
means  for  making  delicate  adjustments  both  for  moving  the  lens 
and  the  object  and  for  obtaining  special  forms  of  illumination. 
It  consists  primarily  of  three  parts — the  body,  which  carries  the 
observing  lenses,  the  stand  or  framework,  and  the  illumination 
apparatus. 

The  body  (M)  carries  an  object  glass  (R),  which  is  attached  to  The  body. 
the  object  end  by  a  standard  size  screw  thread,  and  an  eyepiece 
(0),  which  slips  loosely  into  the  tube  at  the  eye  end  in  a  standard 
size  fitting.  It  has  a  telescopic  tube,  called  a  draw- tube  (N),  for 
varying  the  distance  between  the  object  glass  and  the  eyepiece, 
and  a  diaphragm  (Nl)  to  prevent  reflections  from  the  inner  sur- 
faces of  the  tubes  from  entering  the  eye. 

The  body  and  its  lenses  combined  form  the  magnifying 
apparatus. 

The  object  to  be  examined  is  placed  on  the  stage  (D)  of  the 
microscope.  The  object  glass  if  used  by  itself  acts  in  the  same 
manner  as  a  lantern  lens.  It  throws  an  enlarged  picture  of  the 
object  to  a  position  (U)  at  the  upper  end  of  the  body,  just  as  a 
lantern  lens  throws  an  enlarged  picture  of  a  small  lantern  slide 
upon  a  white  screen,  but  instead  of  its  being  thrown  upon  a 
white  screen  it  is  thrown  into  space.  This  image  is  examined 
with  a  magnifying  lens  called  the  eyepiece  (0),  by  which  it  is 
further  magnified.  If  the  primary  image  were  projected  upon  a 
lantern  screen  and  one  were  to  cut  a  hole  in  the  screen  and  stand 
behind  it  with  a  magnifying  lens  focussed  upon  the  plane  of  the 
screen,  one  would  have  the  same  kind  of  instrument  as  a  micro- 
scope on  a  large  and  inconvenient  scale. 

11 


12 


THE  MICROSCOPE 


of  object 
glass. 


The  object  The  object  glass  (E)  in  the  earliest  instruments  was  a  single 

^^*^'  double  convex  lens  (Fig.  2) ;  it  gave  an  enlarged  but  very  im- 

perfect picture  of  small  objects,  the  outlines  were      ^^777777^ 
surrounded    by   coloured   fringes,    and   the   details      '^ii.r^^^f 
were  fuzzy  and  indistinct.     Such  lenses  were  made        Fia.  2. 
several  hundred  years    ago,   but  in  the    early  part 
of  the  nineteenth  century  is  was  discovered  that  the  defects  of 
a   single  lens  could  be    overcome   by   using   several   lenses   in 
combination,  made  of  different  kinds   of   glass  and  of  suitable 
shapes  and   sizes. 
Focal  length        Modern  object  glasses  are  made  of  different  powers  to  give 
different  magnifications  in  the  primary  image,  and,  in  general, 
the  more  an  object  glass  magnifies,  the  larger  the  number  of  lenses 
that  are  required  to  produce  a  perfect  image-      For  instance, 
Fig.  3  shows  the  optical  construction  of  the  2/3-inch  (16-mm.), 
^  ^  l/6-inch(4-mm.), 

and  1/12  -  inch 
(2-mm.)  object 
glasses. 

The  name 
2/3,  1/6,  or  1/12 
inch,  as  applied 
to  an  object 
glass,  represents 
its  focal  length. 
It  indicates  its 
magnifying 
^  power.  If  an 
ordinary  single 
Fig.  3. — /  =  focal  length  ;   w  =  working  distance,    \qy\s,    of      2-inch 

focal  length  is 
used  as  a  hand  magnifying  glass,  it  has  to  be  placed  about 
2  inches  from  an  object  to  give  a  clear  image,  and  the 
2/3,  1/6,  and  1/12  inch  require  to  be  placed  at  about  these 
respective  distances  from  the  object  when  in  use — thus  the 
higher  the  magnifying  power  of  a  lens,  the  closer  it  must  be 
to  the  object. 

Object  glasses  are  not  single  lenses,  but  are  composed  of 
several,  and  consequently  the  focal  distance  is  measured  from  a 
point  about  half-way  between  the  front  and  back  surfaces  of  the 
component  lenses.  The  distance  between  the  foremost  lens 
and  the  object  is,  therefore,  always  considerably  less  than  the 
true  focal  distance.  This  is  called  the  working  distance  to  signify 
the  space  between  the  end  of  the  microscope  and  the  object 
when  it  is  so  adjusted  that  a  clear  picture  is  obtained,  or  when 
it  is,  as  it  is  called,  "  in  focus."  A  list  of  the  working  distances 
of  different  object  glasses  is  given  on  page  82. 

An  examination  of  the  diagram  (Fig.  1)  on  page  9  illustrates 


Working 
distance. 


Magnifying 
power. 


A  SIMPLE  DESCRIPTION  OF  THE   MICROSCOPE     13 

the  formation  of  the  images.  An  enlarged  pictm-e  of  an  object 
placed  upon  the  stage  (D)  is  formed  in  the  neighbourhood  of 
the  eyepiece  at  U,  and  the  eyepiece  again  magnifies  this  image, 
projecting  the  light  into  the  eye  as  if  it  came  from  an  object 
situated  at  V. 

The  eye,  when  placed  in  a  small  area  (T)  through  which  all  Eyepiece. 
light  passes,  and  which  is  known  as  the  eyepoint,  sees  the  final 
picture  of  the  object  as  if  it  were  a  real  object   placed   at  V, 
10  inches  from  the  eye. 

It  is  assumed  for  convenience  of  measurement  that  this  picture  virtual 
is  actually  10  inches  away,  though  it  may  be  formed  at  a  some-  ^'"*°®' 
what  different  position  according  to  the  adjustment  or  condition 
of  the  observer's  eye.     Whether  the  virtual  image  is  actually 
at  6,   10,   or  20   inches  is  of   no   importance.      It   makes   no. 
difference  to  the  size  of  the  picture,  because  when  the  virtual 
image  is  formed  farther  away  it  becomes  proportionally  larger. 
In  Fig.  4,  if  E  is  the  eye  and  0  0'  0''  are  objects  of  different 
sizes,  they  produce  the  same  size  pictures  in  the  eye  if  placed 
at     such    distances    that    they 
subtend  the  same  angle. 

The  magnifying  power  of 
the  microscope  will  depend 
upon  the  size  of  this  final 
image  formed  at  V  (Fig.  1) 
compared  with  the  size  of  the 
object  being  examined.  In  this 
connection  it  should  be  under- 
stood that  if  a  microscope  is  said  to  magnify  100  diameters, 
it  means  that  the  picture  that  is  seen  is  100  times  as  long  and 
100  times  as  wide  as  the  object  would  appear  if  it  were 
taken  from  the  stage  (D,  Fig.  1)  and  placed  in  the  position  V, 
10  inches  from  the  eye. 

In  order  to  express  how  much  larger  an  object  appears 
when  seen  through  the  microscope  than  when  seen  by  the  naked 
eye,  a  standard  distance  must  be  taken,  because  an  object  appears 
to  the  naked  eye  to  be  of  different  sizes  at  different  distances. 
A  sixpence  is  almost  invisible  at  a  distance  of  100  yards,  but  it 
is  a  large  object  at  8  inches.  Therefore,  some  standard 
must  be  taken  for  comparison  purposes,  and  10  inches  has 
been  universally  adopted.  The  magnifying  power  of  a  micro- 
scope always  denotes  the  relative  size  of  the  picture  com- 
pared with  that  of  the  original  object  when  placed  10  inches 
from  the  eye. 

If  a  microscope  has  a  magnifying  power  of  100,  such  magnifi-  ^^^J^^i^*^ 
cation  may  be  produced  by  different  methods.  The  object  obtaining 
glass  may  magnify  the  object  twenty  times  in  the  primary  image,  ^^s^^^e 
and  the  eyepiece  increasing  the  primary  image  five  times  will 
give  a  total  of  a  hundred.     This  magnification  may  also  be  pro- 

y 


power. 


14  THE  MICROSCOPE 

duced  by  a  lower  power  object  glass  which  magnifies  the  object 
ten  times,  and  a  higher  power  eyepiece  which  magnifies  it 
again  by  ten.  The  same  result  is  obtained  as  far  as  magnifying 
power  is  concerned,  but  a  different  result  as  regards  the  quality 
of  the  image. 

Another  method  of  varying  the  magnifying  power  is  by  increas- 
ing the  distance  between  the  object  glass  and  the  eyepiece.  To 
enable  this  to  be  done  the  microscope  is  supplied  with  a  sliding 
drawtube  (N),  which  allows  the  tube  length  to  be  varied  from 
140  to  200  mm.  The  reason  for  this  increase  in  magnification 
is  well  illustrated  by  reference  to  the  lantern,  in  which  case  the 
lantern  lens  gives  a  larger  picture  when  it  projects  it  upon  a 
screen  that  is  at  a  greater  distance.  In  the  same  way  the  micro- 
scope object  glass  produces  a  more  highly  magnified  primary 
image  if  by  sUght  adjustment  in  the  focussing  of  the  instrument 
the  picture  is  formed  at  a  greater  distance,  and  the  drawtube 
of  the  microscope  is  extended  so  as  to  examine  the  picture 
formed  at  this  greater  distance. 
Field  of  The  "  field  of  view  "  is  a  term  applied  to  the  size  of  the  object 

^^^'  that  can  be  seen  at  one  time  by  means  of  the  microscope.     To 

assist  in  increasing  the  size  of  field  an  eyepiece  is  made  of  two 
lenses  instead  of  a  single  one.  The  lower  field  lens  is  situated 
below  the  position  U  (Fig.  1),  where  the  primary  image  is 
produced,  and  increases  the  field  of  view  while  the  upper  lens 
does  the  magnifying. 

Suppose  that  the  apparent  field  of  view  is  a  circle  of  about 
8  inches  diameter  at  the  position  V,  where  the  final  image 
seen  through  the  microscope  appears  to  be.  It  is  evident  that 
with  a  microscope  magnifying  100  diameters,  the  size  of  the  largest 
object  that  can  be  observed  at  one  time  is  only  1/100  the  size 
of  this  field,  or  about  1/12  inch,  so  that  for  this  reason  alone 
it  is  important  that  a  microscope  should  possess  a  means  of 
varying  the  magnifying  power.  It  is  sometimes  desirable  to 
examine  a  large  area  of  an  object  with  a  small  magnifying  power, 
at  others  a  small  area  with  a  large  magnifying  power.  A  table 
of  the  fields  of  view  given  by  different  lenses  appears  on  page  82. 
The  question  arises  as  to  whether  it  is  preferable  to  vary  this 
magnifying  power  by  means  of  changing  the  eyepiece,  by  means 
of  changing  the  object  glass,  or  by  means  of  lengthening  the 
drawtube. 

This   is   influenced   by   an   optical   consideration   of   great 
importance. 
Aperture.  In  the  early  days,  before  it  was  understood  how  to  correct 

the  errors  of  a  single  lens,  microscopes  were  constructed  in 
which  the  object  glass  was  a  single  lens,  the  defects  of  which 
were  reduced  by  putting  a  very  small  aperture — almost  a  pin- 
hole— in  front  or  behind  this  lens.  This  meant  that  only  an 
extremely  fine  cone  of  light  from  each  point  of  the  object  could 


Small  Aperture     Large  Aperture 
Lens.  Lens. 

Fig.  5. — a  =  angular  aperture. 


A  SIMPLE  DESCRIPTION  OF  THE  MICROSCOPE     15 

enter  the  instrument  (see  Fig.  5).  It  was  soon  found  tliat 
when  this  was  the  case,  although  great  magnifying  power  could 
be  obtained,  fine  detail  could  not  be  seen,  but  merely  a  repre- 
sentation on  a  larger  scale  of  the  coarse  structure  which  could 
readily  be  seen  with  a  lower  magnifying  power. 

In  order  that  an  advantage  should  be  obtained  from  the  use 
of  higher  magnifying  power,  it  was  necessary  to  admit  into  the 
microscope  a  correspondingly  larger  cone  of  light  from  each  point 
of  the  object,  as  unless  this 
were  done,  no  advantage  could 
be  obtained  in  the  observation 
of  fine  details.  Such  a  plan 
had  the  further  advantage  that 
it  collected  a  larger  amount  of 
light  and  rendered  the  object 
more  brilliant.  The  size  of  the 
cone  of  light  admitted  into  the 
microscope  from  each  point  of 
the  object  is  called  the  aperture 
{a,  Fig.  5).  It  is  expressed 
either  by  the  angle  of  the  cone 
of  light  entering  the  micro- 
scope or  by  a  figure  called  the  numerical  aperture,  or  N.A. 

The  aperture  is  of  such  paramount  importance,  that  the  limit 
of  what  can  be  seen  with  the  microscope  does  not  depend  upon 
what  magnifying  power  can  be  obtained,  but  upon  what  size 
cone  of  light  can  be  collected  from  the  object  by  means  of  the 
object  glass  ;  and  lenses  can  be  made  with  a  much  higher  magnify- 
ing power,  but  they  cannot  be  made  with  a  larger  aperture,  than 
those  in  use  at  the  present  time. 

The  aperture,  therefore,  has  a  direct  bearing  upon  the  best 
method  of  increasing  magnifying  power,  because,  if  an  object 
glass  can  only  admit  a  certain  aperture  of  light,  the  use  of  an 
eyepiece  does  not  alter  this  property,  and  therefore  to  increase 
the  magnifying  power  by  high  eyepieces  is  of  no  service,  when 
carried  beyond  that  power  which  is  sufficient  to  enable  the  detail 
that  can  be  shown  by  the  aperture  of  a  particular  object  glass 
to  be  seen. 

The  best  method  of  increasing  the  magnifying  power  is, 
therefore,  by  changing  the  object  glass.  Most  object  glasses 
have  sufficient  aperture  to  allow  of  the  use  of  an  eyepiece  of  as 
high  a  power  as  15,  but,  in  general,  magnification  of  more  than 
10  by  means  of  the  eyepiece  should  only  be  used  in  special  cases, 
and  the  object  glass  should  be  changed  rather  than  the  eyepiece. 

The  same  reason  makes  it  undesirable  to  depend  for  increased 
magnifying  power  upon  extending  the  drawtube  of  the  micro- 
scope, and  the  more  so  in  this  case  because  the  object  glass  can 
only  be  constructed  to  work  at  its  best  with  a  particular  length 


Limit  of 
vision 
dependent 
on  aperture. 


Best  method 
of  increasing 
magnifying 
power. 


standard 
length  of 
body. 


16 


THE  MICROSCOPE 


of  body.  To  obtain  the  most  perfect  results  a  tube  length  of 
160  mm.  should  be  used — the  drawtube  of  the  microscope  is 
graduated,  and  can  be  set  at  this  figure.  If  a  revolving  nosepiece 
is  in  use,  this  lengthens  the  body  15  mm.,  and  the  drawtube  should 
be  set  at  145  mm.  instead  of  160  mm. ;  with  a  Sloan  object  glass 
changer  measuring  10  mm.  it  should  be  set  at  150  mm. 
Thickness  of  The  thickness  of  the  cover  glass  used  over  the  object  has  no 
cover  glass.  g£fgg^  ^^}j  q^j^  immersiou  lens  and  but  slight  influence  with  the 
low  powers,  but  is  a  matter  of  importance  with  a  high-power 
dry  lens.  A  1/6-inch  object  glass  can  only  be  optically  correct 
for  one  thickness  of  cover  glass,  and  it  is  most  important  to 
always  use  those  known  as  No.  1  thickness.  The  object 
glasses,  unless  otherwise  ordered,  are  always  made  for  a  thickness 
of  '007  inch  ('18  mm.),  which  is  the  average  thickness  of  No.  1 
cover  glass.  Thicker  cover  glasses  should  only  be  used  for 
objects  to  be  examined  with  low  powers. 

The  delineation  of  fine  structure  depends  upon  the  aperture 
of  the  object  glass  being  sufficiently  large  to  produce  an  image 
of  this  fine  structure,  but  combined  with  this  it  must  possess  a 
sufficient  degree  of  magnification  to  enable  this  image  to  be  clearly 
seen.  We  may  know  that  the  finest  lines  of  an  etching  or  steel 
engraving  exist  in  a  print,  but  it  may  be  necessary  to  magnify 
the  image  in  order  to  make  them  visible  as  single  lines  to  the 
eye.  If  the  print  is  magnified  further,  the  fine  lines  appear 
thicker,  but  no  further  fine  lines  are  there  to  be  seen.  Thus 
lines  which  are  invisible  require  a  certain  degree  of  magnification 
to  see  them  clearly,  but  extra  magnification  beyond  this  point 
is  useless.  So  with  a  microscope  object  glass,  it  must  possess 
a  large  enough  aperture  to  produce  the  detail  in  the  image,  and 
the  magnifying  power  need  not  be  more  than  enough  to  enable 
the  eye  to  see  it  clearly. 

Each  object  glass  has  a  particular  aperture,  sufficient  to 
form  an  image  of  all  the  detail  that  can  be  seen  with  the  magnify- 
ing power  given  by  it  in  conjunction  with  a  moderate  eyepiece. 
The  following  table  gives  the  apertures  of  standard  object  glasses  : 


Apertures 
suitable  for 
different 
powers. 


Table  of 
apertures 
and  powers. 


Focus. 

Angular 
Aperture. 

Numerical 
Aperture. 

Initial 
Magni- 
fying 
Power. 

Magnj 

fying  Power  with 
Eyepiece. 

42  mm. 

25  mm. 

17  mm. 

1^  in.    =  40  mm.    . 

19'' 

•16 

3 

20 

34 

50 

1^  in.    =  32  mm.    . 

17° 

•15 

4 

25 

45 

65 

2/3  in.  =16  mm.    . 

32° 

•28 

10 

62 

110 

155 

1/3  in.   =  8  mm. 

60° 

•5 

18^5 

115 

200 

285 

1/6  in.  =  4  mm.     . 

116° 

•85 

40 

285 

490 

690 

1/8  in.    oil  immer- 

sion =  3  mm. 

•95 

60 

427 

735 

1,015 

1/12  in.  oil  immer- 

sion =  2  mm. 

1^3 

90 

530 

900 

1,275 

A  SIMPLE  DESCRIPTION  OF  THE  MICROSCOPE     17 

The  1/6-incli   is   receiving   from   the   object,  cones  of  light  Anf>ie  in  air 
of  116°,  as  shown  in  Fig.  6.     It  could  not  be  made  to  collect  wiKaUn 
a  very  much  larger  angle  of  light  because  it  cannot  be  used  in  s^^ss. 
absolute  contact  with  the    object.     Sufficient  space    must   be 
provided  for  a  thin  glass  cover  and  a  small  distance  for  focussing 
adjustment.     It  will  be  noticed  in  Fig.  6  that  the  cone  of  light, 
which   is   116°  as  it  enters   the 

object  glass,  is  only  68°  when  it  ^i^ 

passes  through   the   object.     It  rQbif' 

is  spread  out  by  refraction  as  it  ^^^ 

enters  the  air  between  the  cover  /^^^^^u-      {Ut^l^ 

glass  and  the  lens.     If   the  air  ^^^^^^^''  ^^^•/^ 

space  between  the   cover   glass   •  z::.^::,^^.:.:C:)^^S■:^uC^^■Z::::^    r,, , 
and  the  lens  could  be  filled  up    I-^^vvM^'^v^i'-vSI^ 
with  glass,  this  spreading  out  of  '•  -"■■'-'■^;^j  ••■•••"-  :y^^.^- ■■■;-:_ 

the   cone  would  not  occur,  and  "  UG"*' 

the  cone  of  light  would  remain  Fig.  6. 

68°  when    it    entered    the   lens. 

As  far  as  the  power  of  depicting  detail  is  concerned  it  would  be 

equal  to  a  116°  cone  in  air.     It  is  the  same  body  of  light  and  has 

just  the  same  properties  in  this  respect.     If  therefore  the  space 

between  the  object  and  the  lens  is  glass  throughout,  a  larger 

angled  cone  than  68°  can  be  collected  by  the  object  glass,  and  a 

greater  power  of  depicting  detail,  what  is  known  as  resolution, 

can  be  reached,  and  a  further  power  of  seeing  fine   structure 

obtained. 

Cedar- wood  oil  is  a  liquid  which  has  the  optical  properties  of  immersion 
glass,  and  if  a  drop  of  this  oil  is  placed  between  the  front  of  gfaS. 
the  object  glass  and  the  cover  glass,  the  whole  distance  between 
the  object  and  the  lens  is  equivalent  to  glass.     A  much  larger 
effective   aperture    can   thus   be   obtained   with    corresponding 
increase  in  resolution. 

Thus  object  glasses  of  higher  power  than  1/6  inch  (4  mm.) 
are  generally  what  are  called  immersion  object  glasses.  They  are 
so  constructed  that  a  drop  of  cedar-wood  oil  must  be  placed 
on  the  front  lens  so  that  it  connects  it  to  the  object  being 
examined. 

The  method  of  describing  the  aperture  by  the  term  numerical  Numerical 
aperture  (N.A.)  instead  of  by  the  actual  angle  of  the  cone  is  to  ^^^^  ^^^' 
enable  the  resolving  power  of  a  microscope  to  be  correctly  stated. 
A  1/12-inch  oil-immersion  object  glass  is  generally  made  to  admit 
an  angle  in  glass  of  117°,  which  corresponds  to  an  angle  of  more 
than  180°  in  air.  It  is  almost  the  same  actual  angle  as  the  1/6 -inch 
admits  from  air,  but  the  numerical  aperture  (N.A.)  which  gives 
its  true  resolving  power  is  1*3  N.A.,  while  that  of  the  1/6-inch  is 
only  -85  N.A. 

Dry  lenses  such  as  the  1/6-inch  cannot  be  used  with  cedar- 
wood   oil  as   immersion   lenses,   and    immersion   lenses   cannot 


18 


THE  MICROSCOPE 


Immersion 
fluids. 


Flatness  of 
field. 


Depth  of 
focus. 


Coarse 

focusing 
adjustment. 


be  used  without  the  cedar-wood  oil  because  the  lenses  must 
be  specially  constructed  for  the  conditions  under  which  they 
are  used. 

No  immersion  fluid  but  cedar-wood  oil,  or  a  fluid  sold  for  the 
purpose  with  exactly  the  same  optical  properties,  must  be  used. 
Water,  paraffin,  or  several  other  substitutes  which  are  some- 
times inadvertently  employed,  entirely  destroy  the  fine  quality 
of  the  image  formed  by  an  oil-immersion  lens. 

In  every  optical  instrument  the  centre  of  the  field  gives  the 
finest  definition,  and  the  object  being  examined  should  be  placed 
near  the  centre.  An  absolutely  flat  field  is  incompatible  with 
the  finest  definition  in  the  centre,  and  although  in  certain  types 
of  telescopes  and  photographic  lenses  the  importance  of  a  flat 
field  is  so  great  that  a  compromise  is  made,  no  deterioration  of 
the  central  image  can  be  aUowed  in  the  microscope. 

The  penetration  or  depth  of  an  object  glass  or  the  number  of 
difierent  layers  of  an  object  that  can  be  seen  sharply  at  the 
same  time  with  a  microscope  is  very  small.  With  lenses  of  a 
high  aperture,  and  therefore  in  general  of  a  high  magnifying 
power,  the  penetration  decreases  at  a  very  rapid  rate,  and  the 
power  of  seeing  different  planes  sharply  must  depend  on  adjusting 
the  instrument.  It  has  been  said  that  the  depth  of  focus  of  a 
high-power  microscope  is  really  the  fine  focussing  adjustment. 
The  fine  adjustment  in  the  hands  of  a  skilled  observer  is  in  constant 
motion,  focussing  first  to  one  plane  and  then  to  another  ;  by  this 
means  a  perception  of  depth  is  obtained  which  could  never  be 
given  by  an  object  glass  fixed  at  one  focus. 

■  The  penetration  of  the  microscope  may  be  increased  by  insert- 
ing a  stop  with  a  small  aperture  immediately  behind  the  object 
glass,  but  such  a  method  reduces  the  aperture  and  consequently 
the  detail  that  can  be  seen.  It  is  seldom  adopted  except  for 
photographing  certain  objects  where  the  image  from  the  upper 
or  lower  portion  of  the  object  obscures  the  layer  being 
photographed,  or  for  photographing  objects  with  compara- 
tively coarse  structure.  An  iris  diaphragm  is  made  that  will 
screw  into  the  body  of  the  microscope  between  it  and  the  object 
glass  for  this  purpose. 

There  is  only  one  position  in  relation  to  the  lenses  where  an 
object  can  be  placed  to  give  a  perfectly  clear  picture.  This 
position  is  generally  called  the  focus,  and  the  microscope  is  said 
to  be  '*  in  focus"  when  it  is  so  adjusted  that  the  object  is  in  this 
position.  It  is  more  convenient  to  effect  this  adjustment 
by  moving  the  body  which  carries  the  lenses  rather  than  by  moving 
the  object.  The  coarse  focussing  adjustment  is  actuated  by 
a  helical  rack  and  pinion  which  moves  the  body  along  a  slide 
towards  or  away  from  the  object.  Turning  the  milled  head  so 
that  its  upper  edge  moves  towards  the  observer,  raises  the  body ; 
away  from  the  observer,  lowers  it.     It  is  a  sufficiently  delicate 


A  SIMPLE  DESCKIPTION  OF  THE  MICROSCOPE     19 

motion  for  focussing  with  object  glasses  of  lower  power  than 
1/6-inch  (4  mm.). 

The  fine  focussing  adjustment  does  exactly  the  same  as  the  Fine 
coarse  adjustment,   but   the   movement   is   far   more  delicate :  ^oc'issing 

',    •  ,        .     T  -I  •  T1  '11,     adjustment. 

It  IS  actuated  by  a  micrometer  screw  and  a  lever  moving  the  whole 
body  along  a  second  slide.  A  complete  turn  of  the  screw  moves 
the  body  about  a  quarter  of  a  millimetre.  Turning  the  fine 
adjustment  milled  heads  moves  the  body  in  the  same  direction 
as  those  of  the  coarse  adjustment.  In  the  "  Standard  "  micro- 
scope the  left-hand  milled  head  is  twice  as  delicate  a  motion  as 
that  on  the  right-hand  side.  The  fine  adjustment  is  required 
for  the  focussing  of  high  powers  and  for  examining  the  different 
layers  of  an  object. 

In  moving  the  body  of  the  microscope  up  and  down  to  obtain  The  best 
the  correct  focus,  care  is  required  to  prevent  the  front  of  the  Scu^s(5,°.^ 
object  glass  being  forced  into  contact  with  the  object  by  racking 
it  too  far  down.  It  is  easy  to  break  a  valuable  specimen  by  this 
means ;  and  although  for  its  protection  the  metal  mount  of 
the  object  glass  projects  slightly  in  front  of  the  front  lens,  it  is 
delicate  in  construction,  and  can  be  damaged  by  being  brought 
into  contact  with  the  specimen. 

Experienced  microscopists  can  focus  a  lens  downwards  and 
stop  at  the  position  where  the  object  is  sharply  seen,  but  it  is 
unsafe.  The  correct  method  is  to  set  the  body  of  the  microscope 
so  that  the  front  of  the  object  glass  almost  but  not  quite  touches 
the  object,  and  then  to  rack  backwards,  turning  the  milled  heads 
so  that  the  upper  portion  turns  towards  the  observer,  and  raise 
the  body  until  the  correct  focus  is  found.  With  high-power 
object  glasses,  especially  oil-immersion  lenses,  this  method  is 
not  so  easy  because  the  distance  of  the  correct  focus  may  be 
below  the  point  at  which  the  body  has  been  set  in  the  first  instance. 
If,  however,  the  slow  motion  is  used  to  make  the  final  adjustment, 
damage  is  not  likely  to  occur,  as  it  lowers  the  body  very  gradually, 
and  the  latter  is  only  pressed  down  upon  the  object  by  a  spring. 
When  using  an  oil-immersion  lens  ajdrop  of  cedar -wood  oil  should 
be  placed  on  the  object  glass,  and  the  body  of  the  microscope 
racked  down  until  the  drop  of  oil  touches  the  cover ;  the  final 
focussing  can  then  be  done  with  the  fine  adjustment. 

Some  objects  are  so  transparent  that  it  is  quite  easy  to  pass 
by  the  focus  and  miss  the  correct  position.  In  these  cases  dust  on 
the  cover  glass  maybe  focussed  first,  and  the  fine  adjustment  lowered 
by  an  amount  representing  the  thickness  of  the  cover  glass.  If 
the  slide  be  moved  backwards  and  forwards  on  the  stage  during 
the  process  of  focussing,  the  movement  will  be  seen  directly  the 
correct  position  is  nearly  reached. 

It  may  appear  absurd  to  mention  that  if  a  slide  happens  to 
have  been  placed  on  the  stage  upside  down  a  high-power  object 
glass  will  not  focus  through  the  thick  glass  slip,  but  the  writer 


20 


THE  MICROSCOPE 


ReTolving 
nosepiece. 


Sloan  object 
changer. 


has  more  than  once  made  such  a  mistake  and  wondered  why  he 
could  not  focus  his  specimen. 

The  nosepiece  (S)  of  a  microscope  is  the  lower  end  of  the 
body  (M)  provided  with  a  screw  for  attaching  object  glasses.     A 

revolving  nosepiece  is  an  appliance  which 
screws  into  the  nosepiece  and  which 
carries  a  revolving  plate  into  which  two 
or  three  object  glasses  can  be  fixed, 
known  as  double  or  triple  nosepieces 
respectively.  By  rotating  the  revolving 
plate  each  object  glass  can  be  rapidly 
Fig.  7. — ^No.  3301,  Dust- brought  into  use,  being  held  in  the 
tight  Triple  Nosepiece.  correct  position  by  a  spring  clip.  The 
best  form  is  made  so  that  no  dust  can  drop  into  the  back  of  the 
object  glasses  and  they  can  be  safely  left  attached  to  the  micro- 
scope. The  extra  length  of  the  body  caused  by  the  length  of  a 
nosepiece  is  15  mm.,  and  the  drawtube  should  be  closed  by  that 
amount  or  set  at  145  mm.  instead  of  160  mm. 

An  object  glass  changer  is  an  apparatus  for  rapidly  changing 
the  object  glasses  by  another  method.  Each  object  glass  is 
screwed  into  a  fitting  which  slips  into  an  adapter  that  is  fixed  to 
the  nosepiece  of  the  microscope,  and  as  each  fitting  is  provided 
with  two  adjustable  abutment  screws  the  object  glasses  can  be 
individually  adjusted  so  that  they  exactly  register  as  regards 
the  position  of  the  field  of  view.  Changing  an  object  glass  by 
this  means  is  nearly  as  rapid  and  more  accurate  than  that  of  a 
revolving  nosepiece,  and  is  far  more  convenient  when  the  object 
glasses  are  to  be  used  on 
different  instruments  or  where 
more  than  three  are  used. 

It  consists  of  an  adapter 
which  has  on  one  side  a 
sloping  projection  (A),  and 
on  the  other  a  clamp  screw 
(B)  which  actuates  a  bevelled 
nut  (C).  The  adapter  is 
clamped  to  the  nosepiece  of 
the  microscope  by  a  screwed 
ring  (D),  which  is  provided 
with  slots,  into  which  a  half- 
penny will  fit  for  tightening 
it 'up. 

Loose  fittings  (Fig.  9)  are 
supplied,  one  of  which  is  screwed  on  to  each  object  glass. 

Each  fitting  has  a  bevelled  gap  which  fits  loosely  over  the 
bevelled  nut  (C)  of  the  adapter  and  swings  round  into  position 
when  a  turn  of  the  milled  head  (B)  forces  the  fitting  against  the 
sloping  projection  (A)   and  holds  it  firmly  in  position.     Each 


'•«w..* ^ 

Fig.  8.— No.  3280,  Sloan  Object 
Glass  Changer. 


A  SIMPLE  DESCRIPTION  OF  THE  MICROSCOPE     21 

adapter  has  screwed  studs  with  clamping  screws,  which  form  the 
stops  in  both  directions  when  the  object  glass  is  in  the  correct 
position.     These  can  be  adjusted  by  means  of  a  spanner  supplied 
for  the  purpose,  so  that  each  lens  can  be  centred  with  an  accuracy 
that  is  never  possible  with  a  revolving 
nosepiece,    because '  the    error   of   each 
individual  object  glass  cannot  be  com- 
pensated with  the  latter. 

The  construction  of  this  apparatus 
is  so  simple  and  rigid,  having  no  slides 
to  wear  loose,  that  it  remains  in  ad- 
justment permanently. 

The  total  extra  length  of  the  micro-  

scope  body  caused  by  its  use  is  10  mm.,    -pia.  9 No    3281     Fit- 

and   the   drawtube    should   be   set    at       ting  of    Sloan   Object 
150  mm.  to  obtain  the  standard  tube       Glass  Changer, 
length. 

A  box  is  supplied  to  carry  object  glasses  with  fittings  screwed 
on  ready  for  use,  held  against  dust-tight  pads. 

The  illumination  of  an  object  seen  with  a  microscope  is  ofiiiumina- 
almost  as  much  importance  as  the  quality  of  the  lenses.     It  is*^°°* 
interesting  to  find  that  the  methods  worked  out  by  those  who 
were  enthusiastic  in  the  use  of  the  microscope  as  an  enjoyment, 
and  to  a  great  extent  as  an  amusement,  have  been  one  by  one 
adopted  by  the  more  serious  scientific  worker  who  has  sometimes 
been  ready  to  consider  the  time  spent  on  the  pure  manipulation 
of  the  instrument  to  be  of  little  value.     The  proper  use  of  the 
substage  condenser  to  regulate  the  light  in  viewing  transparent 
objects  is  now  acknowledged  to  be  of  first  importance  for  correct 
observation.    Dark-ground  illumination,  which  has  been  considered 
by  some  to  be  only  useful  to  show  in  an  attractive  manner  what 
could  be  seen  equally  well  by  direct  light,  has  proved  to  be  of 
paramount  importance  for  the  study  of  living  bacteria  and  colloid 
particles.     The  methods  devised  for  illuminating  opaque  objects 
have   formed   the    basis   for   the    observation   of   metallurgical 
specimens,   and  the  much- criticised  study  of  the  markings  of 
diatoms  and  insects'  scales  has  proved  to  be  of  the  greatest  value 
in  enabling  the  images  seen  by  the  microscope  to  be  correctly 
interpreted.    A  bad  lens  can  never  be  made  to  give  a  perfect 
image,  but  a  good  lens  will  only  give  the  best  image  when  the 
illumination  is  satisfactory.     Most  objects  seen  with  the  naked 
eye  only  require  that  a  sufficiently  powerful  light  should  fall  upon 
them.     They  reflect  back  the  light  that  they  receive,   or  the 
greater  portion  of  it,  in  all  directions.     It  is  not  of  importance 
where  the  light  which  illuminates  them  comes  from,  although 
occasionally,  when  the  light  falls  upon  them  from  one  side  only, 
such  deep  shadows  may  be  formed  that  it  is  difficult  to  recogm'se. 
the  true  appearance. 


22 


THE  MICROSCOPE 


Visioa  of 

natural 

objects. 


The  mirror. 


Direction  of 
the  light. 


The  same  holds  true  of  opaque  objects  examined  with  the 
microscope,  but  the  greater  mimber  of  microscope  specimens  are 
either  transparent  or  semi-transparent,  and  must  be  viewed  by 
sending  a  beam  of  light  through  them  from  behind.  This  beam 
then  passes  through  the  microscope  into  the  eye.  Natural  objects 
are  seldom  viewed  in  this  manner,  but  in  order  to  examine  the 
water  mark  of  paper  or  a  photographic  transparency,  they  must 
be  held  between  a  strong  light  and  the  eye,  and  the  ability  to  see 
the  pattern  of  the  water  mark  or  the  view  in  the  transparency 
depends  on  certain  portions  of  the  light  being  blocked  out  which 
would  otherwise  enter  the  eye.  In  the  black  portion  the  whole 
light  is  stopped,  in  others  only  a  portion  is  absorbed,  and  thus  a 
complete  range  of  tone  in  the  picture  may  be  obtained.  This  is 
the  method  by  which  semi-transparent  objects  are  seen  with  the 
microscope. 

The  conditions  are  not  the  same  as  ordinary  vision,  and  the 
direction  and  character  of  the  beam  of  light  used  to  illuminate 
them  are  a  matter  of  great  importance. 

The  mirror  of  the  microscope  (F,  Fig.  1)  is  used  to  direct  a 
beam  of  light  from  some  source  of  illumination  through  the 
object  into  the  microscope.     The  mirror  swings  in  gimbals  and 

can  be  moved  in  all  directions.  It  has 
on  one  side  a  flat,  silvered  surface 
which  gives  a  plane  reflection,  and  on 
the  other  a  concave  surface  which 
concentrates  a  more  powerful  beam 
upon  a  small  area  of  the  object. 

The  direction  of  the  light  should  be 
such  that  it  shines  directly  along  the 
line  that  passes  through  the  centre  of 
the  microscope — the  line  that  is  known 
as  the  optic  axis.  If  the  light  comes 
from  the  side  it  passes  obliquely 
through  the  object,  and  even  if  it  does 
not  give  an  erroneous  appearance  it 
prevents  a  clear  image  being  formed. 
To  illustrate  the  effect  on  the  object, 
one  should  examine  the  appearance  of 
a  fairly  thick  piece  of  wood  which  has 
fine  holes  drilled  in  it :  light  from  one 
side  would  not  pass  through  and  would 
not  reach  the  eye.  The  holes  will  not 
Oblique  light.  Direct  light,  be  visible  unless  the  light  is  passing 
Fig.  10.  through  them  centrally.     The  effect  of 

an  oblique  beam  of  light  as  it  passes 
through  the  microscope  lenses  is  shown  in  Fig.  10. 

The  left-hand  diagram  in  Fig.   10  shows  the  object  glass 
transmitting  oblique  light  only.     The  light  which  actually  forms 


A  SIMPLE  DESCRIPTION  OF  THE  MICROSCOPE    23 

the  image  is  a  fine  bundle  thrown  to  the  edge  of  the  object  glass, 
so  that  the  object  glass  acts  as  if  it  had  only  a  pinhole  aperture 
at  one  side,  and  is  consequently  no  better  for  depicting  detail  than 
the  early  pinhole  lenses  which  were  made  before  the  modern 
achromatic  microscope  was  discovered. 

This  diagram  also  shows  how  the  direction  of  the  light  can  be 
immediately  recognised  by  focussing  the  microscope.  The  only 
light  producing  the  primary  picture  is  shown  in  the  left-hand 
diagram  of  Fig.  10.  It  is  on  the  right  or  the  left  of  the  axis, 
according  to  whether  it  is  above  or  below  the  true  focus.  There- 
fore, by  putting  the  object  in  and  out  of  focus  with  the  focussing 
adjustment,  the  direction  of  the  light  can  be  observed.  When 
the  light  which  illuminates  the  object  is  oblique  instead  of  being 
truly  central,  the  object  will  not  only  become  indistinct  on  either 
side  of  the  focus,  but  will  appear  to  move  from  side  to  side ;  whereas 
if  the  light  is  truly  central,  the  object  will  become  less  distinct 
on  either  side  of  the  focus,  but  will  not  alter  its  position. 

The  mirror  can  always  be  adjusted  until  the  object  remains 
stationary  as  the  microscope  is  being  focussed,  and  the  centring 
of  the  light  is  thus  assured. 

Below  the  stage  of  the  microscope  an  iris  diaphragm  (K,  Fig.  1) 
is  fitted,  and  if  this  is  shut  down  to  a  small  aperture  the  light  will 
not  pass  through  the  microscope  at  all  if  the  light  is  very  far  away 
from  the  axis,  though  this  is  not  in  itself  sufficient  to  make  the 
final  adjustment. 

The  nature  of  the  illumination  may  be  varied  according  to  Nature  of 

whether  it  is  parallel,  divergent,  or  convergent.     If  the  flat  side 

of  the  mirror  be  used  and  the  , 

source  of  light  is  at  a  consider-         -;;;;;;;  !!\  ;''. ';|.'('' 

able  distance,  a  beam  of  nearly        .i,'';:!;  ft  '•,:1;;;:.','.' 

paraUel    light    is     obtained    (a,        v';;';;:  ;Z  ;'¥-,',' 

Fig.  11).  If  the  source  of  iUu-  %^km  ^m^m  Sfe^v..v., 
mination  is  very  close,  a  diver-        ^|j^=-=     ^|^i     ^|^-i-_-----:-v.-:: 

gent  beam  is  obtained  (c,  Fig.  11).        *   ^  ▼  ^ 

If  the  concave  mirror  is  used,  it  a  be 

will    be     a    slightly    convergent   Fig.  11. — Mirror  reflecting  parallel, 
beam  (6,  Fig.  11).     By  means  of      convergent,  or  divergent  light. 
a  substage  condenser  (J,  Fig.  1) 

with  an  iris  diaphragm  below  it— described  later— the  light 
can  be  rendered  still  more  convergent  and  can  be  regulated 
with  accuracy.  As  the  light  which  enters  the  condenser  at  its 
margin  emerges  as  the  outer  portion  of  the  cone,  the  effect  of 
reducing  the  aperture  of  the  diaphragm  of  the  condenser  is  not 
only  to  reduce  the  amount  of  illumination,  but  to  alter  its 
character  by  reducing  the  size  of  the  cone  of  emergent  light. 
Thus  with  a  very  small  aperture  an  almost  parallel  beam  of  light 
can  be  obtained,  and  by  opening  the  iris  diaphragm  a  more  and 
more  highly  convergent  cone  of  light  may  be  used  (see  p.  27). 


24 


THE  MICROSCOPE 


niumina-  As   regards   the   best  kind   of  illumination  for   transparent 

transparent  objects,  the  light  may  be  a  nearly  parallel  beam  from  the  flat 
objects.  mirror,  or  a  slightly  divergent  beam  from  the  flat  mirror  used 
with  a  lamp  near  the  mirror.  The  light  from  a  lamp  may  be 
rendered  nearly  parallel  by  placing  a  bull's-eye  condenser  close 
to  the  lamp.  To  find  the  correct  position  for  the  bull's-eye  to 
give  parallel  light,  an  image  of  the  flame  or  filament  of  the  lamp 
should  be  observed  on  a  distant  wall  and  the  bull's-eye  moved  till 
the  lamp  or  filament  is  in  sharp  focus  on  the  wall.  The  light  is 
then  approximately  parallel,  and  the  microscope  should  be  so 
placed  that  the  mirror  is  in  the  beam  of  light  about  8  or  10  inches 
away  from  the  lamp. 

The  light  may  be  made  slightly  convergent  if  the  bull's-eye 
be  arranged  to  give  parallel  light,  but  the  concave  instead  of  the 
flat  mirror  be  made  use  of. 

The  light  may  be  condensed  by  a  substage  condenser,  which 

not  only  increases  the  brilliancy  of 
the  illumination,  but  also  gives  a 
strongly  convergent  beam  of  light 
which  may  be  modified  to  any  extent 
by  the  use  of  the  iris  diaphragm  and 
stops,  as  described  more  fully  under 
the  description  of  substage  condensers. 
The  question  as  to  whether  the 
best  results  will  be  obtained  by 
parallel,  divergent,  or  convergent  il- 
lumination, depends  to  a  great  extent 
on  the  nature  of  the  object.  When 
light  shines  through  certain  kinds  of 
objects  it  is  distributed  or  scattered 
in  all  directions.  A  cut-glass  lamp- 
shade breaks  up  the  light  that  falls 
upon  it  and  scatters  it  all  round;  the  same  thing  occurs  to 
a  lesser  extent  in  the  case  of  a  botanical  or  histological  section 
of  tissue  when  each  cell  or  irregularity  acts  like  a  facet  of  a  cut- 
glass  lamp-shade.  In  this  case,  whatever  the  nature  of  the 
iUumination,  there  is  a  sufficiently  scattered  light  to  fill  the 
aperture  of  the  object  glass,  and  the  general  structure  of  the  tissue 
will  be  accurately  depicted.  This,  however,  does  not  apply  to 
all  kinds  of  objects.  Some  do  not  scatter  light,  and  the  question 
as  to  whether  the  aperture  of  the  object  glass  is  filled  with  light 
depends  on  the  nature  of  the  illuminating  beam.  If  an  object 
which  does  not  scatter  light  is  illuminated  as  shown  in  Fig.  12  {a), 
the  object  glass  might  just  as  well  have  nothing  but  a  pinhole 
aperture ;  and  to  make  use  of  the  aperture  a  convergent  cone  of 
light  must  be  thrown  upon  the  object,  as  shown  in  Fig.  12  (6). 
oft^Tbj  ^^  ?^^^^  objects  spread  the  light  slightly  by  diflraction, 
object.        though  in  the  case  of  a  single  dark  object  on  a  white  field,  the 


a 


Fig.  12. 


A  SIMPLE  DESCRIPTION  OF  THE  MICROSCOPE     25 

amount  of  such  spreading  is  relatively  small  and  need  not  be 
considered.  If  the  object  is  a  regular  periodic  structure,  like  a 
series  of  dots  and  lines,  the  spreading  due  to  this  cause  may  be 
very  considerable,  and  such  an  object  may  not  require  so  large 
a  beam  to  use  the  full  angle  of  the  object  glass. 

This  is  very  noticeable  in  the  case  of  the  fine  periodic  structure 
of  diatoms,  where  the  structure  may  often  be  shown  when  the 
illuminating  cone  of  light  is  considerably  less  than  that  required 
to  fill  the  whole  of  the  aperture  of  the  microscope.  In  such  cases 
it  will  be  observed,  if  the  eyepiece  of  the  microscope  is  removed, 
that  the  central  direct  beam  illuminates  the  central  portion  of 
the  back  lens  of  the  object  glass,  but  the  rest  of  the  lens  may  be 
illuminated  almost  as  strongly  by  the  large  amount  of  diffracted 
light  scattered  by  the  periodic  structure  of  the  diatom. 

For  the  use  of  the  microscope  with  any  but  the  lowest 
magnifying  powers,  a  substage  condenser  should  be  used  in  order 
that  the  nature  of  the  illumination  may  be  completely  varied  at 
will. 


CHAPTER  II 


Substage 
condensers. 


ILLUMINATING  APPARATUS  AND  SOURCES 
OF  ILLUMINATION 

A  MOST  important  part  of  the  microscope  has  now  to  be  con- 
sidered, namely,  the  substage  condenser,  which  is  essential  with 
all  higher  powers  to  converge  a  beam  of  light  upon  the  object 
in  order  to  illuminate  it  brilliantly  and  to  vary  the  character 
of  the  illumination. 

There  are  three  difierent  kinds  of  substage  condensers.  The 
simple  so-called  Abbe  condenser  consists  of  two  lenses  with  an 
iris  diaphragm  close  behind  the  back  lens  and  a  tray  below  for 
the  insertion  of  patch-stops  or  colour  filters,  as  shown  in  Fig.  13. 
It  was  in  use  under  various  names  long  before  the  time  of  Abbe, 
who,  however,  popularised  it  in  a  particular  form  of  mounting. 
It  does  not  focus  the  rays  correctly  to  one  spot   (see  Fig.  14), 


—  0 


Fia.  13.— No.  3286,  Abbe  Condenser. 


Fig.  14. 


the  oblique  rays  coming  to  a  nearer  focus  than  the  more  direct, 
and  does  not  form  a  definite  image  of  the  source  of  illumination 
due  to  the  uncorrected  lenses  of  which  it  is  constructed.  It 
has  an  aperture  of  1  N.A. — that  is  to  say,  it  will  give  180°  in  air, 
the  maximum  aperture  obtainable  with  a  dry  condenser.  A  large 
beam  of  light  from  the  mirror  thrown  upon  the  back  lens  is 
concentrated  upon  a  small  area  (0).  This  area  is  illuminated  by 
an  imperfect  image  of  the  source  of  light.  The  condenser  fits  into 
the  substage  of  the  microscope  by  which  it  can  be  moved  up  and 
down,  or  "focussed,"  or  can  be  moved  laterally,  or  "centred,"  until 
the  illuminated  area  (0)  coincides  with  the  object  being  examined. 
The  object  is  by  this  means  brilliantly  iUuminated.  A  powerful 
illumination  is  often  required  to  overcome  the  loss  of  light  due 

26 


ILLUMINATING  APPARATUS 


27 


to  the  large  magnification  obtained  with  high-power  lenses ;  but 
this  is  not  the  only  advantage  gained  by  the  use  of  a  condenser, 
as  illumination  might  be  increased  by  other  means — for  instance, 
by  bringing  a  source  of  light  closer  to  the  object.  A  substage 
condenser  receiving  an  approximately  parallel  bundle  of  light 
from  the  mirror  of  the  microscope  converts  it  into  a  wide  angle 
cone  of  light.  When  this  light  is  centred  and  focussed,  the  object 
is  illuminated  by  light  falling  upon  it  in  all  directions. 

The  achromatic  condenser  (Fig.  15)  has  the  same  aperture  as  Achromatic 
the  Abbe  condenser  1  N.A.,  but  it  is  corrected  almost  as  care-  ^^^  ^^^'^' 
fully  as  a  microscope  object 
glass,  so  that  the  rays  come  to 
exact  points,  and  a  very  perfect 
image  of  the  source  of  illumina- 
tion is  formed  in  the  plane  of 
the  object,  much  reduced  in  size. 
It  is  provided  also  with  an  iris 
diaphragm  and  a  tray  for  patch- 
stops  and  filters.  Fig.  16  shows 
a  beam  of  light  (A,  B,  C,  D, 
E,  F)  passing  through  this  con- 
denser to  the  central  point  of 
the  object  at  0.  The  rays 
A,  F,  which  are  at  the   margin 

of  the  beam  of  light  as  it  enters,  emerge  as  the  most  oblique 
rays  falling  upon  the  object  0,  and  the  rays  C,  D,  which  enter 
near  the  centre  of  the  condenser,  emerge  nearly  parallel.  ^  Thus,  if 
the  iris  diaphragm  which  is  placed  below  the  condenser  is  gradu- 
ally closed,  it  excludes  more  and  more  of  the  oblique  rays.  Fig.  16 
shows  a  large  solid  cone  of  light  of  great  angle  converged  upon  the 
object,  the  iris  diaphragm  being  fully  open.  Fig.  17  shows  a  small- 
angled  cone  transmitted  by  the  same  condenser,  the  iris  diaphragm 
being  partially  closed.     Fig.   18  shows  the  same  condenser  in 

which  the  iris  diaphragm  is  fully  open  and 

0        C  an  opaque  patch  or  stop  is  placed  below 

"^  the  condenser  so  that  the  object  is  being 


Fig.  15.- 


No.  3288,  Achromatic 
Condenser. 


m 

^\\ 

s\- 

nN^ 

WW 

m 

m 

m 

A  B  C  D  E 

Fio.   16. 


Fig.  17. 


Fig.  18. 


28 


THE  MICROSCOPE 


Immersion 
achromatic 
condenser. 


Use  of  a 
substage 
condenser. 


To  focus  a 

substage 

condenser. 


Fig.  19.— No.  3291,  Dry  and 
Immersion  Condenser. 


illuminated  by  a  hollow  cone.  Stops  or  patch-stops  with  apertures 
of  different  shapes,  or  in  different  positions  below  the  condenser, 
used  in  combination  with  the  iris  diaphragm,  regulate  the  illumina- 
tion so  that  light  in  any  direction  may  be  passed  through  the 
object. 

The  dry  and  immersion  achromatic  condenser  is  of  even  higher 
quality  than  the  achromatic  condenser,being  equal  in  its  corrections 
to  a  microscope  object  glass,  and  has  an  aperture  of  1*3  N.A.,  so 

that  it  can,  if  used  in  immer- 
sion contact  with  the  under- 
surface  of  the  slide,  fill  the  whole 
aperture  of  an  oil-immersion  lens 
with  light. 

For  transparent  objects  with 
object  glasses  as  low  in  power  as 
a  IJ-inch  (32-mm.)  a  condenser 
is  not  required.  The  aperture  of 
such  a  low- power  lens  is  small, 
and  the  angle  obtained  by  the 
use  of  a  concave  mirror  is  usually 
sufl&cient  to  make  the  best  use  of 
this  lens. 
The  same  applies  to  some  extent  to  the  2/3-inch  (16-mm.) 
object  glass,  but  as  this  lens  is  so  frequently  used  as  a  finder  for 
a  high-power,  it  is  not  always  convenient  to  rapidly  remove  the 
condenser,  and  it  is  customary  to  use  the  condenser,  but  to  put 
it  somewhat  out  of  focus  in  order  to  fill  the  whole  field  with  an 
even  illumination.  A  condenser  should  always  be  used  with 
the  1/3-inch  (8-mm.),  1/6-inch  (4-mm.),  1/8-inch  (3-mm.),  or 
1/12-inch  (2-mm.)  object  glasses. 

A  substage  condenser  is  only  corrected  for  light  which  is 
parallel  or  slightly  divergent ;  therefore  the  flat  mirror  should  be 
used.  The  concave  mirror  giving  convergent  light  is  quite  un- 
suitable for  use  with  a  condenser. 

Daylight  as  a  source  of  light  is  not  recommended  with  a 
condenser,  as  the  finest  detail  cannot  be  shown  bv  its  means. 
For  ordinary  microscopic  examination  of  not  too  critical  a 
nature,  daylight  is  satisfactory,  but  even  then  the  more  delicate 
details  may  escape  notice. 

Assuming  that  the  source  of  light  is  a  paraffin  lamp  with  a 
flat  wick,  or  other  small  source  of  illumination,  the  condenser 
must  first  be  focussed  and  centred. 

In  order  to  focus  the  condenser,  sufficient  light  must  be  thrown 
through  the  object  to  render  it  visible,  and  the  object  glass  must 
be  focussed  upon  the  object.  The  iris  diaphragm  of  the  condenser 
should  then  be  shut  down  to  about  one- quarter  its  size,  and  it 
should  be  focussed  up  and  down  until  an  image  of  the  flame  of 
the  lamp  is  seen  sharply  in  focus  at  the  same  time  as  that  of  the 


ILLUMINATING  APPAEATUS  29 

object ;  the  mirror  will  require  to  be  adjusted  in  order  to  direct 
the  light  through  the  condenser.  If  the  lamp  be  turned  round 
so  that  the  edge  of  the  flame  is  opposite  the  mirror,  it  makes 
an  easier  object  to  focus,  and  its  image  will  appear  as  a  slit 
across  the  field  of  view.  When  focussed  the  lamp  may  be  turned 
round,  with  the  flat  side  of  the  flame  facing  the  mirror,  thus 
illuminating  the  whole  field.  It  is  found  in  practice  that  the  best 
resolution  is  obtained  when  the  source  of  light  is  almost,  but  not 
quite,  in  focus. 

The  accuracy  of  centring  of  the  simple  Abbe  condenser  is  not  To  centre 
important.  The  image  that  it  gives  is  not  accurate,  and  it  is  col'denS 
generally  sufiicient  to  move  the  mirror  slightly  till  the  image  of 
the  object  does  not  move  from  side  to  side,  while  the  body 
of  the  microscope  is  being  focussed  up  and  down  in  a  similar 
manner  to  that  described  on  page  22  for  setting  the  mirror 
when  used  alone.  Microscopes  fitted  with  this  condenser  are 
frequently  not  provided  with  a  centring  adjustment  to  the 
substage. 

When  using  the  achromatic  or  immersion  condenser  centring 
is  a  matter  of  importance.  The  iris  diaphragm  is  placed  in  a 
position  at  such  a  distance  below  the  condenser  lenses  that  if 
the  condenser  be  moved  downwards  the  source  of  light  will  be 
put  out  of  focus  and  an  image  of  the  small  aperture  in  the  iris 
diaphragm  can  be  sharply  focussed.  When  doing  this  the  iris 
diaphragm  should  be  closed  to  its  smallest  aperture,  so  that 
the  image  is  of  a  sufficiently  small  size  to  be  seen  in  the  field  of 
view.  The  condenser  may  then  be  moved  by  the  centring 
screws  until  the  image  of  the  diaphragm  is  in  the  centre  of  the 
field  of  view.  If  the  image  of  the  small  aperture  is  so  far  out  of 
centre  that  it  is  not  in  the  field,  the  aperture  can  be  enlarged 
until  its  edge  begins  to  appear.  When  the  condenser  has  been 
thus  centred  it  should  be  moved  upwards  till  the  image  of  the 
lamp  is  again  in  focus  on  the  object,  and  the  mirror  readjusted 
if  the  light  is  not  in  the  centre  of  the  field. 

The  achromatic  condenser  is  now  in  the  best  position  for  use 
with  a  1/3-inch  (8-mm.),  1/6-inch  (4-mm.),  or  1/12-inch  (2-mm.) 
object  glass,  though  slightly  better  resolution  is  obtained  if  the 
source  of  light  is  a  little  out  of  focus.  If  it  is  required  to  use  a 
2/3-inch  (16-mm.)  object  glass,  the  condenser  can  be  focussed 
down  to  give  an  evenly  illuminated  field,  being  brought  back 
into  focus  when  a  higher  power  is  used. 

The  substage  condenser  having  been  focussed  and  centred.  Effect  of 
the  eyepiece  of  the  microscope  should  be  removed,   and  the  cEi^^iris*^ 
effect  of  opening  and  closing  the  iris  diaphragm  be  observed  by  diaphragm  of 
looking  down  the  tube   of   the  microscope.      This,  effect  is  best 
observed  when  an  object  glass  with  a  fairly  large  aperture  such 
as  1/6-inch  (4-mm.)  is  used.     When  the  iris  diaphragm  is  fully  open 
the  back  lens  of  the  object  glass  will  be  completely  filled  with 


30 


THE  MICEOSCOPE 


Effect  of 
focussing  a 
Bubstage 
condenser. 


light  appearing  as  a  uniform  circular  disc;  closing  the  dia- 
phragm will  at  first  not  make  any  change  in  the  appearance, 
because  the  condenser  is  co verging  upon  the  object  a  beam  of 
light  of  greater  angle  than  can  be  collected  by  the  object  glass, 
and  it  will  receive  the  full  illumination  until  the  light  from  the 
condenser  becomes  smaller  in  angle  than  the  aperture  of  the  object 
glass.  If  the  diaphragm  be  now  closed  to  its  fullest  extent,  the 
back  lens  of  the  object  glass  shows  a  small  spot  of  brilliant  light 
in  the  centre ;  and  as  the  aperture  of  the  iris  diaphragm  is  slowly 
opened,  the  spot  of  light  slowly  increases  in  size  until  the  whole 
of  the  back  lens  of  the  object  glass  is  completely  filled  with 
uniform  light.  As  soon  as  this  is  done  the  whole  of  the  aperture 
of  the  lens  is  receiving  direct  light  from  the  condenser  shining 
through  the  object  as  shown  in  Fig.  16. 

While  the  microscope  is  in  this  condition,  the  effect  on  the 
appearance  of  the  back  lens  of  the  object  glass,  which  is  produced 
by  putting  the  condenser  in  and  out  of  focus,  should  be 
observed.  The  iris  diaphragm  being  open  to  the  full  extent,  it 
will  be  found  that  unless  the  condenser  is  in  correct  focus  the 
whole  area  of  the  back  lens  will  not  be  equally  filled  with  light, 
and  from  the  appearance  so  observed  it  can  be  realised  why  an 
uncorrected  condenser  like  the  Abbe  condenser  cannot  readily 
be  made  to  fill  the  whole  area  of  the  object  glass  with  uniform 
light,  the  reason  being  that  the  light  of  difierent  obliquity  is 
brought  to  a  focus  at  different  positions.  It  may  be  that  only 
one  ring  of  light  at  the  edge  of  the  object  glass,  or  a  small  area 
in  the  centre,  or  a  combination  of  both,  is  being  illuminated. 

The  importance  of  the  character  of  the  illumination  has  been 
ufmninatSig  referred  to,  and  the  question  arises  as  to  what  aperture  cone  of 
light  it  is  best  to  use.  A  well-corrected  substage  condenser 
centred  and  in  focus  gives  the  means  of  completely  controlling 
this,  and  from  what  has  so  far  been  said  it  might  be  supposed 
that  the  full  aperture  of  the  object  glass  should  be  always  illu- 
minated ;  this  is  not  necessarily  the  case,  further  research  is 
required  before  definite  rules  can  be  laid  down  to  meet  all  condi- 
tions. For  the  best  resolution  the  diaphragm  in  the  condenser 
should  never  be  opened  to  admit  a  larger  angle  of  light  than  that 
of  the  object  glass,  or  "  glare  "  may  produce  a  misty  appearance 
which  will  destroy  the  crispness  of  the  image. 

This  can  be  avoided  by  reducing  the  size  of  the  diaphragm 
until  the  aperture  just  becomes  visible  at  the  edge  of  the  back 
lens  of  the  object  glass.  To  what  extent  the  angle  of  light 
admitted  by  the  condenser  should  be  smaller  than  the  aperture 
of  the  object  glass  depends  upon  the  nature  of  the  object.  Full 
resolution  will  only  be  obtained  if  the  whole  aperture  of  the 
object  glass  is  transmitting  light,  for  reasons  previously  explained ; 
and  if  the  light  from  the  condenser  were  not  filling  the  aperture 
of  the  object  glass,  and  there  were  no  object  on  the  stage,  the 


Best 


cone 


ILLUMINATING  APPARATUS  31 

aperture  condition  would    not  be    fulfilled  until  the    iris    dia- 
phragm was  open  to  the  desired  amount  to  fill  the  back  lens  of 
the  object  glass;  but  the  object  itself  always  has  some,  and  oftert 
a  very  great,  power  of  scattering  light.    Even  when  the  condenser 
diaphragm  is  cut  down  to  a  minute  aperture,  such  an  object  as 
a  diatom  or  a  podura  scale  scatters  so  much  light  that  on  look-  Resolution 
ing  at  the  back  of  the  object  glass  it  is  almost  as  bright  over  its  s,fatSred 
whole  surface  as  at  the  spot  where  the  direct  light  passes  through,  ^y  object. 
and  the  image  is  formed  by  the  scattered  quite  as  much  as  by  the 
direct  light. 

The  markings  of  the  podura  scale  form  a  good  illustration 
of  this  point.  The  scales  of  this  small  insect  appear  to  have 
markings  somewhat  like  small  quills.  If  the  aperture  of  the  con- 
denser is  reduced  so  as  to  send  direct  light  through  only  a  com- 
paratively small  fraction  of  the  object  glass,  the  best  image  of 
these  quills  is  formed  somewhat  as  Fig.  20  (A).  If  the  diaphragm 
be  opened  beyond  a  certain  amount  the  clearness  of  this  image 
is  reduced.  If  a  diaphragm  is  inserted  at  the  back  of  the  object 
glass  to  cut  off  all  the  scattered  light,  and  only  let 
the  direct  light  from  the  condenser  through,  the  image 
will  be  absolutely  fuzzy  and  indistinct,  somewhat  as 
Fig.  20  (B),  shomng  that  in  the  delineation  of  this 
object  the  light  scattered  by  the  object  is  doing  the 
work  of  resolution,  and  that  it  is  not  done  so  well  if 
the  full  cone  of  light  is  passed  through  the  object  by  the 
condenser.  The  same  applies  to  a  lesser  extent  to 
some  diatom  structure,  though  in  this  case  the  finest 
structure  is  always  best  shown  by  a  large  cone  of  light  from 
the  condenser. 

The  tubercle  bacillus  embedded  in  tissue  is  extremely  difficult 
to  see  unless  a  large  cone  of  light  is  used,  and  in  the  early  days  of 
its  discovery  its  existence  in  the  tissues  themselves  was  doubted 
by  some  of  those  who  were  not  in  the  habit  of  using  a  condenser 
to  its  best  advantage.  Some  delicate  semi-transparent  objects 
are  quite  invisible  when  a  large  cone  of  light  is  used,  but  can 
be  seen  with  a  pinhole  aperture  in  the  condenser.  It  should  be 
understood,  however,  that  appearances  created  by  the  use 
of  very  small  apertures  may  be  incorrect  representations  of  the 
correct  structure.  In  the  case  of  the  podura  scale  and  the 
structure  of  diatoms,  the  nature  of  the  actual  structure  is  not 
definitely  known,  and  it  is  not  certain  that  the  images  obtained 
of  these  apparently  well-marked  structures  are  even  approximately 
correct.  Observers  have  frequently  claimed  the  discovery  of 
delicate  envelopes  around  bacilli  and  micrococci  which  a  skilful 
microscopist  can  at  once  refer  to  the  result  of  incorrect  illumina- 
tion. It  is  generally  best  to  use  the  largest  aperture  in  the  con- 
denser that  does  not  produce  indistinctness,  for  the  image  is 
more  likely  to  be  a  correct  representation ;  and  it  is  well  to  try 


32 


THE  MICROSCOPE 


Variation 
of  light 
intensity 
when  iris 
diaphragm 
of  condenser 
is  altered. 


Double 
wedge  light 
moderator. 


I 


different  apertures  when  it  is  required  to  observe  very  fine  struc- 
ture. 

There  is  a  difficulty  in  making  use  of  different  apertures  because 
the  light  varies  so  greatly  by  the  change  in  the  aperture  of  the 
condenser,  that  the  variation  in  intensity  becomes  a  serious  incon- 
venience. The  observer  should  never  be  tempted  to  overdo  the 
brilliancy  of  the  illumination ;  several  pieces  of  ground  glass  should 
be  at  hand  to  place  between  the  lamp  and  the  mirror  to  modify 
the  light  as  the  condenser  is  opened  up. 

The  most  satisfactory  appliance  for  critical  work  is  an  adjust- 
able pair  of  neutral  tint  glass  wedges  mounted  on  a  stand  which 
can  be  placed  between  the  light  and  the  mirror  of  the  microscope 

to  vary  the  intensity  of  the 
light  at  will.  The  apparatus 
consists  of  a  frame  which 
carries  two  neutral  glass 
wedges  which  slide  in  fit- 
tings and  are  connected 
together  so  that  they  move 
over  one  another  in  oppo- 
site directions  by  sliding 
a  knob.  The  total  thick- 
ness of  the  neutral  tint 
glass  is  varied  and  the 
brilliancy  of  the  illumina- 
tion increased  or  decreased 
within  very  considerable 
limits.  The  frame  is  at- 
tached by  means  of  a  clamp 
to  a  rod  fixed  to  a  strong 
stand.  Its  height  from  the 
table  may  be  varied  from 
2  inches  to  8  inches,  and 
two  slides  are  provided  in 
front  of  the  wedges  for  the 
reception  of  colour  filters  or 
ground  glass. 
As  the  illumination  is  increased  by  opening  the  diaphragm 
of  the  condenser,  it  can  be  reduced  by  thickening  the  neutral 
tint  layer  of  glass  by  sliding  the  two  wedges  over  each  other. 
This  apparatus  can  be  provided  with  wedges  of  different  intensities 
according  to  the  strength  of  the  illuminant  with  which  it  is  to 
be  used. 

The  use  of  a  small  bundle  of  oblique  light  directed  upon  the 
object  at  a  particular  angle  has  been  studied  in  connection  with 
the  delineation  of  line  structure.  It  is  accomplished  by  cutting 
a  small  hole  in  an  opaque  sheet  of  card,  metal,  or  celluloid,  and 
placing  this  aperture  in  different  positions  under  the  condenser, 


Fig.  21.— No.  3328,  Double  Wedge 
Moderator. 


ILLUMINATING  APPARATUS  33 

so  that  one  or  more  selected  beams  of  oblique  light  may  be  used  to 
illuminate  the  object.  An  experiment  with  finely  ruled  parallel 
lines  shows  that  if  a  small  oblique  beam  of  light  is  used  to  illu- 
minate them  at  right  angles  to  their  length,  finer  lines  can 
be  distinguished  than  with  direct  light ;  but  it  is  doubtful 
whether  this  method  is  of  any  advantage  for  ordinary  objects, 
and  it  is  liable  to  give  rise  to  quite  erroneous  impressions  of 
structure. 

Dark-ground  illumination  is  obtained  by  throwing  light  upon 
the  object  in  such  a  manner  that  the  object  is  illuminated,  but 
that  none  of  the  light  enters  the  microscope  except  that  reflected 
by  the  object  itself. 

The  illuminator  must  be  capable  of  throwing  light  upon  the 
object  at  a  greater  angle  than  can  be  received  by  the  object  glass  in 
use.  The  illuminator  must  have  a  larger  aperture  than  the 
object  glass.  Dark-ground  illumination  with  a  substage  con-  park-ground 
denser  in  which  the  object  is  illuminated  by  light  that  is  so  oblique  '^'th^*^^^" 
that  it  cannot  enter  the  object  glass  is  accomplished  by  placing  substage 
a  glass  with  a  central  black  patch  below  the  condenser  and  by 
opening  the  iris  diaphragm  to  its  full  extent,  as  shown  in  Fig.  18, 
page  27.  With  the  Abbe  form  of  condenser  this  method  is  useful 
for  low  powers — IJ-inch  (32-mm.)  and  2/3-inch  (16-mm.) — but  is 
not  sufficiently  corrected  to  cut  off  the  central  light  with  the 
accuracy  required  for  a  high  power.  It  will  perform  fairly  well  with 
a  1/3-inch  (8  mm.)  achromatic,  which  has  an  aperture  of  '5  N.A., 
but  not  for  lenses  with  a  larger  aperture.  With  the  achromatic 
or  immersion  condenser,  dark-ground  illumination  can  with  care 
be  used  with  a  1/6-inch  (4-mm.)  object  glass,  but  it  is  better  to  use 
a  special  liigh-power  illuminator  described  later  for  the  1/6-inch 
(4-mm.),  1/8-inch  (3-mm.),  and  the  1/12-inch  (2-mm.)  object  glass. 
This  is  partly  because  the  high-power  illuminator  is  of  shorter  focal 
length  and  gives  more  brilliant  illumination,  and  partly  because 
the  stop  of  a  substage  condenser  is 
some  distance  below  the  lenses  and 
allows  some  light  to  spread  round  the 
stop  employed.  It  does  not  produce 
so  black  a  background. 


For   dark-ground  illumination  with    ^^^^""^f^^;^    |  pat'htp'ot 
a    condenser,   an    adjustable    stop    in- 
vented by  Mr.  Traviss   (Fig.   22),    on 
the    principle   of   a   reversed   iris    dia- 
phragm, is  a  very  convenient  appliance.    Fig.  22. — No.  3284,  Tra- 
By  moving  the  handle  the  size  of  the  viss  Patch-stop, 

central  patch  is  enlarged  or  diminished. 

The  high-power  dark-ground  illuminator  is  a  reflecting  device  High-power 

or  o  J!  "11         '    n    dark-ground 

by  means  of  which  a  very  small  image  or  trie  source  or  luurmna-  iuomiuator. 
tion  is  focussed  upon  the  object,  and  this  image  is  formed  by  rays 
of  light  which  fall  upon  the  object  at  a  very  oblique  angle.     Fig.  23 

3 


34 


THE  MICROSCOPE 


Fig.  23.— No.  3295,  High-power  Dark- 
ground  Illuminator. 


shows  the  optical  portion  of  this  illuminator.  The  light  from 
the  mirror  being  thrown  upon  the  under- surface  of  the  glass 
reflector,  the  light  is  reflected  by  two  curved  surfaces  so  that 
a  ring  of  light  is  focussed  to  a  point  upon  the  object  at  a  very 

oblique  angle,  as  shown 
by  the  white  portion  of 
the  diagram.     The  whole 
of  this  light  is  so  oblique 
that  it  will  all  be  totally 
reflected  inside  the  glass 
and  will  not  emerge  from 
the     illuminator     unless 
the  latter  is  brought  into 
immersion   contact   with 
the  under-surface  of  the 
slide  by  placing  a  drop 
of    cedar-wood     oil    be- 
tween      the       top       of 
the  illuminator  and    the 
slide.    It  must  be  used 
in     immersion      contact 
with    the    slide    in   the 
same  way  that  an  oil-immersion  object  glass  is  used  in  immersion 
contact  with  the  cover  glass.     With  this  iUuminator  any  dry 
lens  or  an  immersion  lens  with  an  aperture  of  less  than  1  N.A. 
can  be  used,  and  no  direct  light,  but  only  that  reflected  by  the 
Use  of  oil-    object,    enters   the   microscope   (see    Fig.    24).      A    special    oil- 
objUcr^^"^    immersion  1/8-inch  (3-mm.)  focus,  with  an  aperture  of  '95  N.A., 
glasses  with  jg  made  for  work  with  this  illuminator ;  or  an  immersion  lens 
groun  .  ^^^^   ^  larger  aperture  can  be  used  if  it  be  stopped  down  by 
means  of  a  small  diaphragm  placed  behind  the  back  lens  of  the 
-  object  glass. 

In  the  latter  case  the  object  glass  must  be  stopped  down  to 
a  considerably  smaller  aperture  than  1  N.A.,  because  the  stop 
cannot  be  placed  in  the  best  position,  which  is  between  the  lenses 
themselves,  and  with  a  stop  behind  the  back  lens  a  certain  amount 
of  direct  light  is  not  properly  excluded  by  a  stop  of  the  theoretical 
size,  because  it  is  not  in  the  correct  position. 

There  is  a  peculiarity  in  dark-ground  illumination.  The 
object  must  be  exactly  at  the  crossing  point  of  the  beams  of  light — 
that  is,  in  its  focus — or  it  will  not  be  illuminated  at  aU  (see  Fig. 
24),  whereas  with  an  ordinary  condenser,  even  if  the  object  is 
not  in  the  exact  focus,  it  will  still  be  illuminated,  though,  perhaps, 
not  so  brifliantly.  The  non-focussing  dark-ground  illuminator 
has  no  adjustment ;  as  the  front  portion  of  the  illuminator  must 
be  in  immersion  contact  with  the  under-surface  of  the  slide,  it 
cannot  be  moved  up  and  down,  and  therefore  the  slides  used 
with  this  illuminator  must  be  1  mm.  thick.    SUdes  of  this  thick- 


ILLUMINATING  APPARATUS 


35 


Fig. 


24.— No.  3294,  Focussing  Dark- 
ground  Illuminator. 


ness  can  be  selected  for  examining  living  specimens,  but  mounted 
objects  can  seldom  be  examined. 

To  overcome  this  difficulty,  R.  &  J.  Beck,  Ltd.,  have  designed  Focussing 
the  dark-ground  iUuminator  (Figs.  24  and  25)  with  a  focussing  lurj^Stor."^ 
adjustment.  The  upper 
lens  (C)  remains  in  im- 
mersion contact  with  the 
slide,  but  the  reflector 
(D)  can  be  moved  up 
and  down,  which  raises 
and  lowers  the  illumi- 
nated point,  enabling 
any  slide  of  from  J  mm. 
to  IJ  mm.  thickness  to 
be  used. 

A  convenient  means 
of  setting  the  focus  for 
a  particular  slide  is 
arranged  for  in  the 
mount  of  the  illumi- 
nator. Fig.  25  shows 
the  focussing  illuminator 

mounted  for  use  on  the  Standard  London  Microscope.  Turning 
the  lever  (C),  which  projects  from  the  lower  portion  of  the  mount, 
moves  the  focussing  lens  (D,  Fig.  24) ;  in  doing  this  it  also 
moves  the  pin  (A,  Fig.  25)  up  and  down,  and  alters  its  distance 
from  a  flange  (B,  Fig.  25)  on  the  mount. 

If  the  lever  (C)  is  moved  till  this  pin  (A,  Fig.  25)  is  at  its  farthest 
distance  away  from  the  ring  (B,  Fig.  25),  the  slip  that  is  to  be  used 
may  be  placed  between  the  ring  (B)  and  the  pin  (A) ;  and  if  the 
lever  (C)  be  again  moved  till  the  pin  (A)  just  clamps  the  slip, 
the  illuminator  will  be  approximately  set  to  the  correct  focus 

for  this  thickness  of  sHp.  The 
final  adjustment  may  then  be  made 
when  the  object  is  in  position. 

The  high-power  dark-ground 
illuminator  has  been  found  specially 
valuable  for  the  examination  of 
living  bacteria,  rhizopods,  and  other 
transparent  and  unstained  speci- 
mens that  are  difficult  to  see  with 
direct  light.  Such  objects,  however, 
due  to  their  transparency,  reflect 
only  a  small  portion  of  the  light 
that  falls  upon  them,  and  a  strong  illumination  is  necessary.  To 
accomplish  this  the  high-power  illuminator  is  made  to  produce 
a  very  minute  image  of  the  source  of  light,  so  that  all  the  light 
may  be  concentrated  on  a  very  small  area,  almost  a  point. 


Fig.  25.— No.  3294,  Focussing 
Dark-ground  Illuminator. 


36  THE  MICROSCOPE 

To  centre  It  is  therefore  necessary  that  a  centring  adjustment^should 

dark'-^ound  ^^  provided,  SO  that  the  illuminated  point  may  be  exactly  in 
illuminator,  the  field  of  view.     If  the  substage  of  the  microscope  is  not  pro- 
vided with  a    centring   adjustment,    the   form   of   dark- ground 
illuminator  mount  which  has  centring  screws  should  be  used. 

To  centre  the  illuminator  the  following  is  a  satisfactory  method 
of  procedure  : 

Remove  the  eyepiece  and  object  glass  from  the  microscope 
and  swing  out  the  substage.  Place  the  eye  six  or  eight  inches 
above  the  tube  of  the  microscope,  and  move  the  eye  until  the 
lower  end  of  the  microscope  tube,  where  the  object  glass  screws  in, 
is  central  with  the  upper  edge  of  the  drawtube.  Then,  without 
moving  the  eye,  move  the  mirror  until  the  light  appears  in  the 
centre  of  these  apertures.  Swing  in  the  substage  with  the  dark- 
ground  illuminator  without  moving  the  microscope  or  the  mirror, 
place  a  drop  of  cedar-wood  oil  upon  the  upper  surface  of  the 
illuminator,  put  the  object  to  be  examined  on  the  stage,  and  move 
the  substage  up  till  the  illuminator  is  in  immersion  contact  with 
the  sHp.  This  having  been  done,  put  a  low-power  object  glass, 
say  2/3-inch,  into  the  microscope,  use  a  low-power  eyepiece,  and 
focus  the  slide.  There  will  be  sufficient  dirt  or  particles  on  the 
slide  to  show  the  small  illuminated  point,  which  will  probably 
not  be  in  the  centre  of  the  field.  By  means  of  the  centring 
screws  of  the  substage  or  the  illuminator  mount,  this  illuminated 
patch  may  be  brought  into  the  centre  of  the  field,  and  the  2/3-inch 
object  glass  may  then  be  replaced  by  the  object  glass  which  it  is 
desired  to  use.  A  further  slight  adjustment  may  be  made 
for  the  new  object  glass  if  necessary,  after  which  the  centring 
screws  should  not  be  touched,  but  slight  alterations  should  be 
made  by  altering  the  position  of  the  mirror.  Until  the  centring 
of  the  illuminator  has  been  completed,  the  position  of  the  lamp, 
the  microscope,  or  the  mirror,  should  not  be  altered. 
To  focus  Focussing,    as   previously   mentioned,    is    almost   impossible 

d?rk'-ground  '^^^^  *^®  nou-focussiug  illumiuator,  although  a  very  small  move- 
iiiuminator.  ment  of  the  substage  is  possible  without  breaking  the  film  of  oil 
between  the  illuminator  and  the  slip. 

With  the  focussing  model  it  is  best  to  set  the  focus  approxi- 
mately by  the  pin  on  the  mount,  as  described,  but  a  small 
movement  of  the  adjusting  lever  while  the  object  is  being 
observed  is  particularly  useful  in  obtaining  the  best  result. 

If  the  illuminator  is  out  of  focus,  a  dark  circular  patch  will 
appear  in  the  centre  of  the  field  surrounded  by  a  bright  ring ; 
when  focussed,  the  central  dark  patch  disappears. 

Objects  mounted  dry  cannot  be  examined  by  this  illuminator. 

They  must  be  in  some  fluid  or  medium,  as  no  light  will  reach  the 

object  if  there  is  any  layer  of  air  between  it  and  the  illuminator. 

It  is  important  to  see  that  the  slips  and  cover  glasses  are 

thorpughly  clean  and  that  there  are  no  air  bubbles  in  the  oil  or 


ILLUMINATING  APPARATUS  37 

the  fluid  containing  the  object,  as  reflections  from  dirt  or  bubbles 
may  cause  a  glare  that  destroys  the  black  background  against 
which  the  illuminated  objects  stand  out. 

A  strong  source  of  light  for  use  with  this  illuminator  is  essential,  intensity 
It  is  referred  to  under  the  heading  of  "  Illuminants."  If  the°^"=^^- 
light  is  of  only  moderate  intensity,  a  bull's-eye  condenser  should 
be  used.  It  should  be  placed  at  such  a  distance  that  an  image 
of  the  lamp  is  formed  in  the  centre  and  on  the  surface  of  the 
mirror.  The  correct  position  is  best  ascertained  by  holding  a 
white  card  on  the  mirror  while  the  bull's-eye  is  adjusted  between 
the  lamp  and  the  mirror. 

When  a  colour  filter  is  used,  a  stronger  light  than  would  other- 
wise be  required  should  be  available.  The  light,  however,  must 
not  be  too  strong,  for  although  a  weak  light  will  not  illuminate 
such  transparent  structures  as  bacteria  sufficiently  to  render 
them  visible,  too  strong  a  light  shows  up  certain  diffraction 
images  and  destroys  definition. 

It  is,  therefore,  necessary  to  reduce  the  illumination  to  just 
such  an  extent  that  these  diffraction  effects  are  not  aggressively 
apparent. 

A  very  brilliant  source  of  illumination,  such  as  an  electric 
"  Pointolite  "  lamp,  gives  more  light  than  is  required,  except  for 
use  with  colour  filters ;  but  used  in  combination  with  the  adjustable 
neutral  tint  wedges  (p.  32),  gives  every  intensity  of  illumination 
that  is  required  for  this  and  all  other  classes  of  microscopic  work. 

Those  who  have  not  used  this  form  of  illumination  cannot  Resolution 
realise  the  large  amount  of  extra  structure  that  can  be  recognised  ^^^^^^  ^^^J^^^ 
by   its    means    in    certain    classes    of    objects.     The    spines    or  illumination, 
pseudopodia  of  Coscinodiscus  were  discovered  by  dark-ground 
illumination.     Hidden  structure  in  bacteria  has  been  revealed, 
and  the  markings  of  diatoms  are  shown  with  greater  brilliancy 
by  this  illuminator  than  by  any  other  means.     The  resolution  of 
diatoms  is  much  easier  with  dark-ground  illumination  because  it 
gives  far  greater  contrast.     If  an  object  is  not  opaque  and  only 
appears  slightly  darker  than  the  background,  it  is   but  faintly 
seen  when  viewed  by  transmitted  light ;  but  if  it  has  even  a  small 
power  of  reflecting  light  it  can  be  made  to  show  brilliantly  upon 
a  black  ground  provided  the  illuminating  source  is  sufficiently 
powerful. 

Reflection  of  light  takes  place  from  any  object  that  has  a 
different  refractive^ index  or  density  from  that  of  the  material  in 
which  it  is  situated.  A  very  small  difference  of  density  only  is 
required  to  give  reflection.  An  interesting  experiment  to  illus- 
trate this  consists  of  taking  two  plates  of  glass  with  a  layer  of 
water  between  them,  and  flowing  in  from  one  side  a  few  drops  of 
a  highly  refracting  fluid,  which  gradually  mix  with  the  water 
and  raise  its  density  above  that  of  glass.  As  the  fluid  mixes, 
the  density  is  gradually  raised  from  point  to  point,  and  at  the 


38 


THE  MICEOSCOPE 


Bull's-eye 
condenser. 


Fig.  2b.— No.  3215, 
Bull's-eye  Con- 
denser on  Stand. 


Increase  in 
illumination. 


position  where  it  actually  readies  exactly  that  of  the  glass  an 
extremely  fine  line  shows  on  looking  at  the  surface  obliquely, 
indicating  that  only  at  that  exact  point  where  the  density  of  the 
fluid  and  the  glass  are  absolutely  the  same  is  the  grey  effect  pro- 
duced by  reflection  destroyed  and  turned  into  black. 

A  bull's-eye  lens  on  a  stand  used  in  conjunction  with  a  lamp, 
or  attached  to  the  lamp  itself,  is  required  for  the  illumination  of 

opaque  objects.  It  is  also  useful  for  in- 
creasing the  illumination  with  a  dark- 
ground  illuminator  or  for  obtaining  a 
moderately  convergent  beam  of  light  in 
combination  with  the  mirror  when  a  sub- 
stage  condenser  is  not  used.  It  may  be 
used  in  connection  with  a  substage  condenser 
for  increasing  the  size  of  the  image  of  the 
source  of  light.  This  is  of  service  when  a 
very  small  luminous  source  of  light,  such  as 
the  electric  "  Pointolite,"  is  employed. 

On  an  optical  bench  a  condenser  will 
transfer  the  light  source  to  a  position  nearer 
the  microscope.  Except  for  these  purposes, 
it  should  not  be  employed  with  a  good 
substage  condenser.  It  does  not  give 
additional  light,  and  it  generally  ruins  the  performance  of  a 
perfectly  made  condenser  due  to  its  own  lack  of  correction. 

Its  action  in  increasing  the  illumination  of  opaque  objects  can 
be  explained  by  Fig.  27.  A  bull's-eye  lens  placed  between  the 
source  of  light  (S)  and  the  object  (0)  produces  a  small  image  of 
the  source  of  light  at  (0).  The  upper  diagram  in  Fig.  27  shows 
the  lens  (L)  forming  a  somewhat  reduced  picture  of  the  source  of 
light.  The  lower  diagram  shows  the  distances  so  arranged  that 
it  is  forming  a  much  reduced  image.  The  size  of  the  image  will 
depend  on  the  relative  distances  from  the  light  source  to  the 
lens  and  the  image  to  the  lens.  In  the  upper  diagram  L  0  is 
half  L  S,  and  the  image  is  half  the  size  of  the  luminous  source  ; 
in  the  lower  diagram  L  0  is  one- quarter  L  S,  and  the  image  is 
one-quarter  the  size  of  the  luminous  source.  The  lens  does  not 
only  collect  a  much  larger  amount  of  light  than  would  other- 
wise reach  the 
object  at  0,  but 
it  also  compresses 
it  into  a  very 
small  size  image, 
especially  in  the 
case  of  the  lower 
diagram,  and 
therefore  it  pro- 
duces a  very  con-  Fig.  27. 


ILLUMINATING  APPARATUS 


39 


centrated  and  brilliant  small  patch  of  light  which  is  made  use 
of  for  the  illumination  of  opaque  objects  viewed  with  low 
powers. 

Fig.  28  shows  the  Beck  electric  lamp  used  with  a  bull's-  Condenser 
eye  condenser  in  this  manner  for  condensing  a  powerful  beam  °°  ^^^' 
of  light  upon  the 
top  of  an  object  on 
the  stage  of  the 
microscope  (see  also 
p.  46). 

A  method  of 
displaying  the  struc- 
ture of  opaque  ob- 
jects is  sometimes 
adopted  in  which 
two  lamps  and  two 
bull's-eye  condensers 
are  used,  one  of 
which  has  a  blue 
and  the  other  a  red 
glass  to  colour  the 
light.  If  these  are 
used  at  different 
angles,  difficult 
structure  may  be 
more  easily  inter- 
preted by  observing 
the  different  colours 
of  the  shadows. 

A  suitable  illumination  of    opaque   objects  is  required  for  illumination 
botanical,  entomological,  and  general  work,  and  is  of  paramount  objects  vSth 
importance  for  metallurgy.     When  low-power  object  glasses  are  ^"(^g'^Jer. 
used   there  is  sufficient  working   distance  (see  Fig.  3)  between 
the  front  of  the  object  glass  and  the  object  to  throw  light  in  from 
one  side  by  means  of  a  bull's-eye  condenser  either  attached  to  a 
lamp,   as  shown  in  Fig.   28,   or  on  a  separate  stand  (Fig.  26). 
"When  a  bull's-eye  is  used  on  a  separate  stand  there  is  another 
method  of  using  it  which  is  useful  for  even  high  powers.     If  the 

bull's-eye  condenser  is  placed 
with  its  flat  surface  upwards, 
nearly  parallel  with  the  direc- 
tion of  the  light,  as  in  Fig.  29, 
the  light  enters  the  curved 
surface  and  is  condensed ;  when 
it  meets  the  flat  surface  it  is 
reflected  back  in  such  a  way 
that  a  very  powerful  narrow  ribbon  of  light  is  emitted ;  this 
band  is  so  narrow  that  it  can  be  made  to  illuminate  the  object 


Fig.  28.— No.  3332. 


Fjg.  29. 


40 


THE  MICROSCOPE 


Parabolic 
reflector. 


Sorby 
reflector. 


Glass 
reflector. 


Vertical 
illiiminator. 


Fig.  30.— No.  3360, 
Parabolic  Reflector. 


even  when  a  moderately  high  power,  such  as  a  1/6-inch  (4-nim.) 
is  used,  because  the  band  of  light  is  sufficiently  narrow  to  be 
directed  through  the  small  working  distance  between  the  object 
glass  and  the  object.  This  method  is  particularly  useful  for  the 
examination  of  alloys  of  metals  or  substances  with  fine  laminae, 
as  the  heavy  shadows  shown  by  such  oblique  illumination 
indicate  the  character  of  the  structure. 

Another  method  of  illuminating  opaque  objects  is  by  means 
of  a  silvered  parabolic  mirror,  which  can  be  attached  to  either  a 

IJ-inch  (40-mm.  or  32-mm.)  or  a  2/3-inch 
(16-mm.  or  14-mm.)  object  glass.  The  front 
lens  of  the  object  glass  being  removed,  the 
tubular  portion  of  the  reflector  can  be  slid 
on  to  the  cylindrical  part  of  the  object  glass, 
which  is  of  a  standard  size.  This  may  be 
lightly  clamped  in  position  by  the  milled 
head  and  the  front  lens  replaced.  The 
object  glass  having  been  screwed  into  the 
microscope  and  focussed,  the  reflector,  which 
has  an  adjustment  up  and  down,  should  be  placed  so  that 
its  lower  edge  almost  touches  the  object.  The  light  should 
then  be  directed  by  a  bull's-eye  condenser  in  a  horizontal 
direction  parallel  with  the  stage,  so  that  it  illuminates  the 
whole  of  the  reflector  (Fig.  30).  The  reflector  condenses  it  to 
a  focus  on  the  object,  and  a  slight  movement  of  the  reflector 
up  or  down  or  a  slight  turn  will  give  the  best  result.  This 
produces  a  very  brilliant  illumination,  and  as  the  light  falls  upon 
the  object  from  a  large  number  of  directions,  the  shadows 
produced  are  not,  as  a  rule,  misleading  in  interpreting  structure. 

Mr.  Sorby  devised  an  addition  to  this  reflector,  which 
can  be  used  with  the  IJ-inch  (32-mm.  and  40-mm.)  object  glass, 
which  consists  of  a  small,  flat,  silvered  mirror  which  swings  in  and 
out  of  the  optic  axis,  and  when  it  is  in  position  it  covers  half 
the  object  glass.  It  reflects  a  beam  of 
light  directly  downwards  upon  the  object, 
which  illuminates  it  in  such  a  manner  that 
no  shadows  are  produced. 

A  modification  of  this  apparatus  is  also 
used  for  metallurgy  in  which  a  thin  transparent 
plate  of  glass  is  placed  at  45°  between  a  low- 
power  object  glass  and  the  object  (see  Fig.  31). 
This  has  the  advantage  that  it  does  not  reduce 
the  aperture  of  the  object  glass.  It  can  be 
used  with  either  a  IJ-inch  (40-mm.  or  32-mm.)  or  a  2/3-inch 
(16-mm.)  object  glass. 

For  the  illumination  of  opaque  objects  viewed  with  high 
powers,  a  system  was  first  invented  by  Mr.  Richard  Beck  in  which 
a  thin  glass  disc  was  placed  behind  the  object  glass,  and  a  beam 


Fig.     31.  —No. 

3362,  Thin  Glass 

Reflector. 


ILLUMINATING  APPARATUS 


41 


No.  3363. 


of  light  reflected  througli  the  object  glass  itself  upon  the 
object. 

This  illuminator  is  generally  known  as  a  vertical  illuminator,  use  of 
It  is  screwed  into  the  nosepiece  of  the  microscope,  and  the  object  Ju^^nator. 
glass   screwed  into  the  illuminator   mount.     The  body  of  the 
microscope    is    then    racked    into    the 
position  where   the  object  glass  is  ap- 
proximately in  focus.     The  illuminator, 
which  can  be  turned  round,  should  be 
rotated  until  the   two  apertures  in  the 
mount  are  pointing  one  to   each  side, 
while  the    milled    head,    which   carries 
the  thin  glass  reflector,  is  at  the  front 

of  the  instrument.  A  lamp  is  now  set  up  on  one  side,  pre- 
ferably the  left-hand,  at  the  height  of  the  apertures  in  the 
illuminator,  so  that  light  will  shine  through  the  two  apertures 
upon  a  card  held  on  the  right-hand  side.  A  bull's-eye  con- 
denser may  now  be  placed  in  front  of  the  lamp,  and  the  beam 
of  light  concentrated  by  this  means.  The  milled  head,  which 
carries  the  thin  glass  reflector,  should  be  turned  round  until 
the  light  is  reflected  downwards  through  the  object  glass 
upon  the  object.  The  milled  head  has  engraved  on  it  a  line 
parallel  with  the  reflector,  and  this  enables  the  reflector  to  be  set 
to  approximately  the  correct  angle  (45°)  before  commencing  work. 
The  microscope  can  now  be  accurately  focussed,  and  a  slight 
alteration  of  the  position  of  the  lamp  or  the  reflector  will  throw 
the  light  in  one  direction  or  the  other. 

For  critical  work  the  lamp-flame  or  source  of  illumination 
should  appear,  if  the  object  be  flat,  as  a  small  sharp  image  on  its 
surface.  This  can  be  effected  by  having  the  lamp,  if  used  alone, 
about  6  inches  away  from  the  microscope,  or,  if  a  bull's-eye 
condenser  is  used,  by  adjusting  the  distance  of  the  bull's-eye 

from  the  lamp. 

A  disadvantage  of  this  form  of  illumi- 
nation is  that  the  surfaces  of  the  lenses  of 
an  object  glass  are  convex  and  that  a  certain 
amount  of  light  is  reflected  back  into  the 
eye  by  these  surfaces,  tending  to  produce  a 
glare  ;  but  this  can  frequently  be  overcome 
by  small  adjustments  in  the  position  of  the 
lamp  or  reflector  so  that  such  reflections 
are  directed  on  the  sides  of  the  interior  of 
the  microscope. 
Another  form  of  this  illuminator  is  made  in  which  the  trans-  Prism 
parent  glass  reflector  is  replaced  by  a  small  prism  which  occupies  illuminator. 
half  the  aperture  of  the  object  glass.     This  method  reflects  more 
light,  but  reduces  the  aperture  and  resolution  of  the  object  glass. 
The  prism  illuminator  gives  more  light  when  used  with  low  powers 


Fig.  33.— No.  3364, 
Prism  Uluminator. 


42 


THE  MICROSCOPE 


Colour 

screens. 


than  the  Beck  illuminator,  but  most  prefer  the  thin  glass  form 
for  high  powers,  and  the  Sorby  reflector  for  low-power  work. 
As  a  convenient  and  universal  illuminator  for  all  powers  where 
the  highest  resolution  is  not  required,  the  prism  illuminator, 
especially  with  an  electric  light  bulb  attached  to  it,  is  popular 
for  metallurgical  work.  Fig.  Ill  (p.  120)  shows  this  illuminator 
provided  with  a  small  focussing  lens,  a  receptacle  for  colour 
filters,  and  a  16-candle-power  electric  light  to  suit  either  the 
100-  or  200-volt  current  in  a  metal  casing. 

Colour  screens  are  of  use  for  several  important  purposes. 
They  are  either  coloured  glasses  or  coloured  gelatine  mounted 
between  two  glasses.  They  give  greater  contrast  where  objects 
being   examined   are   stained    or    are   naturally   coloured,    and 

give  truer  rendering  of 
natural  colours  where 
artificial  light  is  em- 
ployed. 

Every  substage  con- 
denser should  be  sup- 
plied with  a  green  glass, 
but  a  set  of  different 
coloured  screens  is 
very  useful  for  increas- 
ing colour  contrasts, 
both  for  visual  and 
photomicrographic  work. 
If  a  specimen  of 
bacteria    is    stained 


KODAK 


Fig.  34. 


faintly    with     red,    the 
use    of    a    green    screen 
will  make  them  appear  almost  black  and  much  more  distinct. 
If  a  specimen  is  stained — 


a  red 
a  red 


filter  should  be  used. 


55 


55 


55 


55 


55 


55 


55 


5  5 


Blue, 

Green, 

Red,      a  green  ,, 

Yellow,  a  blue 

Brown,  a  blue 

Purple,  a  green 

Violet,   a  yellow 

If  the  screen  is  too  dark,  and  the  light  and  shade  contrast 
too  great  in  consequence,  fine  detail  in  the  structure  may  be 
somewhat  clogged  or  obscured,  but  faintly  stained  or  coloured 
specimens  are  rendered  much  more  visible  by  the  use  of  the 
correct  colour  filter. 

Another  marked  advantage  in  the  use  of  colour  filters  of  green 
or  blue  is  obtained  by  the  greater  power  they  give  to  an  object 
glass  of  resolving  fine  structure. 


ILLUMINATING  APPARATUS  43 

When  discussing  the  aperture  of  an  object  glass,  the  resolution 
was  stated  to  be  dependent  on  its  aperture,  but  it  is  also  depen- 
dent on  the  colour  of  the  light.  White  light,  when  split  into  its 
component  parts  by,  for  instance,  a  rainbow  or  a  prism,  con- 
sists of  certain  pure  spectrum  colours — red,  yellow,  orange,  green, 
blue,  and  violet.  The  resolution  obtained  with  white  light  is  that 
due  to  the  orange-coloured  portion  of  its  component  parts,  because 
this  coloured  light  is  more  powerful  than  any  of  the  others  that 
go  to  make  it  up.  If  a  green  light  be  used,  the  resolution  of  a 
microscope  can  be  increased  about  15  per  cent.,  and  if  a  purple 
be  used,  about  25  per  cent. 

A  purple  light,  or  even  a  very  dark  blue,  is  unpleasant  to  the 
eyes  of  most  observers,  but  a  bluish-green  light  is  very  restful, 
and  is  the  best  colour  to  use  as  regards 
microscope  resolution. 

Apochromatic  object  glasses  are  so 
perfectly  corrected  for  colour  that  the 
brilliancy  of  their  images,  quite  apart 
from  resolution,  will  not  be  improved 
by  the  use  of  colour  screens ;  but 
achromatic  object  glasses  are  always 
slightly,  and  under  some  conditions 
considerably,  improved  in  their  per- 
formance by  the  use  of  a  screen 
which  transmits  a  pure  colour. 

The  green  glass  supplied  with 
the  substage  condensers  trans- 
mits a  moderately  pure  colour, 
but  is  not  so  good  as  the  special 
Wratten  &;  Wainwright  gelatine 
filters. 

A  glass  trough  about  3/4  inch 
thick,   filled   with    a    nearly    saturated   solution   of    acetate   of  colour 
copper,  makes  a  fairly  pure  blue-green  screen.    It  is  rather  more  ^^°^^^' 
transparent  than  a  gelatine  filter. 

Various  fluid  screens  have  been  used,  but  they  are  so  much  less 
convenient  than  the  Wratten  gelatine  screens  that  they  are  not 
so  frequently  employed. 

The  best  form  of  illuminant  for  the  microscope  depends  upon  sources  of 
many  circumstances.  The  author  is  of  opinion  that  daylight  is  illumination. 
the  worst  form  for  accurate  observation,  but  that  when  it  is  used 
a  screen  or  card  with  a  hole  in  it  about  2J  inches  diameter  should 
be  placed  in  front  of  the  mirror  of  the  microscope  about  8  or 
10  inches  away.  This  ensures  a  moderately  parallel  beam  of  light 
falling  upon  the  mirror.  A  paraffin  lamp  with  a  flat  flame  is 
probably  the  most  convenient  light  for  general  purposes, 
but  it  is  not  powerful  enough  for  the  use  of  colour  screens  or  for 
high-power    dark-ground    illumination.     The    electric   light   of 


Fig.  35.— No.  3366,  Monochro- 
matic Light  Trough. 


44 


THE  MICROSCOPE 


Relative 
intensities 
of  different 
sources  of 
illamination. 


the  ordinary  type  is  unsatisfactory  unless  used  with  a  ground 
glass  or  tissue  paper  in  front  of  it.  A  form  of  1/2- watt  bulb 
called  the  "  Grid  "  is  a  good  light,  as  the  filaments,  when  looked 
at  from  the  correct  side,  appear  as  a  fairly  large  ribbon  of  almost 
homogeneous  light.  The  "  Pointolite  "  electric  arc  is  extremely 
good  for  the  highest  power  work.  The  incandescent  gas  mantle 
lamp  is  a  useful  illuminant,  and  a  modification  of  this,  heated 
by  a  methylated  spirit  lamp,  is  an  excellent  light  for  those  who 
have  not  gas  or  electricity. 

The  relative  intensities  of  a  similar  size  small  area  of  different 
illuminants  are  approximately  according  to  the  following  table 
taken  from  Mr.  A.  P.  Trotter'^  book  on  Illuminating  Engineering  : 


Candle 

Daylight  (blue  sky) 

Paraffin  lamp 

Incandescent  gas  mantle 

Carbon  electric  filament 

Metal  electric  filament 

1/2-watt  electric  biilb  . 

"  Pointolite  "  electric  bulb 

Arc  lamp   , 

Direct  light  from  the  sun 


2i 
2 

4  to  9 

50 

300 

1,000 

5,250 

12,000 

80,000  to  110,000 

.  800,000 


From  the  above  table  it  is  evident  why  ordinary  daylight  is 

not  sufficiently  powerful  for  high- power 
microscope  work.  The  sun,  even  in  a  clear 
climate,  requires  the  use  of  a  heliostat,  and 
the  arc  lamp  requires  a  special  equipment. 
It  is  of  great  advantage  to  use  a  very  in- 
tense light  modified  with  the  neutral  tint 
wedge  moderator  described  on  page  32. 
The  brightness  of  the  light  can  then  be 
perfectly  regulated  to  meet  requirements. 

This  advantage  of  a  very  powerful  illu- 
minant has  been  referred  to  in  connection 
with  substage  condensers  and  dark-ground 
illuminators,   but  care   should   be   exercised 
in  its  use.      When  direct  light 
is   being   used    through   a   con- 
denser, it  is   damaging    to    the 
eyes  if  too  strong  a  light  is  em- 
ployed.    A  strong  illuminant  is 
necessary   for   high- power  dark- 
ground  or  opaque  illumination, 
but  it  must  be  modified  when 
direct  light  is   thrown   through 
the  object.     Some  colour  screens 
require  a  strong  light,  but  imme- 
diately  they   are   removed    the 
Just   enough   light   to   show   the 


Fig.  36. 


-No.  3335,  Paraffin 
Lamp. 


hght  should    be   cut  down. 


ILLUMINATING  APPARATUS 


45 


object  readily  should  be  used,  and  no  more.  If  this  precaution 
is  taken,  microscopists  need  have  no  fear  of  injuring  their  eyes, 
however  long  they  work.  The  light  should  be  more  powerful 
than  is  required  for  general  purposes,  it  should  be  powerful 
enough  for  dark-ground  illumination  and  to  allow  of  the  use  of  . 
colour  filters. 

Fig.  36  shows  a  good  form  of  parafhn  lamp  for  microscopic  Paraffin 
work.  It  has  a  single  flat  wick  5/8  inch  wide.  The  burner  ^^™p* 
has  a  revolving  motion  and  may  be  used  with  its  edge  facing  the 
mirror  to  give  a  strong  illumination,  and  with  the  flat  surface 
facing  the  mirror  for  a  softer  light.  It  has  a  means  of  raising 
and  lowering  it  from  the  table  to  enable  it  to  be  used  for  illumina- 
ting opaque  objects  with  a  bull's-eye  condenser  or  parabolic 
reflector,  or  for  setting  it  to  the  correct  height  for  using  the 
vertical  illuminator  described  on  page  41. 

The  reservoir  and  burner  are  carried  on  a  support  which 
passes  through  the  centre  of  the  reservoir  so  that  the  weight  is 
well  balanced  over  the  centre  of  the  stand.  The  lamp  glass  is 
simply  a  3  X  1-inch  microscope  slip  carried  in  a  thin  metal 
chimney.  The  burner  is  insulated  from  the  reservoir  by  a  fibre 
ring,  which  is  always  cool  enough  to  touch  for  turning  the  burner 
round.  The  metal  chimney  can  be  removed  and  the  burner 
hinged  back  for  trimming  the  wick.  The  reservoir  has  a  large 
screw  stopper  for  filling.  A  bull's-eye  condenser  on  a  separate 
stand  may  be  used  in  combination  with  this  lamp  for  illuminating 
opaque  objects  or 
for  high  -  power 
dark- ground  illu- 
mination, al- 
though this  lamp 
is  not  recom- 
mended for  the 
latter  purpose. 

The  ordinary 
electric  incan- 
descent lamp  pro- 
vided with  a 
frosted  or  ground 
glass  bulb  is  a 
handy  lamp  for 
ordinary  observa- 
tion, but  is  not 
sufficiently  bril- 
liant for  many  purposes.     It  is  supplied  on  an  adjustable  table 

stand. 

The  best  equipment  is  the  "  Pointolite,"  or  1/2-watt  "  Grid" 
lamp,  with  a  neutral  glass  double  wedge,  a  set  of  colour  screens, 
and  a  bull's-eye.     It  does  everything  that  is  required  for  every 


Electric 
lamp. 


Fig.  37. — No.  3336,  Electric  Lamp  on  Stand. 


46 


THE  MICROSCOPE 


Electric 
lamp. 


Fig.  38.— No.  3332,  "  Pointolite  "  Lamp 


class  of  illumination ;  and  the  Beck  electric  lamp  is  a  convenient 

form  which  takes  either  kind  of  electric  bulb. 

The  lamp  has  adjustment  so  that  the  beam  of  light  can  be 

placed  at  any  height 
between  3  and  9  inches 
above  the  level  of  the 
table. 

The  stand  (A)  is  in 
the  form  of  a  heavy 
ring  with  a  section  cut 
ofi  so  that  it  can  be 
placed  close  to  the 
microscope. 

A  vertical  rod  (B)  is 
fixed  into  this  ring.  On 
this  rod  a  bracket  (C) 
slides  up  and  down 
and  can  be  clamped  in 
position  at  any  point 
by  a  milled  head  (E). 
This  bracket   can   also 

be  inclined  at  any  angle  and  clamped  by  another  milled  head  (F). 

These  clamps  are  independent  of   each   other,  and   either    can 

be  used  without  disturbing  the  other  adjustment. 

On  the  bracket  (C)  is  fixed  another  vertical  rod  (G),  which,  by 

means  of  the  arm  (H),  carries  the  electric  light  bulb.     This  can 

be   moved   up   and   down  the   vertical   rod  G   and  fixed  by  a 

clamp  screw  (J)  in  such  a 

position    that   the    incan- 
descent     point      of      the 

"  Pointolite  "  or  the  most 

luminous    portion    of    the 

filament     of     an    electric 

bulb  can  be  placed  in  the 

optic  axis  of  the  bull's-eye 

condenser  (K). 

The  arm   (H)   has   at- 
tached to  it  a  thin  metal 

cylindrical    tube    with    a 

circular     aperture     which 

forms  a  shield  to  cut  off 

stray  light  from  the  room, 

and  when  the  electric  bulb 

has  been   adjusted  to  the   Fig. 39. — No. 3332,  "Pointolite"  Lamp. 

optic  axis  of  the  condenser, 

this  tube  can  be  moved  up  and  down  till  the  aperture  in  the 

metal  casing  is  also  opposite  the  condenser  (K). 

The  condenser  (K)  is  carried  in  a  mount  which  has  two  slides 


ILLUMINATING  APPAKATUS 


47 


for  colour  screens  or  ground  glass.  It  is  supported  on  a  rod  (L) 
which  moves  backwards  and  forwards  parallel  with  the  optic 
axis  for  obtaining  either  parallel  or  convergent  light,  and  can  be 
clamped  in  any  position. 

If  the  condenser  is  not  required  it  can  be  swung  to  one  side ; 
or  if  it  is  required  to  use  colour  screens  alone,  the  lens  of  the  con- 
denser can  be  removed  from  its  mount. 

The  illustrations  show  the  lamp  (Fig.  38)  for  use  with  the  mirror 
of  the  microscope  for  transparent  or  dark- ground  illumination 
by  means  of  a  dark-ground  condenser,  or  for  metallurgical  or 
photomicrographic  work.  Fig.  39  shows  it  tilted  for  use  without 
a  mirror,  or  Fig.  40 
shows  it  arranged  for 
the  illumination  of 
opaque  objects  from 
above. 

The  lamp  is  pro- 
vided with  a  ground 
glass  and  a  signal- 
green  glass  ;  it  is  com- 
pleted by  the  addi- 
tion of  the  Wratten 
&  Wainwright's  colour 
filters  and  the  neutral 
glass  moderator.  It  is 
provided  with  12  feet 
of  cable  and  an  at- 
tachment for  fitting  it 
to  a  lamp  fitting  of 
an  ordinary  house 
supply.  For  use 
with  the  "  Pointolite  " 
lamp,  which  is  an  in- 
candescent disc  about 
the    size    of    a    small 

peppercorn,  a  direct  Current  of  any  voltage  from  100  to  250 
volts  is  equally  satisfactory,  a  variable  resistance  being  supplied 
to  adapt  it  to  any  current  between  these  limits.  The  candle- 
power  is  100,  but  as  it  is  all  concentrated  in  the  one  point  it  is 
at  least  twenty  times  as  powerful  as  the  filament  lamp  focussed 
with  the  condenser. 

If  a  100-candle-power  1/2- watt  lamp  or  40-  or  60-candle- 
power  metal  filament  lamp  is  used,  it  is  suitable  for  either 
direct  or  alternating  currents,  and  for  a  voltage  from  100  to 
200  volts,  although  a  lamp  suitable  for  the  voltage  must  be 
selected. 

No   special  wiring  is  required,  any  ordinary  house   current 

being  sufficient. 


Fig.  40.— No.  3332,  "Pointolite"  Lamp. 


48 


THE  MICROSCOPE 


Incandes- 
cent gas 
launp. 


Incandes- 
cent spirit 
lamp. 


An  incandescent  gas  mantle,  either  of  the  ordinary  or  inverted 
type,  makes  a  good  light.     Its    only    disadvantage   is   that   it 

cannot  be  conveniently  used 
with  its  image  exactly  in  focus, 
because  the  fine  mesh  of  the 
mantle  does  not  then  give  a 
continuous  surface.  This  light 
is  sufficiently  powerful  for  high- 
power  dark- ground  illumination 
if  dark  colour  screens  are  not 
used. 

For  those  who  have  not  gas 
or  electric  light,  but  who  require 
a  more  powerful  light  than  a 
paraffin  lamp,  an  extremely  use- 
ful lamp,  which  is  quite  simple 
to  use  and  gives  excellent  re- 
sults, consists  of  an  incandescent 
mantle  heated  by  a  methylated 
spirit  flame.  The  reservoir 
having  been  filled  with  spirit^ 
the  method  of  lighting  the  lamp 
is  as  follows.  The  cap  of  the 
reservoir  must  be  screwed  off,  and  the  bellows  attached  by  screwing 
in  the  nipple  at  the  end  of  the  tube.     The  bellows  must  be 


j^ 


Fig.  41.— No.  3337. 


Fig.  42.— No.  3338. 


squeezed   till  the   burner   is   hot.     The   U-shaped   metal   piece 
covered  with  asbestos  should  now  be  soaked  in  spirit  and  placed 


ILLUMINATING   APPARATUS  49 

on  the  supporting  tube  below  the  burner,  as  shown  in  Fig.  42, 
and  ignited.  This  will  heat  the  burner  which  is  inside  the 
hanging  incandescent  mantle.  When  the  asbestos-covered 
U-piece  has  almost  burnt  out,  the  bellows  should  be  gently 
squeezed  two  or  three  times,  which  will  drive  the  spirit  from  the 
reservoir  to  the  burner,  where  it  will  become  volatilised  and 
burn  with  a  steady  flame.  The  bellows  may  be  gently  squeezed 
every  five  minutes  if  the  light  appears  to  be  failing.  The  handle 
below  the  burner  regulates  the  air  supply,  and  should  be  adjusted 
till  the  best  illumination  is  obtained. 

The   electric    arc   lamp   is   useful    for  photomicrography  or 
projection,  but  is  troublesome  for  general  use. 


CHAPTER  III 

APPARATUS  FOR  HOLDING  SPECIMENS  FOR 

EXAMINATION 


stage  clips. 


Vertical 
position  of 
microscope. 


Sliding 
ledge. 


Fig.  43.— stage  Clips. 


All  microscopes  are  provided  with  some  means  of  attaching 
slips  of  glass  or  similar  appliances  to  the  stage,  and  with  a  means 
of  moving  them  about  in  order  to  bring  difierent  portions  of  the 
slide  into  the  optic  axis  of  the  instrument. 

If  the  microscope  is  placed  in  a  vertical  position,  the  stage 

then  forms  a  horizontal  table,  and  the  slide  or  slip  may  be  allowed 

simply  to  rest  on  the  surface,  but  it  is  difi&cult  to  move  it  about 

with  a  regular  and  even  motion  unless  it  is  held  in  some  way. 

The  simplest  holding  device  is  a  pair  of  springs  called  "  stage 

cHps"  (Fig.  43),  which  fit  into  two 
holes  in  the  stage  and  press  the  slide 
down  upon  its  surface.  They  give 
sufficient  friction  to  enable  the  slide 
to  be  pushed  with  the  fingers  with  an 
even  and  steady  movement. 
The  microscope  should,  however,  not  be  used  with  its  body  in 
a  vertical  position  unless  it  is  necessary.  It  causes  the  observer 
to  bend  down  in  an  unnatural  manner,  which  is  fatiguing,  and  is 
said  to  interfere  with  the  proper  circulation  of  the  blood,  and  it 
allows  the  fluid  on  the  surface  of  the  eye  to  collect  in  the  line 
of  sight,  interfering  with  perfect  vision.  If  the  microscope  is 
inclined  to  a  suitable  angle,  so  that  the  observer  can  use  it 
comfortably,  most  objects  can  be  as  readily  examined  as  is  the 
case  when  the  instrument  is  in  a  vertical  position.  Even  those 
objects  which  are  mounted  in  fluid  can  be  used  in  this  manner, 
as  they  are  almost  invariably  enclosed  between  a  glass  slip  and 
a  cover  glass.  When  used  in  this  position  stage  clips  or  some  other 
holding  device  are  essenital. 

A  sliding  ledge  (Fig.  44),  which  fits  on  to  the  edge  of  a  square 
stage  and  can  be  slid  up  and  down,  is  the  most  convenient  simple 
apparatus  for  holding  a  3  X  1  slip,  as  a  very  even  motion  vertically 
can  be  obtained  by  pushing  the  ledge  up  and  down,  and  laterally 
by  pushing  the  slip  to  and  fro  along  the  edge  of  the  ledge.  There 
are  two  springs  on  the  ledge  which  press  the  slip  on  to  the  stage 
which  can,  however,  be  turned  aside  if  not  required.  A  slide 
can  be  searched  in  this  way,  as  the  object  can  be  raised  by  an 

50 


APPARATUS  FOR  HOLDING  SPECIMENS 


51 


^  te  & 


Mechanical 
stage. 


Fig.  44.— No.  3307,  Sliding  Ledge. 


amount  equal  to  the  field  of  view  of  the  microscope,  and  the 
specimens  pushed  all  the  way  along.  It  can  then  be  raised  a 
similar  amount  and 
pushed  back,  and 
so  on  till  the 
whole  area  has 
been  searched.  It 
is  not  so  conveni- 
ent as  a  mechani- 
cal  stage ,  but 
makes  an  inex- 
pensive  substitute. 

A  mechanical 
stage  is  an  appa- 
ratus which  holds 
a  slide  or  object- 
holder,  and  bv 
me  ans  of  two 
racks  actuated  by 
milled  heads  moves 

it  in  a  dehcate  manner  in  either  direction.  One  milled  head 
travels  the  object  laterally,  the  other  longitudinally.  This 
appliance  is  almost  essential  for  the  delicate  movement  of  the 
object  when  exacting  work  is  being  performed,  and  it  has 
other  important  uses.  It  enables  the  whole  of  a  specimen  to 
be  systematically  examined  over  its  entire  surface  step  by 
step,  in  a  manner  that  is  impossible  by  hand.  It  is  provided 
with  scales  and  verniers,  so  that  any  position  in  a  specimen  can 
be  recorded.  The  readings  of  the  scale  may  be  written  upon  a 
label  on  the  slide,  and  the  specimen  found  at  any  future  time  by 
setting  the  stage  to  the  same  reading  (see  page  72). 

Fig.  94,  page  99,  shows  a  form  of  mechanical  stage  that  is 
very  popular.  It  can  be  attached  to  a  microscope  and  removed 
at  will,  and  it  does  not  interfere  with  the  adjustments  of  the 
substage  apparatus  or  alter  the  level  of  the  stage.  It  consists 
of  a  frame  which  holds  the  ends  of  a  3  x  1-inch  sHp,  and  moves 
it  on  the  flat  stage  of  the  microscope,  with  which  the  slip  is  always 
in  contact.  A  spring  presses  the  slip  down  on  to  the  stage  and 
may  be  turned  aside  when  not  required.  It  has  a  lateral  travel 
of  2 J  inches  (65  mm.)  and  a  vertical  travel  of  1  inch  (25  mm.) 

Fig.  45  shows  a  concentric  rotating  stage,  with  a  mechanical  Rotating 
stage  built  into  its  surface.  In  this  case  the  slide  moves  longitudin-  ^gg^"'*^^^ 
ally  along  the  base-plate  of  the  mechanical  stage,  but  the  whole 
base-plate  moves  laterally  to  and  fro.  The  mechanical  portion  of 
the  stage  can  be  racked  completely  off  the  circular  stage,  leaving 
a  plain  stage- plate  for  the  examination  of  large  objects  ;  but  in 
this  case  a  small  readjustment  of  substage  apparatus  is  required 
to  compensate  for  the  thickness  of  the  travelling  base-plate  which 


52 


THE  MICROSCOPE 


has  been  removed.     This  form  has  adjustable  slide- holders,  so 
that  slides  of  any  length  between  2  and  4  inches  can  be  held. 


Fig.  45. — No.  3306,  Rotating  Mechanical  Stage. 


Centring 
rotating 
stage. 


Glass  slip. 


2J  inches 


The  ordinary  mechanical  stage  is  made  to  take  only  the  3-inch 
standard   length   slide.      It   has   a  lateral   travel  of 
(55  mm.)  and  a  vertical  travel  of  1  inch  (25  mm.). 

Fig.  98,  page  105,  shows  another  form  of  mechanical  stage,  in 
which  the  actuating  milled  heads  are  at  the  side  instead  of 
being  vertically  over  the  stage.  It  can  also  be  removed  from  the 
large  square  stage.  It  has  a  lateral  travel  of  3  inches  (75  mm.) 
and  a  vertical  travel  of  \\  inches  (30  mm.). 

Fig.  97,  page  103,  shows  a  plain  rotating  stage  with  two  stage 
clips  and  two  centring  screws.  The  centring  screws  are  primarily 
intended  for  moving  the  axis  of  rotation  so  as  to  adjust  it  to  the 
exact  optic  axis,  but  they  may  also  be  used  as  a  means  of  adjusting 
the  position  of  the  object  to  a  small  extent  in  both  directions. 
This  stage  has  only  a  travel  of  about  1/6  inch  in  either  direction, 
and  cannot  be  used  for  searching  a  specimen  or  for  registering 
positions  on  the  slide.  It  forms  a  means  of  finally  adjusting  a 
specimen  that  has  been  roughly  adjusted  by  the  fingers. 

A  revolving  stage  is  necessary  for  petrological  work.  It  is 
very  useful  in  observing  opaque  objects  illuminated  with  oblique 
light,  as  the  behaviour  of  the  shadows,  where  the  stage  is  rotated, 
assists  in  the  interpretation  of  the 
structure.  It  is  of  great  service  in 
adjusting  an  object  into  the  correct 
position  for  drawing,  measuring,  or 
photomicrography. 

Holding  an  object  for  examina- 
tion under  the  microscope  calls  for  various  appliances,  according 
to  the  nature  of  the  object.     The  most  universal  method  consists 


Fig.  46.— No.  3400,  Glass  Slip. 


APPAKATUS  FOR  HOLDING  SPECIMENS 


53 


of  placing  the  object  between  a  glass  slip  and  a  tbin  cover 
glass.  Such  glass  slips  are  made  3  inches  long  and  1  inch 
wide,  and  it  is  only  for  a  few  special  pur- 
poses that  slips  of  any  other  size  are  used. 
The  thickness  of  such  slips  varies  from  J  to 
1 J  mm.  Most  forms  of  illuminating  apparatus 
can  be  adjusted  to  focus  through  slips  of 
such  thickness,  but  apparatus  which  cannot  be 
focussed  is  constructed  for  slips  of  a  thickness 
of  1  mm.,  which  must  be  specially  selected. 

Cover  glass  is  a  specially  thin  form  of  glass 
prepared  for  use  with  the  microscope.  It  is 
made  in  squares  or  circles  of  5/8  to  7/8  inch 
diameter,  or  can  be  cut  to  any  particular  size 
required.     It  is  made  in  three  thicknesses  : 

No.  1.  Average  thickness  .     "006  in.    '15  mm. 
2.  „  „         .     -008  „    -2     „ 

o.  ,,  ,,  .       *Ui       ,,      'JiO    ,, 


Cover  glass. 


Fig.    47.  —  Thin 
Glass. 


The  thickness  varies  about  20  per  cent,  in  different  individual  Measuring 
pieces,  and  absolute  uniformity  of  thickness  can  only  be  obtained  "''^^^  ^^^^* 
by  selection.     A  screw  micrometer  is  the  most  useful  form  of 
appliance  for  measuring  cover  glasses. 

Cover  glass  can  also  be  measured  by  the  microscope  itself. 
The  fine  adjustment  milled  head  of  a  microscope  is  provided 
with  a  series  of  divisions,  and  the  amount  that  the  body  tube  of 
the  microscope  is  moved  by  the  revolution  of  the  milled  head  for 
one  division  is  given  on  page  96.  A  high-power  dry  object 
glass  should  be  used,  and  the  cover  glass  to  be  measured  placed 
under  the  microscope,  resting  on  a  glass  slip  so  that  one  edge 
of  the  cover  glass  appears  near  the  centre  of  the  field  of 
view.  The  microscope  should  now  be  carefully  focussed  on 
to  specks  of  dust  on  the  upper  surface  of  the  cover  glass 
and  the  position  of  the  fine  adjustment  milled  head  ob- 
served. The  milled  head  provided  with  the  divisions  should 
then  be  turned  till  the  dust  on  the  slip  is  in  focus  and  the 
number  of  divisions  that  the  milled  head  has  moved  to  make 
the  alteration  noted.  This  number  multiplied  by  the  value 
of  one  division  gives  the  thickness  of  the  cover  glass.  It  is 
necessary  to  focus  particles  of  dust  which  are  situated  on 
the  slip  to  one  side  of  the  cover  glass,  and  not  seen  through 
it,  as  the  optical  path  seen  through  glass  is  not  the  same  as  that 
in  air. 

If  it  is  desired  to  ascertain  the  thickness  of  a  cover  glass  of  Measuring 
a  mounted  specimen  where  the  edge  of  the  cover  glass  cannot  ^^^e^jP^sg^ 
be  observed,  the  microscope  may  be  focussed  to  the  dust  on  the  ot  mounted 
surface  of  the  cover  glass  and  then  to  the  object  itself,  but  the^^^^^'" 
result  so  obtained  will  be  too  small,  and  must  have  one-half  as 


54 


THE  MICROSCOPE 


Cleansing 

cover 

''lasses. 


Tliickness 
of  cover 
glass. 


Glass  slip 
with  ledge. 


Blood  films. 


much  added  to  it.    If  the  motion  of  the  adjustment  is  ten  divisions, 
the  true  thickness  is  15. 

It  is  essential  that  cover  glasses  before  use  should  be  thoroughly- 
cleansed,  and  all  specks,  hairs,  and  fibres  be  removed.  In  most 
cases  a  little  soapy  water  will  remove  all  dirt  and  grease,  after 
which  they  should  be  rinsed  in  clean  water  and  dried  with  a  clean 
linen  duster  or  chamois  leather.  Some  microscopists  use  two 
flat  boards  covered  with  chamois  leather,  between  which  the 
cover  glasses  are  rubbed,  reversing  the  glass  during  the  process 
to  make  sure  that  it  does  not  adhere  to  one  board,  thus  cleaning 
only  one  side. 

With  low-power  object  glasses — IJ  inch  (32  mm.),  2/3  inch 
(16  mm.),  1/3  inch  (8  mm.) — the  thickness  of  cover  glass  used  is  of 
little  importance  ;  but  for  high  powers — 1/6  inch  (4  mm.)  or  higher 
power  dry  lenses — it  is  most  important  to  always  use  the  thinnest 
covers  (No.  1),  because  with  high-power  object  glasses  which  are 
not  immersion  lenses  a  variation  in  the  thickness  of  the  cover 
glass  affects  the  correction  of  the  object  glass.  An  object 
glass  can  only  give  the  most  perfect  image  when  used  with  a  cover 
glass  of  a  particular  thickness,  and  they  are  always  adjusted  for 
the  No.  1  cover  glass  (see  page  81).  The  1/6-inch  (4-mm.)  object 
glass  is  very  sensitive  in  this  respect,  and  one  apochromatic 
lens  of  this  power  is  provided  with  a  correction  collar  to  adjust 
for  cover  glass  of  different  thicknesses.  As  microscopic  cover 
glass  is  sold  by  weight,  the  cost  of  the  No.  1  glass  is  not  materially 
more  than  the  No.  2  or  3,  because  a  larger  number  go  to  the 
ounce. 

If  a  specimen  in  the  nature  of  a  leaf,  a  fibre,  or  powder,  is  to 
be  examined  under  a  high  power,  it  is  best  to  place  such  a  specimen 
on  a  glass  sUp  and  place  a  cover  glass  over  it  to  flatten  it  out  and 
hold  it  in  position,  preferably  in  a  drop  of  water. 

In  this  case  a  slip  with  a  ledge  against  which  the  cover 
glass  may  rest  is  a  convenience. 

If  the  specimen  is  to  be  ex- 
amined  in   fluid,  a   drop   should 
be  placed  on  the  slip  and  a  cover 
glass   put   down   over    it    at    an 
angle  in  such  a  manner  that  the 
cover   glass   touches   one   side  of 
the  drop  first,  and  is  then  allowed 
to  gradually  fall  so   as 
to   prevent  air  bubbles 
being  enclosed  (Fig.  49). 
Blood  films,  or  speci- 
mens of  bacteria  which 
are     to    be    examined 
and  then  destroyed,  may  be   dried  by  heating   over    a   spirit 
lamp  upon  the  slip  or  cover  glass.     If  they  are  to  be  examined 


Fig.  48.— No.  3406,  Slip  with 
Ledge. 


Fig.  49. 


APPARATUS  FOR  HOLDING  SPECIMENS 


55 


witli  a  dry  object  glass,  they  should  be  dried  upon  the  cover 
glass  and  placed  film  downwards  upon  the  slip.  If  an  immersion 
object  glass  is  used  they  may  be  dried  on  to  the  slip  and  the 
use  of  a  cover  glass  dispensed  with,  for  the  whole  space  between 
the  object  and  the  lens  is  filled  with  what  corresponds  to  glass. 
The  thickness  of  the  cover  glass,  therefore,  makes  no  difference 
optically,  but  unless  the  object  is  thoroughly  dried  a  cover  glass 
may  be  required  to  prevent  the  object  from  floating  off  into  the 
immersion  fluid.  By  putting  a  drop  of  Canada  balsam  between 
the  cover  glass  and  the  slip,  and  firmly  pressing  them  together,  a 
permanent  mount  may  be  prepared. 

When  objects  in  a  drop  of  fresh  or  salt  water  are  placed  between  Examination 
a  cover  glass  and  a  slip,  the  superfluous  fluid  around  the  cover  fluj^^^*^**  ^ 
glass  should  be  removed  with  blotting  or  filter  paper,  and  capillary 
attraction  wiU  hold  the  cover  glass  in  position  when  the  slide  is 
placed  at  an  angle. 

If  a  specimen  is  to  be  examined  for  a  long  period,  a  piece  of 
cotton  may  be  placed  between  the  cover  glass  and  the  slip,  one 
end  of  which  dips  into  a  bottle  or  capsule  of  water  at  a  higher  level 
than  the  slip,  and  the  other  in  a  similar  bottle  at  a  lower  level. 
By  this  means  the  slide  will  be  kept  moist  and  objects  can  be  kept 
alive  for  a  considerable  period. 

Small  organisms,  such  as  infusoria,  bacteria,  or  protozoa,  Slip  with 
have  sufficient  room  in  the  thin  layer  of  water  between  the  cover 
glass  and  the  slip  to  live  and  move 
freely,  but  larger  objects,  such  as 
rotifers,  entomostraca,  etc.,  require 
more  room.  For  use  with  such 
objects,  slips  are  made  with  cavi- 
ties, and  are  known  as  slips  with 
hollows.      They    are    used    in    the 


Fig.  50.— No.  3405. 


Fig.  51.— Cells. 


same  way  as  ordinary  slips,  the  water  which 
fifls  the  cavity  holding  the  cover  glass  in 
position  by  capillary  attraction. 

CeUs  or  rings  of  vulcanite  metal  or  glass  Ceiis. 
may  be  cemented  to  3  X  1-inch  slips  with 
Hollis  glue,  forming  deeper  cavities  for  the 
reception  of  large  specimens  (see  page  58). 
When  such  objects  are  in  fluid,  the  removal 
of  the  superfluous  water  is  sufficient  to  make 

the  cover  glass~adhere  to  the  cell. 

If  insects  are  to  be  examined  dry, 

the  cover  glass  may   be   made   to 

adhere  to  the    ceUs    by  placing    a 

smear  of  grease  or  vaseline  around 

the  upper  edge. 

For  the  examination  of  aquatic  weeds,  algse,  and  animalcula  Trough. 

with  low  powers,  a  trough  is  a  useful  apparatus.    Fig.  53  shows  a 


m 


a. 


Fig. 


52.— Slip  with  Cell 
and  Cover. 


56 


THE  MICROSCOPE 


Adjustable 
trough. 


Live  box. 


Fia.  53.— No.  3413,  Slip  with 


Trough. 


convenient  form  mounted  on  a  3  X  1  slip  ;  it  has  usually  a  space 

for  water  about  2  mm.  tliick.     It  is  made  with  an  upper  glass 

either  of  thin  microscope  cover 
glass,  about  "25  mm.  thick,  or  a 
thicker  glass  about  1  mm.  thick. 

Fig.  54  shows  a  glass  trough 
cemented  together.  This  has 
dimensions  of  IJ  xlj-  xj  inches, 
and  is  made  of  glass  about 
1  mm.  thick.  It  is  only  suitable, 
owing  to  the  thickness  of  the 
glass,  for  use  with  low  powers. 
A  very  useful  form  of  trough,  known  as  Beck's  glass  trough, 

is  made  of  a  3  X  1  glass  plate,  into  which  are  fixed  two  screv/s 

and  milled  nuts,  each  holding  a  clamping  plate.      A  half-circle 

of  indiarubber  made  from  an  elastic  band  is  laid  on  the  3x1 

slip,   and   a  glass  cover  plate  of    any   re- 
quired   thickness   is   placed    on    the   top. 

The   whole  is    clamped   together   by  the 

milled   nuts.     As  all    the    parts    take    to 

pieces,    it   can   be    readily    cleaned,    and 

cover  glasses  or  separating  bands  of  any 

thickness  can  be  used.     Separating  bands 

of   the   very   thinnest    material,   such    as 

dental    rubber,    or    even    paper,    can   be 

used,  so  that  the  layer  of  material  being 

examined  is  exceedingly  thin.     This  is  of 

great  convenience  when  it  is  desirable  to 

examine    the    specimens    by   dark-ground 

illumination  or  with  high  powers.     It  is  a 

very   convenient   appliance    also    for    the 

examination  of  aquatic  specimens.     These 

can  be  first  arranged  in  position  on  the  lower  3  X  1-inch  slip 

within   the    area   surrounded    by    the    rubber   band,  the  cover 

may  then  be  placed  in  position  and  sufiicient  fluid  dropped  in. 

If  a  small  circular  cover  glass  be  cemented  in  the  centre  of  the 

3  X  1-inch  lower  glass,  a  small 
drop  of  fluid  can  be  confined  to 
the  centre  of  the  field  for  ex- 
amination. It  can  be  used  with 
substage  condensers  or  dark- 
ground  illuminators. 

A  live  box  consists  of  a  plate 
3x1  inches,  with  an  aperture 
in  the  centre  of  wliich  is  fixed  a 
short  brass  tube  carrying  at  the 

top  a  glass  plate.     Over  this  tube  sUdes  a  cap,  in  the  top  of  which 

a  cover  glass  is  held  by  a  screwed  cell.     The  object  to  be  examined 


Fig.  54.— No.  3415, 
Trough. 


Fig. 


55 


No.  341G,  Bsck's 
Glass  Trough. 


APPARATUS  FOR  HOLDING  SPECIMENS 


57 


i^l 


1 


VAY///m///.','ZM 


Wy/'jjwj/fjj/f^ 


is  held  between  the  two  glasses.  This  appliance  is  useful  for 
examining  living  insects  or  for  flattening  out  thin,  uneven  objects, 
such  as  a  piece  of  a  leaf  or 
fabric.  It  is  chiefly  used  with  low 
powers,  as  substage  illuminating 
apparatus  cannot  be  readily  used. 
A  form  of  live  box  known  as 
the  Rousselet  live  box  is  useful  for 

high  powers.  The  principle  is  that  fig.  56.— No.  3420,  Live  Box. 
of  an   ordinary  live  box,  but  the 

fixed  lower  glass  plate  is  on  the  level  of  the  stage,  and  a 
substage  condenser  or  high-power  illuminator  can  be  used  with 
this  live  box.  When  a  very  small  object  is  to  be  examined,  a 
still  smaller  cover  glass  may  be  cemented  with  Canada  balsam 
to  the  centre  of  the  lower  glass  plate,  and  the  object  is  thus 
confined  to  the  centre  of  the  field.  It  is  2|  X  If  inches,  and  is 
not  suitable  for  use  on  a  mechanical  stage. 

The  Beck  compressor  is  a  3  X  1-inch  plate  of  glass  at  one  end  Compressor, 
of  which  a  circular  pillar  is  fij?:ed.     This  pillar  carries  an  arm 
which  holds  a  thin   cover  glass  3/4  X  If  inches.     The  arm  is 

raised  or  lowered  by  a  screw 
at  the  top  of  the  pillar,  which 
mechanically  varies  the  space  for 
holding  the  specimen.  The  arm 
carrying  the  thin  glass  can  be 
swung  to  one  side  for  placing  the 
specimen  in  position  and  then 
lowered  to  the  required  amount. 
For  many  purposes  this  compressor  is  more  convenient  than  a 
live  box,  for  by  means  of  the  delicate  screw  motion  a  living  object 
may  be  held  stationary  without  being  crushed.  Also,  the  slip 
being  made  of  glass,  it  can  be  kept  clean,  and  the  thin  glass  which 
is  attached  to  the  arm  by  spring  clips  can  be  readily  removed 
for  cleaning,  or  replaced  if  broken.  It  can  be  used  with  substage 
condensers  and  dark- ground  illuminators. 

A   convenient    method   of   holding   small   solid   objects   for  stage 
observation  under  the  microscope  is  by  means  of  a  pair  of  stage  ^o^^^^p^ 
forceps,  which  are  attached  to  a  3  X  1-inch  ebonite  plate.     The 
plate  is  either  held  by  the  mechanical  stage  or,  if  the  microscope 
is  not  fitted  with  the  latter, 
by  means  of  the  spring  stage 
clips.      On    the    plate    is    a 
metal  fitting  holding  a  rod, 
which  has  at  one  end  a  small 
pair  of  spring  forceps  opened 
by    pressing     the    two    pins 

together,  while  at  the  other  end  is  a  cork  into  which  specimens 
may  be  pinned.     The  forceps  can  be  unscrewed  from  the  rod, 


Fig.  57.— No.  3421,  Beck's 
Compressor. 


3S. 


Fig.  58.— No.  3422,  Stage  Forceps. 


58 


THE  MICEOSCOPE 


Mounting 
specimens. 


Turntable. 


Haema- 
cytometer. 


which  can  then  be  reversed  in  its  fitting  so  that  either  the  forceps 
or  the  cork  can  be  brought  into  the  centre  of  the  field ;  they  can 
also  be  rotated  so  that  all  parts  of  the  object  they  hold  can  be 
examined.  This  apparatus  is  useful  for  the  examination  of  small 
insects,  botanical  specimens,  fragments  of  rock,  tissues,  and  other 
small  solid  bodies. 

A  case  can  be  supplied  containing  apparatus  for  holding 
objects,  which  includes  3  X  1-inch  slips,  a  slip  with  ledge,  a  sUp 
with  hollow,  a  trough  on  slip,  a  Beck  glass  trough,  a  live  box, 
a  Beck  compressor,  stage  forceps,  and  a  supply  of  thin  glass. 
The  Rousselet  live  box  is  not  included,  as  it  is  not  of  the  standard 
3  X  1-inch  size. 

Mounting  permanent  specimens  for  the  microscope  is  a  subject 
that  is  beyond  the  scope  of  this  book.  The  microscopist  should 
be  pro^dded  with  a  bottle  of  Canada  balsam  dissolved  in  benzol 
or  xylol,  which  is  a  transparent  cement,  and  a  bottle  of  HoUis  glue, 
which  is  a  brown  shellac  cement.  Many  objects  can  be  mounted 
by  means  of  these  two  cements.  Small  shells,  botanical  and 
entomological  specimens,  diatoms,  and  other  small  objects  may 
be  attached  to  a  3  X  1-inch  slip  with  gum  or  Canada  balsam  inside 
a  cell  of  paper,  vulcanite,  or  glass  of  a  thickness  sufficient  to 
protect  them,  and  with  a  cover  glass  cemented  to  the  cell.  A 
narrow  ring  of  cement  of  the  diameter  of  the  cover  glass,  dried 
upon  the  slip,  is  often  sufficiently  thick  to  protect  small  objects 
when  a  cover  glass  is  cemented  to  the  surface  of  this  ring. 

A  turntable  (Fig.  59)  is  an  appliance  for  making  rings  of 
cement  on  a  3  X  1-inch  glass  slip  and  for  placing  a  protecting  ring 

of  cement  round  a  circular  cover  glass  or 
cell  after  it  has  been  cemented  on.  The 
slip  is  held  on  to  the  circular  revolving 
table  by  spring  clips,  and  by  holding  a 
camel's-hair  brush,  which  has  been  dipped 
into  cement,  against  the  slip  as  this  table 
spins  round,  a  layer  of  cement  is  left 
in   a  neat  circular  ring. 

Many  objects  can  be  placed  on  a  slip  and  a  drop  of  Canada 
balsam  dropped  upon  them,  a  cover  glass  being  then  placed  over 
the  drop  before  it  has  set.  The  specimen  is  thus  permanently 
preserved.  This  is  all  that  is  required  with  such  specimens  as 
dried  blood  films  or  stained  bacteria. 

It  is  essential,  however,  that  such  specimens  should  not  be 
moist,  as  water  will  not  mix  with  Canada  balsam,  and  some  objects 
require  to  be  first  soaked  in  absolute  alcohol  or  turpentine  to 
remove  the  water  or  air. 

The  hsemacytometer  is  an  appara,tus  for  counting  the  blood 
corpuscles,  and  consists  of  a  counting  chamber,  two  mixing  pipettes, 
and  suitable  optically  plane  cover  glasses.  The  blood  is  first 
diluted  with  a  solution  known  as  "  Toisson's"  solution,  for  either 


Fig.  59.— No.  3386, 
Turntable. 


APPARATUS  FOR  HOLDING  SPECIMENS  59 

the  red  or  white,  or  with  a  solution  of  acetic  acid  when  a  count 
of  white  cells  only  is  being  made.  In  counting  red  corpuscles 
a  dilution  of  1-200  is  generally  used,  but  in  certain  cases  1-100 
may  be  employed.  The  blood  is  drawn  into  the  pipette  up  to  the 
mark  0*5  in  the  case  of  1-200  dilutions,  and  up  to  the  mark  1  for 
1-100  dilutions.     The  pipette  is  then  immediately  placed  in  the 


Fig.  60.— No.  3325a,  Pipettes  for  Red  and  White  Corpuscles. 

diluting  fluid,  which  is  drawn  up  to  the  mark  101  above  the  bulb. 
Both  ends  of  the  pipette  are  then  closed  with  the  fingers,  and  the 
pipette  shaken  to  ensure  an  even  mixing,  the  glass  bead  in  the 
bulb  facilitating  this.  For  white  corpuscles,  a  dilution  of  1-10  is 
employed  and  the  other  pipette  is  used.  For  filling  the  counting 
chamber,  a  drop  of  the  mixture  is  blown  out  of  the  pipette,  after 
allowing  several  drops  to  go  waste,  into  the  centre  of  the  counting 
chamber.  The  cover  glass  is  then  placed  over  the  cell.  The  drop 
of  blood  must  not  be  allowed  to  overflow  the  platform  into  the 
groove  which  surrounds  it,  and  the  cover  glass  must  be  in  perfect 
contact  with  the  object  slide,  and  all  must  be  scrupulously  clean. 

A  B    C 


C 


Fig.  61. 


DeptK 
•lOin.in. 

1 

400  sqm.m. 


THOKA 
HAWKSLEY 


Fig.  62. — No.  3325a,  Thoma-Hawksley  Counting  Chamber. 


The  counting  chamber  consists  of  a  plate  of  glass  with  an 
annular  groove  ground  upon  it.  The  circular  portion  inside  the 
groove  is  ground  and  polished  to  a  distance  of  '1  mm.  below  the 
level  of  the  plate  of  glass. 

In  the  Thoma  haemacytometer  this  portion  is  ruled  with  a 
diamond  into  squares  l/400th  of  a  square  mm.  each  in  area.     It 


Tiirck. 


Thoma. 


Elzholz. 


Centre  Ruling  of  Thoma. 


Biirker. 


Nebauer, 


j^^-^^-. 

h=^H**"=^ 

ir       - 

J 

1 

1                1 

I        1 

5  = — ,  -—=4= 

-    : 

;Eii:-ii: 

T        

1    

1 

Tr F~^^ 

-====^=^. 

Fiichs  and  Rosenthal. 
Fig.  63.— Rulings. 


Breuer. 


60 


APPARATUS  FOR  HOLDING  SPECIMENS  61 

will  therefore  be  seen  that  the  amount  of  liquid  resting  upon 
each  square  has  a  cubic  capacity  of  l/4,000th  of  a  cubic  mm.  The 
hquid  which  has  been  placed  in  the  counting  chamber  is  allowed 
to  settle,  and  the  corpuscles  will  therefore  be  in  contact  with  the 
bottom  of  the  cell.  It  will  be  found  that  it  is  a  simple  matter 
to  count  the  corpuscles  contained  in  each  square.  The  usual 
method  is  to  count,  say,  100  squares,  and  it  must  be  noted  that 
m  dealing  with  those  actually  on  the  lines,  only  those  on  two  sides 
of  the  square  should  be  counted,  and  this  rule  should  be  applied 
throughout. 

The  number  of  corpuscles  in  1  cubic  mm.  "of  undiluted  blood 
is  then  obtained  by  multiplying  together  the  rate  of  dilution,  the 
number  of  corpuscles  counted,  the  volume  of  each  square  (l/4,000th 
of  a  cubic  mm.),  and  dividing  by  the  number  of  squares  counted. 
The  above  is  the  general  method  of  counting  the  red  corpuscles ; 
but  in  the  case  of  the  white  corpuscles,  as  there  are  a  very  much 
smaller  number  of  these,  the  method  generally  employed  is  to 
count  the  total  number  of  the  whole  ri3ed  area  of  the  counting 
chamber,  which  is  1  sq.  mm. 

^^  There  are  other  forms  of  counting  chambers,  such  as  the 
Biirker,  Fiichs -Rosenthal,  Breuer,  and  Zapperts  ;  the  method  of 
employment  in  all  these  is  the  same,  but  the  ruHng  and  also  the 
counting  are  different  in  each  case. 

The  use  of  a  mechanical  stage  greatly  assists  the  counting. 

A  simpler  form  of  haemacytometer  can  be  used  which  depends 
for  its  action  on  Mr.  Rheinberg's  beautiful  process  of  making 
graticules  and  glass  scales.  A  glass 
plate  is  photographed  with  squares 
in  the  pattern  of  a  chess-board,  so 
that  alternate  squares  are  tinted,  al- 
though they  are  transparent.  This 
plate  is  dropped  into  an  eyepiece  be-  Fig.  64. 

tween  the  lenses,  and  by  means  of  a 

stage  micrometer  the  drawtube  can  be  varied  until  a  definite 
number  of  squares  are  equal  to  '1  of  a  millimetre  in  the  mi- 
crometer. The  chess-board  glass  plates  are  supplied  with  squares 
either  1/4,  1/2,  1,  or  2  mm.  in  size.  They  are  made  to  cover  the 
whole  field  of  view,  or  as  a  small  block  of  squares  in  the  centre 
of  the  field.     The  latter  are  to  be  preferred  for  blood  counts. 

The  only  other  requirement  is  a  3  X  1-inch  sHp  with  a  metal 
ring  cemented  to  it  which  is  '1  mm.  thick,  into  which  the  blood 
is  placed  covered  with  an  ordinary  cover  glass.  Suppose  a  1/6-inch 
object  glass  is  being  used,  a  1-mm.  chess-board  plate  dropped  into 
the  eyepiece  can  be  made  by  drawing  out  the  drawtube  to  the 
required  position  according  to  the  eyepiece  and  object  glass 
employed,  of  such  an  apparent  size  that  nine  squares,  three  each 
way,  correspond  to  '1  mm.,  and  the  count  of  nine  squares  will 
give  the  number  in  a  cubic  tenth  of  a  millimetre.     If  a  1/2-nmi. 


62 


THE  MICROSCOPE 


Culture 
plates. 


Warm  stage 


chess-board  plate  be  used,  then  thirty-six  squares,  six  eacb  way, 
correspond  to  a  cubic  millimetre.  The  most  convenient  size  to 
select  will  depend  upon  the  class  of  object  to  be  counted  and  the 
object  glass  that  is  used. 

Due  to  the  alternate  squares  being  tinted,  a  count  can  be  made 
with  much  less  eye-strain  than  with  the  ordinary  haemacytometer, 
and  this  method  is  preferred  by  some  apart  from  the  question  of 
the  cost  of  the  apparatus. 

The  preparation  of  culture  plates  and  the  methods  of  cultiva- 
tion will  be  found  in  text-books  on  bacteriology.  They  are  large 
square  plates  covered  on  one  surface  with  nutrient  gelatine,  upon 
which  isolated  colonies  of  bacteria  are  growing.  They  should 
be  examined  with  a  low-power  IJ-inch  (32-mm.)  object  glass, 
the  mechanical  stage  having  been  removed  from  the  surface  of 
the  stage  for  the  purpose.  The  required  colonies  having  been 
recognised,  a  morsel  of  the  gelatine  can  be  removed  with  a 
platinum  needle,  while  the  colony  is  in  the  field  of  the  microscope, 
and  can  be  smeared  on  a  cover  glass.  A  drop  of  distilled  water 
having  been  added,  it  can  be  spread  out  on  the  cover  glass  and 
examined  in  a  living  state  with  the  high- power  dark- ground 
illuminator  or  dried  in  a  spirit  flame,  stained  and  mounted  on 
a  3  X  1-inch  slip  with  a  drop  of  Canada  balsam. 

A  warm  stage  is  an  apparatus  for  applying  warmth  to  a  speci- 
men  under    continuous    observation.     A   simple   form   consists 

of  an  oblong  copper  plate  3x1  inches, 
from  one  side  of  which  projects  a  long 
narrow  strip  and  which  has  an  aperture 
1/2  inch  diameter  in  the  centre  of  the 
3  X  1-inch  portion.  It  is  placed  on  the 
stage  of  the  microscope  and  held  like  an 
ordinary  3x1  glass  slip  in  such  a  posi- 
tion that  the  long  strip  projects  in  front 
of  the  microscope.  A  spirit  lamp  is  placed 
under  the  far  end  of  the  projecting  strip 
and  adjusted  so  that  its  flame  impinges 
on  the  strip,  or  is  slightly  to  one  side, 
until  the  portion  of  the  copper  plate 
which  is  near  the  1/2-inch  aperture  is  at 
blood  heat.  The  correct  temperature  is 
readily  ascertained  if  a  small  piece  of  a 
mixture  of  cacao  butter  and  wax  is  placed 
on  the  copper  near  the  aperture.  The 
mixture  is  made  in  such  proportions  that 
it  melts  at  blood  heat,  and  when  the  piece  melts  on  the  copper 
the  correct  temperature  has  been  reached. 

The  drop  of  fluid  to  be  examined  is  placed  on  a  large  cover 
glass  and  a  smaller  cover  glass  is  placed  over  it,  and  the  two  laid 
upon   the    copper    plate.     To    prevent   evaporation   the    upper 


Fig.  t)o.— No.    3384, 
Warm  Stage. 


APPARATUS  FOR  HOLDING  SPECIMENS  G3 

cover  glass   should   be  smeared   round   its  edge  with  olive  oil 
or  vaseline. 

A  centrifuge  is  a  small  hand  machine  for  revolving  test  tubes  centrifuge. 
of  fluid  at  a  very  rapid  speed,  so  that  the  heavy  portions  of 
sediment  may  be  rapidly  separated  from  the  fluid.  Two  glass 
test  tubes  encased  in  aluminium  covers  are  revolved  at  a  speed 
of  about  2,500  revolutions  per  minute  by  turning  a  handle. 
The  examination  of  urine  is  greatly  facilitated  by  this  method, 
and  hyaline  cysts  can  be  deposited  without  breaking  them  or 
altering  their  form.  Milk  is  separated  by  the  centrifuge  so  as  to 
give  the  percentage  of  fat,  and  micro-organisms  can  be  readily 
concentrated  to  the  bottom  of  the  test  tube,  from  which  they  may 
be  extracted  with  a  pipette. 

A  simple  microspectroscope  for  the  examination  of  blood  has  Micro- 
been  designed  by  Mr.  Rheinberg;  it  consists  of  the  micrometer  ^p'^''*''"°^^°p^' 
eyepiece,  as  described  on  page  68,  with  a  slit  in  the  position 
where  the  divided  glass  plate  is  generally  placed,  and  a  diffrac- 
tion grating  placed  in  the  eyepiece.  On  looking  through  the  eye- 
piece, the  slit  is  observed,  while  to  one  side  a  spectrum  is  formed. 
If  a  low-power  object,glassbeused,and  the  object  to  be  examined 
placed  on  the  stage  of  the  microscope,  its  spectrum  will  be  seen 
some  little  distance  to  one  side  of  the  slit.  If  a  comparison 
slide  of  a  fluid  be  prepared  close  to  the  edge  of  a  glass  slip, 
it  can  be  placed  on  the  stage  in  contact  with  the  fluid  to  be 
examined  on  the  edge  of  another  slip,  and  the  two  spectra  can 
be  seen  at  the  same  time  one  above  the  other.  If  colour  filters 
are  to  be  examined  they  can  also  be  compared  by  this 
method.  It  is  very  useful  for  the  examination  of  blood,  chloro- 
phyll, dyes,  or  other  colouring  matter. 

The  preparation  of  metaUurgical  specimens  for  examination  Metai- 
under  the  microscope  consists  of  cutting  ofi  a  smaU  piece  of  the  gpeSeas. 
metal  to  be  examined  with  a  hacksaw  and  grinding  a  small 
portion  to  a  flat  surface  and  polishing  it.  It  is  then  etched  with 
such  solution  as  will  remove  certain  constituents  from  the 
surface,  leaving  the  rest  unaffected.  Where  a  fracture  of  steel 
is  to  be  examined,  it  is  sometimes  advantageous  to  cover  it,  before 
grinding  and  polishing,  with  a  coating  of  copper  by  electro- 
plating, as  by  this  means  a  fractured  edge  shows  up  very  clearly 
against  the  different  colour  of  the  copper. 

The  piece  so  polished  is  then  mounted  by  embedding  it  in  a  ^oj"*^^^'^^ 
lump  of  wax  placed  on  the  slide.  The  best  wax  for  this  purpose  ^^th^ax, 
is  one  prepared  in  such  a  manner  that  it  will  hold  its  position 
for  a  long  period  and  yet  remain  plastic  under  pressure.  It  is 
known  as  S.I.R.A.  wax.  The  specimen  should  be  attached  so 
that  its  surface  is  paraUel  to  the  slip  upon  which  it  is  mounted, 
and  this  is  done  most  readily  as  follows : 

Cut  two  square  or  circular  pieces  of  wood  or  vulcanite  from 
the  same  piece  of  material  of  a  thickness  greater  than  the  specimen 


64 


THE  i\IICIlOSCOPE 


Grinding 
and  polishing 
specimens. 


Grinding 
and 

polishing 
machine. 


m 


and  about  1  inch  diameter.     Lay  one  of  these  at  each  end  of  a 
3x1  glass  slip,  and  lay  the  metal  specimen  face  downwards 

on  the  glass  slip  in  the  space  be- 
'      tween   the   two    pieces   of    wood. 
Take   another  3x1    slip  with   a 

lump    of    wax    adhering    to    the 

Y^Q    66  centre,   and,   holding  it  with   the 

wax  downwards,  press  it  down 
upon  the  wooden  plates  until  it  is  in  contact  with  them ;  the  wax 
will  adhere  to  the  metal  specimen  and  cement  it  to  the  upper 
slip.  This  can  now  be  removed  and  turned  over,  and  the  speci- 
men is  ready  for  examination  (see  Fig.  66). 

The  grinding  and  polishing  is  generally  done  on  a  machine 
with  a  horizontal  revolving  disc  with  carborundum  and  emery, 
and  polished  on  the  same  machine  with  rouge  or  diamantine.  The 
following  describes 
a  special  machine 
made  for  the 
purpose  which  is 
driven  with  an 
electrometer  from 
the  ordinary  light- 
ing circuit. 

It  is  com- 
plete and  self- 
contained,  and 
only    requires    to 

be  connected  with  the  electric  current  supply  by  the  usual  fittings 
to  be  ready  for  immediate  use. 

Fig.  67  gives  a  general  view  of  the  machine,  which  consists  of  a 
vertical  spindle  carrying  a  grinding  or  polishing  disc,  driven  by 
a  small  electric  motor. 

The  machine  consists  of  a  vertical  spindle  (A)  carrying  a 
grinding  or  polishing  disc  (B)  driven  by  a  small  electric  motor  (L), 
and  gives  in  a  compact,  convenient  form  all  that  is  required  for 

preparing  metal  specimens 
for  examination. 

The  spindle  (A)  is  made 
of  steel,  and  is  bored  out 
at  the  upper  end  to  re- 
ceive the  disc  upon  which 
the  polishing  or  grinding 
material  is  to  be  placed. 
The  lower  end  is  hard- 
ened to  prevent  undue 
wear.  This  spindle  is 
with  pulleys  of  varying 
of   a  belt  from  the  driving 


Fig.  68.— No.  1292. 


furnished    with 
diameters,  and 


a    speed    cone    (F), 
is  driven  by  means 


APPAEATUS  FOR  HOLDING  SPECIMENS 


65 


,1 


cone  (G),  whicli  in  its  turn  is  driven  from  the  motor.  By  shifting 
the  belt  on  the  speed  cone,  a  range  of  speeds  varying  from 
about  300  to  1,000  revolutions  per  minute  can  be  obtained. 

The  disc  B  is  made  of  brass,  and  fits,  by  means  of  a  tapered 
fitting,  into  the  spindle  A,  which  allows  of  its  easy  removal 
and  at  the  same  time  ensures  accuracy  in  the  running. 

A  lip  (E)  projects  downwards  and  prevents  any  grinding  or 
polishing  material  reaching  the  bearing. 

The  cloth  for  polishing,  or  emery  paper  for  grinding,  is 
secured  to  the  disc  by  a  simple  but  very  effective  device.  A 
groove  (K)  is  made  in  the  edge  of  the  disc,  and  the  paper  or 
cloth  is  stretched  over  the  surface  of  the  disc  and  is  held  in 
position  by  means  of  a  garter  made  of  a  stiff 
brass  spiral  spring,  which  presses  the  material 
into  the  groove.  In  this  way  the  cloth,  or  paper, 
is  held  in  contact  with  the  disc,  no  matter  what 
its  thickness  may  be  (see  Fig.  68). 

In  order  to  collect  the  spent  polishing 
materials,  the  disc  is  surrounded  by  a  catcher 
(C),  which  can  be  easily  removed  for  cleaning. 
In  the  top  of  the  catcher  is  fitted  a  guard 
ring  (D)  which,  being  wide,  forms  a  rest  for 
the  hand,  and  by  being  continued  downwards 
below  the  surface  of  the  disc,  and  nearly  touch- 
ing the  edge,  prevents  any  specimens  that  are 
being  polished  from  falling  into  the  catcher 
should  they  be  let  slip  from  the  fingers. 

This  ring  is  also  used  for  stretching  the  paper 
or  other  material  on  the  disc  in  the  following 
manner : 

The  catcher  (C)  being  removed,  the  paper  or 
material  is  placed  on  the  disc  (B)  and  the  ring 
(D)  pressed  over  the  paper  until  the  ring  (D)  is 
about  half-way  down  the  edge  of  the  disc  (B). 
garter  is  stretched  over  the  edge.  The  ring  (D)  is  now  pressed 
right  down  over  the  disc,  and  the  garter  spring  is  pressed  home 
into  the  groove. 

If  it  is  desired  to  remove  a  piece  of  paper  that  has  been  fitted 
to  the  disc  so  as  not  to  disturb  the  folds  of  the  paper,  the  garter 
spring  should  be  removed  downwards.  The  paper  should  be 
replaced  in  the  manner  described  above.  . 

Should  the  disc  at  any  time  become  so  firmly  nxed  m 
the  spindle  that  it  cannot  be  removed  by  hand,  a  pair  of  liftmg 
levers  are  suppHed,  which  can  be  placed  resting  on  the  edge  of 
the  catcher  with  one  end  under  the  disc  ;  a  steady  pressure  on 
the  other  end  wiU  raise  the  disc  from  its  fitting  in  the  spnidle 

A  cover  is  provided  to  protect  the  revolving  disc  from  dust 
when  it  is  not  in  use. 


Fig.     69.  — 
Pipettes. 

The   spring 


66 


THE  MICROSCOPE 


li 


'^■■■:'i 


The  motor  is  supplied  with  flexible  connecting  wire  and 
plug  adapter,  so  that  it  can  be  connected  with  any  ordinary 
lamp  fitting. 

The  machine  can  be  made  to  suit  any  voltage  specified,  and 

for  direct  or  alternating  current;  in  the  latter  case,  the  phase, 

cycle,  etc.,  must  be  given. 

Pipettes.  Pipettes  (P'ig.  69)  are  small  glass  tubes  of  various  shapes,  and 

are  useful  for  taking  specimens  out  of  fluid  and  transferring  to  the 

slip  or  object-holder 
for  examination.  If 
the  upper  end  of 
the  tube  be  closed 
with  the  finger,  the 
lower  end  can  be 
immersed  in  a  fluid, 
and  the  air  within 
the  tube  prevents 
the  entrance  of  the 
liquid.  On  removal 
of  the  finger  from 
the  upper  end,  the 
fluid  enters  the  glass 
tube,  carrying  with 
it  small  bodies  sus- 
pended in  it;  by 
replacing  the  finger, 
the  fluid  will  be  re- 
tained in  the  tube, 
and  thus  transferred 
to  a  slip,  live  box, 
or  compressor.  Two 
Instruments,  or  three  ucedles,  a  pair  of  fine  forceps,  a  pair  of  scissors,  and  a 
scalpel  are  required  for  the  manipulation  of  unmounted  objects 
before  examination.  For  the  collection  of  aquatic  organisms 
from  either  fresh  or  salt  water,  a  collecting  stick  and  net  are 
of  great  use.  The  net  is  made  of  fine  bolting  cloth,  and  is  of  a 
conical  shape  with  a  glass  bottle  secured  to  its  apex  (Fig.  71).  A 
surface  net  that  is  towed  behind  a  boat  may  be  made  in  a  similar 
manner,  and  should  be  provided  with  a 
smaU  calico  bag  attached  to  its  front  edge 
which  may  be  fiUed  with  stones  to  enable 
it  to  be  towed  along  when  sunk  below  the 
surface  of  the  water. 

Most    of    the    free   swimming    fauna    in 
open  water  are  near  the  surface  during  the 
day,  but  there  is  often  a  great  variation   in   the  fauna  to  be 
found  at  different  levels. 


Fig. 


70. — Dissecting  Instruments. 


Collecting 
net. 


Fig.  71.— No.  3460, 
Collecting  Net. 


CHAPTER  IV 


Fig.  72.  — 
No.  3279, 
Rilled  Eye- 
piece Plate. 


SUNDRY  APPARATUS 

The  drawing  of  specimens  seen  under  the  microscope  by  free-  Drawing 

hand  suffers  from  the  disadvantage  that  it  is  difficult  to  obtain  Jecimena* 

accuracy  in  dimensions   and  relative   proportions.     Microscope 

drawings  are  seldom  required  as  works  of  art,   but  must  be 

accurate.     The  simplest  aid  to  accurate  drawing  is  paper  ruled  ituied 

with  lines  in  squares  used  in  combination  with  a  glass  plate  ruled  ^^i"*""^- 

into  squares  dropped  into  the  eyepiece  of  the  microscope.     If 

the  top  lens  of  the  eyepiece  be  unscrewed,  it  will 

be  seen  that  about  half-way  down  the  tube  there 

is  a  stop ;   a  ruled  plate  (Fig.  72)  can  be  dropped 

upon  this  stop,  when  it  will  be  found  to  be  in  the 

focus  of  the  top  lens.     If  the  lines  are  not  quite 

distinct  when  the  top  lens   is   screwed   home,  the 

latter  may  be  slightly  unscrewed  till  the  lines  come 

sharply  into  focus. 

This  method  of  drawing  is  popular  because  the 
position  of  the  main  outlines  and  salient  features 
of  an  object  can  be  accurately  ascertained,  and  as  much  of 
the  detail  as  is  desired  filled  in  freehand.  A  sketch  showing  the 
points  of  importance,  leaving  out  much  of  the  extraneous  detail, 
is  sometimes  of  more  scientific  value  than  a  photograph,  which 
shows  so  much  detail  that  it  is  difficult  to  pick  out  the  features 
of  special  interest. 

Objects  sketched  in  this  manner  may  be  measured  by  reference  Measuring 
to  a  stage  micrometer.  This  is  a  3  X  1-inch  glass  sHp  with  lines  specunena. 
ruled  on  it  1/10  and  1/100  of  a  millimetre,  or  1/100  and  1/1000 
of  an  inch ;  and  if  it  be  placed  on  the  stage  of  the  microscope 
and  viewed  under  the  same  conditions  as  the  object  that  has  been 
drawn  by  means  of  the  squared  paper,  it  is  easy  to  see  how  many 
1/lOOths  of  a  millimetre  or  1/lOOOths  of  an  inch  are  included  in 
each  square.  This  can  be  noted  on  the  paper,  and  the  dimensions 
of  the  object  may  be  obtained  by  measuring  the  drawing. 

A  glass  plate  4x1  inches,  with  divisions  etched  on  its  lower 
surface,  is  the  most  convenient  scale  for  making  such  measure- 
ments on  the  drawing. 

The  measurement  can  also  be  made  without  making  a  drawing, 
for  once  the  value  of  a  square  in  hundredths  of  a  miUimetre  or 

67 


68 


THE  MICROSCOPE 


Ruled 
micrometer. 


Micrometer 
eyepiece. 


thousandths  of  an  inch  has  been  ascertained  with  a  particular 
object  glass  and  a  particular  tube  length,  the  measurement  can 
be  made  direct  in  the  microscope.      For  this  pur- 
pose ruled  squares  in  the  eyepiece  are  not  always 
convenient.     An  eyepiece  micrometer  is  a  plate  of 
glass  ruled  with  a  finer  series  of  hues  (Fig.  73).    It 
drops  into  the  eyepiece  in  a  similar  manner  to  the 
square  ruling.     An  even  better  method  of  making 
such  measurements  is  by  means  of  the  Beck  mi- 
Eyepiece     crometer  eyepiece. 
Micrometer.  This   consists   of   a   complete   eyepiece  with   a 

magnifying  power  X  8,  and  a  special  vernier  milli- 
metre scale  (A,  Fig.  75)  placed  in  its  focus  which  is  outside  the 
lenses. 

It  is  provided  with  a  coUar  (B)  which  fits  over  the  draw- 
tube  and  can  be  clamped  in  position  by  a  milled  head  (C).  The 
eyepiece  itself  can  be  focussed  up  and  down  by  revolving  it  in 
its  fitting  till  the  scale  A  is  in  exact  focus  for  the  observer's  eye. 

The  scale  (Fig.  76)  is  in  millimetres  with  a  vernier  reading  to 
1/lOth  of  a  mm. 

On  the  left  is  a  vertical  series  of  divisions  divided  in  half- 
millimetres  for  rough  measurement.  For  fine  measurement  the 
object  to  be  measured  is  placed  in  a  horizontal  position,  and  the 
length  is  measured  in  1/lOth  mm.  by  use  of  the  slanting  line  on 
the  right.  The  image  of  an  object  as  shown  in  the  diagram 
measures  3*25  mm.,  because  it  covers  three  large  divisions 
and  extends  to  the  oblique  line  at  a  point  half-way  between  the 
•2  and  '3  of  tenth -millimetre  vernier  divisions.     ' 

To  obtain  the  actual  size  of  the  obiect  itself,  this  result  has 


^ 


Fig.  74.  — No.  3275, 
Micrometer  Eyepiece. 


^S^ 


^ 


21223  - 


^^l 


1— -A 


Fig.  75.  —No.  3275, 
Micrometer  Eyepiece. 


'^:V' 


4     3     2      10 


Fig.  76.— Scale 

of    Micrometer 

Eyepiece. 


merely  to  be  divided  by  the  initial  magnifying  power  of  the  object 
glass.     (See  table  of  magnifying  power  on  page  77.) 

In  cases  where  great  accuracy  is  required,  each  object  glass 
can  be  verified  as  to  its  initial  magnifying  power  by  the  use  of  a 


SUNDRY  APPARATUS  69 

stage  micrometer.  For  this  purpose,  focus  the  scale  of  a  stage 
micrometer  carefully ;  if  1/lOth  of  a  millimetre  now  measures 
2-5  mm.  in  the  scale  with  the  correct  tube  length  of  160  mm. 
and  a  particular  object  glass,  the  magnifying  power  of  that  object 
glass  is  25. 

The  first  image  formed  by  a  microscope  is  produced  by  the  initial 
object  glass  at  a  position  rather  above  the  stop  of  the  eyepiece.  Sfwer^^'"^ 
This  initial  magnification  depends  on  the  focal  length  of  the  object 
glass,  and  also  the  position  of  this  image,  which  is  governed  by 
the  length  of  tube  of  the  microscope.  The  approximate  initial 
magnifying  power  of  each  object  glass  or  the  enlargement  pro- 
duced in  the  first  image  is  engraved  on  each  Beck  object  glass 
for  a  standard  tube  length  of  160  mm.  It  can  only  be  approxi- 
mate, because  different  eyepieces  have  their  stops  in  slightly 
different  positions,  and  therefore  a  small  variation  in  the  theoreti- 
cal tube  lengths  is  caused  by  the  use  of  different  eyepieces. 
The  eyepiece  magnifies  the  first  image  formed  by  the  object  glass 
by  a  fixed  amount,  according  to  the  focal  length  of  the  eyepiece, 
and  does  not  vary,  and  the  total  magnifying  power  at  the  160- 
mm.  tube  length  is  obtained  by  multiplying  the  power  of  the 
object  glass  by  the  power  of  the  eyepiece. 

The  Beck  micrometer  eyepiece  measures  the  size  of  the  first 
image  formed  by  the  object  glass  in  millimetres  and  tenths  of 
a  millimetre.  The  result  obtained  when  the  drawtube  has  been 
set  at  160  mm.  has  only  to  be  divided  by  the  initial  magnifying 
power  of  the  object  glass  to  give  the  actual  size  of  the  object 
being  measured. 

Small  variations  may  occur  in  individual  lenses,  but  they 
are  usually  not  sufficiently  great  to  be  of  consequence  in  ordinary 
work. 

The  camera  lucida  is  an  apparatus  for  making  correct  drawings.  Horizontal 
It  is  made  in  four  models,  suitable  for  three  different  positions  Sda^ 
of  the  microscope.  ' 

The  Beck  horizontal  camera  lucida  (Fig.  77)  requires  the 
tube  of  the  microscope  to  be  in  a  horizontal  position,  and  the 
paper     upon    which  , 

the    drawing    is    to        j^^^^^  ^^^^r  ' 

eyepiece       of       the  ^l^^MLg^y                                   "J" 

microscope.      The  ^^^^^^^^^                                     « 

camera    lucida   is   a  • 

small      half  -  silvered  Fig.  77. — ^No.  3368,  Horizontal  Camera  Lucida. 

prism     held     in     a 

mount  which  fits  on  to  the  drawtube  of  the  microscope  in 
such  a  position  that  one  surface  is  close  to  the  front  lens  of 
the  eyepiece.  The  observer  places  his  eye  immediately  above 
the  prism,  and  the  image  seen  in  the  microscope  is  reflected 


70 


THE  MICROSCOPE 


Vertical 

camera 

lucida. 


Abbe 

camera 
lucida. 


i 

I 


<^- 


(Ar 
( 


Upwards  into  his  eye  by  means  of  a  reflection  in  the  prism  from 
a  half- silvered  surface.  The  eye  also  sees  the  paper  and  pencil 
through  the  half- silvered  surface,  and  can  draw  the  object 
seen  through  the  microscope  accurately  and  rapidly,  because  it 
appears  to  be  superimposed  on  the  paper. 

If  the  eyepiece  of  the  microscope  is  closer  to  the  paper  than 
about  10  inches  (the  near  point  of  vision),  the  pencil  will  not 
appear  sharp  ;  and  to  obviate  the  necessity  of  raising  the  micro- 
scope, a  lens  is  supplied  below  the  prism  which  enables  the  pencil 
and  paper  to  be  clearly  seen  at  a  distance  of  about  6  inches,  which 
is  the  usual  height  of  a  microscope  body.  The  lens  is  also  a 
great  assistance  even  when  the  paper  is  10  inches  away.  It 
fits  into  a  recess  in  the  mount  and  is  held  in  by  a  turn-button. 

The  Beck  horizontal  camera  lucida  is  superior  to  the  old 
WoUaston  form,  as  the  eye  does  not  require  to  be  held  in  an 
exact  position  during  the  process,  and  there  is  no  training  required 
for  its  use.     The^only^care  that  is  required  is  to  see  that  neither 

the  illumination  of 
the  object  nor  the 
paper  is  so  brilliant 
as  to  obscure  the 
one  or  the  other. 
The  relative  illumi- 
nation can  be  easily 
regulated  by  a  neu- 
tral tint  glass  placed 
either  between  the 
prism  and  the  paper 
to  reduce  the  apparent  brightness  of  the  paper,  or  between  the 
microscope  eyepiece  and  the  prism  to  reduce  the  apparent  bright- 
ness of  the  microscope  image,  or  the  illumination  of  the  micro- 
scope may  be  varied  by  any  of  the  means  previously  referred 
to.  A  slot  is  provided  in  the  two  positions  to  receive  the  neutral 
glass. 

The  Beck  vertical  camera  lucida  (Fig.  78)  is  a  prism  which 
acts  in  a  similar  manner  except  that  the  microscope  must  be 
placed  in  a  vertical  or  an  inclined  position.  When  the  microscope 
is  in  a  vertical  position,  the  drawing  paper  must  be  placed  on  a 
slanting  board  at  an  angle  of  30°  in  front  of  the  microscope. 
In  other  respects  the  manipulation  is  the  same.  When  the 
instrument  is  used  in  an  inclined  position,  the  tube  of  the  micro- 
scope must  be  set  at  an  angle  of  60°  and  the  paper  may  be  placed 
upon  the  table.  The  same  arrangements  are  made  for  the 
reception  of  the  lens  and  neutral  glass. 

The  Abbe  camera  lucida  (Fig.  79)  consists  of  a  prism  over  the 
eyepiece  and  a  large  mirror  placed  a  few  inches  to  one  side 
in  a  horizontal  direction.  The  prism  has  a  completely  silvered 
surface,  with  a  small  aperture  in  the  centre,  and  is  not  so  easily 


Fig.'  78.- 


-No.   3369,  Beck  Vertical  Camera 
Lucida. 


SUNDRY  APPARATUS 


71 


used    as    any    of    tlie    other    forms.     With   this  apparatus  the 
instrument  is  placed  in  a  vertical  position,  and  the  drawing  paper 


Fig.  79.— No.  3370,  Abbe  Camera  Lucida. 


Fig.  80.- 


-No.  3371,  Modified  Abbe  Camera 
Lucida. 


placed  on  the  table  at  one  side.  The  mirror  must  be  inclined 
at  such  an  angle  that  the  centre  of  the  field  of  view  appears 
below  the  centre  of  the  mirror,  or  a  distortion  in  the  picture  will 
be  caused.  This  generally  limits  the  size  of  the  drawing  to  a  small 
portion  of  the  centre  of  the  field  of  view,  because  of  the  closeness 
of  the  mirror  to  the  side  of  the  microscope.     This  can  be  remedied 

if  the  paper  on  which 
the   drawing   is    to    be 
I  made    be   tilted   up  so 

1  that    the    distortion   is 

S'lf  ^^  corrected,  for  the  image 

I  \  can  then  be  thrown  to 

I  V        a    greater    distance   to 

the  side  of  the   instru- 
ment.    In  order  to  find 
the    correct     angle    at 
which  the  paper  should 
be  tilted  to  avoid  distortion,  the  circular  margin  of  the  field  of 
view  as  seen  upon  the  paper  may  be  measured  in  two  directions, 
sideways  and  fore  and  aft,  and  the  angle  of  the  paper  altered 
till  the  two  measurements  are  the  same.     This  method  can  also 
be  adopted  with  the    Beck   vertical   camera   lucida,  when  it  is 
required  to  set  the  inclination  of  the  microscope  or  drawing-board 
to  the  correct  angle  experimentally. 
A  camera  lucida  (Fig.  80)  of  the 
Abbe    type   is   made  in   which   the 
bulky  mirror  is  replaced  by  a  small 
tilting   prism  attached   close  to  the 
eyepiece,     and    the    prism    is    half- 
silvered.     In  this  case  the  drawing- 
board  must  always  be  placed  at  an     Fig. 
angle  which  can  be  ascertained  as  ex- 
plained above.     A  lens  and  neutral 
tint  glass  can  be  used  in  the  same  manner  as  previously  described.  Drawing 

A  table  (Fig.  81)  which  can  be  set  at  any  desired  angle  is  ^^^®' 
supplied  which  is  a  convenience  where  the  drawing  paper  requires 


Modified 
Abbe 
camera 
lucida. 


81.— No.  3375,  Draw- 
ing Table. 


72 


THE  MICROSCOPE 


Finders. 


Vernier. 


Polarising 
apparatus. 


•o 


to  be  at  an  inclination.  It  is  marked  for  the  correct  position 
for  the  use  of  the  Beck  vertical  camera,  or  may  be  set  at  any 
other  position. 

A  large  amount  of  time  is  saved  in  examining  specimens  if 
the  position  of  a  particular  object  or  of  a  portion  of  a  slide  can 
be  recorded  for  future  reference.  For  this  reason  mechanical 
stages  are  provided  with  divided  scales  and  verniers.  The 
readings  of  these  scales  are  taken  when  the  desired  object  is  in 
the  centre  of  the  field.  These  readings  can  be  written  on  the 
label  on  the  slide,  and  the  object  in  question  can  always  be 
found  again  by  setting  the  stage  so  that  the  scales  read  these 
numbers. 

For  those  who  are  not  familiar  with  the  use  of  a  vernier,  the 
following  description  may  be  useful.  The  scales  of  the  mechanical 
stages  are  all  divided  in  millimetres  with  a  vernier  which  reads 
to  1/10  of  a  mm.  For  a  rough  reading  the  first  line  with  arrow- 
head on  the  right-hand  scale  (Fig.  82)  may  be  used 
as  an  index,  and  the  distance  which  it  is  beyond  one 
of  the  lines  estimated,  thus  the  reading  of  the  scale 
as  shown  in  the  figure  would  be  about  13 J.  For  a 
more  accurate  reading  the  other  lines  on  the  right- 
hand  scale,  which  form  what  is  known  as  the  vernier, 
should  be  examined.  The  line  with  arrowhead  is 
not  opposite  any  division  on  the  long  scale,  but  it 
will  be  found  that  one  of  the  lines  on  this  scale 
is  opposite  a  division — in  the  case  illustrated  it  is 
the  fourth  line — this  shows  that  the  true  reading 
is  not  13 J,  but  is  134.  If  it  had  been  the  eighth 
line  that  was  opposite  a  division,  it  would  have 
been  13-8,  and  so  on. 

Polarising  apparatus  consists  of  a  polarising  (Nicol)  prism  in 
a  revolving  fitting  which  pushes  into  the  substage  of  the  micro- 
scope, a  plate  of  selenite  in  a  detachable  tube  sliding  over  the 
polarising  prism,  and  an  analysing  (Nicol)  prism  in  a  revolving 
mount  which  screws  into  the  nosepiece  of  the  microscope  between 
the  body  of  the  microscope  and  the  object  glass.  An  analysing 
prism  in  a  special  eyepiece  may  be  used  instead  of  the  analyser 
over  the  object  glass  if  preferred.  A  polarising  apparatus  is 
essential  for  the  study  of  rocks,  and  is  always  supplied  in  petrologi- 
cal  microscopes  ;  but  it  is  used  on  an  ordinary  microscope  for  the 
study  of  crystals,  starch,  and  many  organic  substances.  A  starch 
granule  can  always  be  recognised  by  its  means,  as  it  shows  under 
polarised  light  a  black  or  coloured  cross,  due  to  the  crystalline 
refraction  of  the  material.  Sugar  and  other  crystals  display 
brilliant  colours,  and  such  materials  as  horses'  hoofs,  wax,  or 
finger-nails,  show  the  structure  in  a  manner  that  is  not  other- 
wise seen.  An  explanation  of  the  reason  for  the  appear- 
ances  obtained  with  polarised   light  involve  a  full    discussion 


Fia.  82.— 
Vernier. 


SUNDRY  APPARATUS 


73 


Eyepiece 
with  cross 
lines. 


of  the  theory  of  light,  which  is  not  within  the  scope  of  this 
book. 

An  eyepiece  with  a  movable  pointer  or  indicator  (Fig.  83)  is  Demon- 
a  useful  aid  to  teaching.  It  consists  of  an  eyepiece  magnifying  f  "g**!?^ 
X  10,   which   has  a  fine   movable  index  eyepece 

which  can  be  made  to  point  to  any  portion 
of  an  object  under  consideration,  or  can 
be  turned  out  of  the  field  when  not  re- 
quired. It  is  invaluable  for  demonstra- 
ting cell-structure,  crystals,  etc. 

An  eyepiece  with  a  pair  of  cross  lines 
(Fig.  84)  is  necessary  for  petrology  where 
angles  are  to  be  measured  by  means  of  a 
rotating  stage,  and  is  useful  for  other 
purposes. 

An    eyeshade    (Fig.    85)    which    clips    Fig.  83.— No.  3263,  Eye- Eyeshade. 
on    to  the  drawtube   of   the   microscope      P^®^®  ^^^^  Indicator, 
obscures  the  unemployed  eye  and  saves 

much  inconvenience  and  eye-strain  with  a  monocular  microscope. 
It  enables  the  observer  to  keep  both  eyes  open  without  his 
attention  being  diverted. 

An  erecting  eyepiece  is  a  very  low-power  eyepiece  which  Erecting 
does  not  invert  the  image.  It  drops  into  the  tube  ^^^p^^^®* 
of  the  microscope  in  the  ordinary  way,  and  is 
made  for  use  with  a  2/3-inch  object  glass.  It 
gives  a  magnifying  power  from  10  to  40  diameters 
by  extending  the  drawtube.  It  gives  a  very  large 
field  of  view  and  an  erect  image,  so  that  it  at 
once  converts  an  ordinary  microscope  into  a  thor- 
oughly efficient  dissecting  microscope ;  and  a  slight 
alteration  in  the  length  of  tube  gives  great  varia- 
tion in  magnifying  power. 

To    take   photographs    through    the   microscope   which    are  Photo- 
entirely    satisfactory   for    most   purposes   is   simple,    and    does  °^"°°'"^p^^ 
not  require  much  apparatus  or  special  appliances. 

The  microscope  is  first 
arranged  to  give  the  best 
visual  image,  the  particulars 
as  regards  illumination  given 
in  the  earlier  part  of  the  book 
having  been  carefully  followed. 
The  ordinary  eyepiece  is  re- 
placed by  the  30-mm.  focus 
compensating  eyepiece,  and  the 

photomicrographic  camera  is  attached  to  the  tube  of  the  micro- 
scope. The  image  must  now  be  carefully  re-focussed  upon  the 
ground  glass  and  the  plate-holder  inserted.  The  light  is  cut 
off  from  the  microscope  by  placing  a  card  between  the  light  and 


Fig.  84.— 
No.  3264, 
Cross  Lines 
of  Eye- 
piece. 


Fig.  85.— No.  3257,  Eyeshade. 


74 


THE  MICROSCOPE 


Vertical 
photo- 
micro  - 
graphic 
camera. 


the  stage  of  the  instrument,  and  the  slide  of  the  plate-holder 
is  drawn.  The  exposure  may  now  be  made  by  withdrawing  the 
card,  replacing  it  and  closing  the  plate-holder. 

The  use  of  colour  screens  (see  page  42)  is  of  great  service  in 
photography  to  increase  contrasts,  but  the  student  is  referred 
to  books  on  this  subject  for  detailed  information  as  to  photo- 
graphing difficult  objects. 

At  the   same  time,   the   photography  of  most   microscopic 

objects  is  so  simple  that 
the  ordinary  observer 
need  not  be  deterred  by 
the  complexity  of  the 
instruction  given  for  the 
most  advanced  work. 

There  are  two  general 
forms  of  photomicro- 
graphic  cameras.  One  is 
vertical  and  is  used  with 
the  microscope  in  a  ver- 
tical position.  The  other 
requires  the  microscope 
to  be  placed  in  a  hori- 
zontal position,  and  con- 
sists of  a  metal  bar 
on  raising  and  lowering 
screws  which  carries  a 
camera  with  a  variable 
extension  adjusted  by 
means  of  bellows. 

The  vertical  camera 
consists  of  a  frame  stand- 
ing on  three  strong  legs 
splayed  out  to  give  sta- 
bility. It  has  a  slide 
Fig.  86. — No.  3342,  Vertical  Photomicro-  on  its  upper  surface  into 
graphic  Camera.  ^j^j^h    either    a     ground 

glass  screen  or  a  double 
plate-holder  is  inserted.  Below  this  frame  is  a  flexible  bag 
which  fits  over  the  upper  end  of  the  drawtube  of  the  microscope 
and  can  be  attached  by  a  cord.  The  size  of  plate  used  is  4^  X  3J 
inches  (quarter-plate),  and  the  distance  from  the  upper  end  of 
the  standard  microscope  is  such  that,  with  the  30- mm.  compen- 
sating eyepiece,  it  gives  a  circular  picture  of  about  3  inches.  It 
is  rigid,  and  light,  and  extremely  convenient.  When  the  micro- 
scope is  adjusted  with  all  the  care  required  to  obtain  the  best 
image  the  camera  is  placed  over  it,  attached  to  the  tube  by  means 
of  the  bag,  and  a  touch  of  the  fine  adjustment  is  all  that  is  neces- 
sary.    In  order  to  focus  the  image  on  the  ground  glass  accurately, 


SUNDRY  APPARATUS 


75 


Fig.    87. — Focus- 
sing Glass. 


a  focussing  glass  should  be  used  ;    this  consists  of  a  high-power  Focussing 
lens  niounted  in  an  adjustable  tube,  which  can  be  set  so  that^^^^* 
when  it  is  stood    upon  the  ground  glass    the  latter    is  sharply 
focussed.     A  small  portion  of  the  ground  glass  screen  in  the  centre 
is    left   clear    so    that   the    image    can    be 
viewed   with   the    focussing    glass   without 
being  partially  obscured  by  the  ground  glass. 

The  method  of  setting  the  focussing  glass 
is  as  follows :  Loosen  the  top  cell  which 
hold  s  the  lens  combination  by  slightly  un- 
screwing it,  then  screw  the  outer  one  of 
the  tubes  downwards  away  from  the  cell, 
leaving  the  screw  of  the  top  cell  exposed. 
Now  hold  the  inner  tube  and  screw  the  top  cell  backwards  and 
forwards  until  a  pencil  mark  on  the  lower  side  of  the  ground  glass 
is  sharply  defined,  while  the  focussing  glass  is  held  against  the 
upper  side.  Then  screw  up  the  outer  tube,  which  will  form  a 
lock  nut  and  fix  the  top  cell  in  the  correct  position. 

The    horizontal    pattern    of    photomicrographic    camera    is  Horizontal 
illustrated  in  Fig.  88.     When  this  is  used,  the  microscope  must  Stro- 
be placed  with  its  tube  in  a  horizontal  position.     It  enables  a  ^^p^° 

...         •      j_i         •  r     1  .  1  1       •       ^  T  camera. 

variation  in  the  size  oi  the  picture  to  be  obtained  according  to 
the  extension  of  the  bellows.  It  is'  of  unusually  solid  construction. 
It  has  an  extension  of  30  inches  and  takes  a  4|-  X  3J  inches 
(quarter-plate)  size  negative.  It  consists  of  a  heavy  steel 
hexagonal  bar  fix:ed  to  two  steel  cross  bars  which  are  supported 
on  four  levelling  screws.  Along  this  bar  slide  three  frames  with 
connecting  bellows,  each  frame  being  provided  with  a  clamp 
screw.  The  frame  at  one  end  holds  the  ground  glass  or  double 
plate-holder,  the  frame  at  the  other  end  carries  a  flexible  bag  to 
attach  to  the  microscope.  It  can  be  adjusted  up  and  down  so 
that  its  centre  is  at  any  distance  from  the  ground  between 
5|  and  TJ  inches,  and  it  can  be  raised  to  a  higher  level  for  use 


Fig.  88. — No.  3340,  Horizontal  Photomicrographic  Camera. 


with  large  microscopes  by  putting  four  feet  under  the  levelling 
screws. 

There  are  but  few  purposes  for  which  a  larger  photomicroscope 
than  4J-  X  3J  inches  isjequired,  and  for  this  size  30  inches  is 
ample  extension. 


CHAPTER  V 

OBJECT  GLASSES  AND  EYEPIECES 

The  object  glasses  and  eyepieces  are  of  such  paramount  impor- 
tance in  the  performance  of  a  microscope  that  their  use  and 
selection  is  a  matter  which  should  receive  the  careful  consideration 
of  the  microscopist.  Each  object  glass  is  a  complicated  combina- 
tion of  lenses  and  metal  parts.  In  some  as  many  as  ten,  and  in 
none  less  than  four,  lenses,  mounted  in  their  cells  at  specified 
distances  apart,  form  the  complete  whole.  The  adjustment 
and  setting  of  these  demands  the  utmost  skill  and  care  in 
manufacture  ;  an  error  of  1/10000  of  an  inch  may  damage  the 
quality  of  a  high-power  lens. 
Scratches  Scratches  upon  the.  surf  aces  of  the  lens,  or  dust  either  on  or 

on*object  between  the  components,  unless  in  an  aggravated  form,  do  not 
glasses.  interfere  with  its  performance  beyond  stopping  or  scattering  a 
little  light,  but  the  slightest  shifting  of  one  of  the  lenses  or  the 
least  smear  of  grease  or  moisture  will  entirely  upset  the  corrections 
and  ruin  its  performance.  No  glass  surface  should  ever  be  touched 
by  the  fingers,  as  they  always  leave  a  smear  of  grease.  It  is, 
therefore,  of  the  utmost  importance  to  treat  all  object  glasses, 
eyepieces,  and  condensers  with  care,  and  to  keep  them  free  from 
moisture,  dirt,  or  grease.  They  will,  even  with  the  greatest  care, 
collect  dust  from  the  atmosphere  in  time,  but  they  should  always 
be  kept  in  a  dry  place,  especially  when  in  a  moist  climate. 
Dirt  on  Dirt  in  the  eyepieces  shows  in  the  field  of  view,  that  on  the 

eyepieces,  object  glasses  is  uot  clcarly  visible,  but  may  make  the  image 
hazy  and  indistinct.  It  is  quite  readily  detected  in  the  eyepieces, 
as  by  revolving  the  eyepiece  in  the  drawtube  the  specks  due  to 
dirt  in  the  eyepiece  will  revolve.  If  the  specks  are  on  the  object 
or  in  the  observer's  eye,  they  will  remain  stationary. 
Cleaning  To  icmovc  dirt  from  the  eyepiece,  the  surfaces  should  be 

carefully  cleaned  with  a  very  soft  piece  of  well- washed  silk ;  and 
if  after  this  any  still  adheres,  the  silk  should  be  moistened  with  a 
little  xylol  or  alcohol.  For  this  purpose  it  is  quite  safe  for  the 
microscopist  to  unscrew  the  cells,  which  hold  the  lenses,  from 
the  eyepiece  tube,  provided  that  care  is  taken  to  replace  them  at 
the  right  ends.  It  is  best  to  only  unscrew  one  at  a  time.  It  is 
inadvisable  for  the  microscopist  to  attempt  to  clean  the  internal 
lens  surfaces  of  an  object  glass ;  the  interior  surfaces  do  not  readily 

76 


lenses. 


OBJECT  GLASSES  AND  EYEPIECES 


77 


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CO 


•r<       CD 

OH 


78 


THE  MICROSCOPE 


Removing 
oil  from 
immersion 
lenses. 


Keeping 

object 

glasses. 


Corrections 
of  object 
glasses. 


become  dirtv,  and  in  most  cases  the  dirt  will  be  on  the  front 
surface.  This  should  be  cleaned  with  soft  silk  or  very  soft 
chamois  leather,  but  it  must  always  be  remembered  that  dust 
consists  in  many  cases  of  hard  particles,  often  harder  than  glass, 
and  if  these  are  rubbed  upon  the  surface  of  the  lenses  they  will 
leave  fine  scratches.  Hence  the  correct  method  is  to  wipe  very 
gently  and  so  to  remove  the  smaU  particles  and  not  to  grind  them 
on  to  the  surfaces. 

The  lens  which  requires  most  cleaning  is  the  front  of  an  oil 
immersion,  which  is  necessarily  continually  covered  with  cedar- 
wood  oil.  The  oil  should  always  be  removed  when  the  lens 
is  put  away  after  use.  Oil  can  be  removed  with  xylol, 
benzol,  or  spirits  of  wine,  but  care  should  be  taken  not  to 
use  too  much  of  this  liquid,  so  that  there  is  no  danger  of 
its  getting  into  the  interior  of  the  lenses.  A  piece  of  filter 
paper  or  blotting  paper  moistened  with  xylol  or  benzol  and 
lightly  wiped  over  the  front  surface  will  remove  the  oil  with- 
out rubbing. 

It  is  advisable  to  keep  object  glasses  in  the  dust- tight  metal 
boxes  in  which  they  are  supplied  when  they  are  not  in  use.  They 
will  be  safe  except  in  a  very  moist  atmosphere  in  a  dust-tight 
nosepiece  or  in  the  boxes  of  the  Sloan  object  glass  changer.  If 
object  glasses  show  dirt  on  the  interior  surfaces  or  any  other 
defects,  they  should  be  returned  to  the  manufacturers,  who  alone 
can  satisfactorily  put  these  matters  right  and  see  that  the  lenses 
are  in  adjustment.  High-power  object  glasses  can  be  put  out  of 
order  by  the  slightest  error  in  putting  the  component  lenses 
together.  If  a  piece  of  dirt  prevents  one  of  the  cells  from  screwing 
quite  home,  it  is  sufficient  to  destroy  its  performance.  An  object 
glass  on  the  table  when  not  in  use  should  always  be  stood  with 
its  front  lens  upwards  to  prevent  dust  from  accumulating  on  its 
back  surface. 

The  reasons  for  constructing  an  object  glass  out  of  a  number 
of  separate  lenses  in  order  to  correct  its  aberrations  will  be 
discussed  in  a  more  complete  treatise  referred  to  in  the  preface, 
but  one  characteristic  of  a  corrected  lens  should  be  thoroughly 
grasped.  Any  lens  or  combination  of  lenses  can  be  made  to  form 
an  image  of  an  object  at  many  different  positions.  If  a  lens,  such 
as  a  bulFs-eye,  be  put  in  front  of  a  lamp,  it  can  be  moved  to  and 
fro  from  the  lamp  till  a  position  is  found  where  it  will  form  a 
picture  of  the  lamp  on  a  wall  ten  feet  away.  If  a  card  be  now 
interposed  at  a  distance  of  only  two  feet  from  the  lamp,  a  slight 
movement  of  the  lens  away  from  the  lamp  will  form  the  picture 
upon  the  card  instead  of  the  wall.  In  the  same  way,  if  the  length 
of  the  drawtube  of  a  microscope  be  altered,  a  slight  movement  of 
the  object  glass  wiU  bring  the  image  which  it  forms  to  the  correct 
position  for  the  eyepiece  to  render  it  sharply  defined.  Any  two 
positions  where  an  object  and  its  image  are  situated  are  called 


OBJECT  GLASSES  AND  EYEPIECES  79 

a  pair  of  conjugate  foci.  The  important  characteristic  of  an  conjugate 
object  glass  is  that  it  can  only  be  absolutely  correct  for  one  pair  »"^^-<^- 
of  conjugate  foci  which,  as  applied  to  the  use  of  the  microscope, 
means  that  at  one  length  of  drawtube  (160  mm.)  the  image  will 
be  clearer  than  in  any  other  position.  It  is  a  point  that  is 
sometimes  considerably  exaggerated  by  writers  on  the  microscope, 
who  give  the  impression  that  if  the  wrong  length  of  drawtube  is 
used,  the  object  glass  is  almost  as  bad  as  an  uncorrected  lens. 
The  truth  is  that,  especially  with  high-power  lenses  used  with  high 
eyepieces,  for  examining  the  finest  details  it  becomes  a  factor  of 
importance;  but  with  moderate  power  eyepieces  for  general 
observation,  a  considerable  variation  of  the  length  of  tube  makes  Tube  length 
no  noticeable  deterioration  in  the  sharpness  of  the  picture  and  ^^"'^^ction. 
forms  an  exceedingly  useful  means  of  altering  the  magnifying 
power.  Low-power  lenses,  on  account  of  their  smaller  aperture, 
are  much  less  sensitive  to  a  change  in  the  length  of  the  drawtube, 
and  when  using  the  IJ-inch  (40- nam.  and  32-mm.),  2/3-inch 
(16-mm.),  or  even  1/3-inch  (8-min.)  object  glasses  with  eyepieces 
not  higher  in  power  than  X  10,  a  variation  of  30  or  40  mm.  in 
the  drawtube  is  difficult  to  notice.  With  1/6-inch  (4-mm.), 
1/8-inch  (3-nmi.),  1/12-inch  (2-mm.)  object  glasses,  it  is  best 
to  use  the  standard  160- mm.  tube  length,  and  if  a  1/3-inch  (8-mm.) 
is  being  used  with  high  eyepiece,  the  drawtube  should  be  set 
at  its  correct  length. 

The  other  important  factor  in  the  best  performance  of  an  Thickness  of 
object  glass  is  the  thickness  of  the  cover  glass  which  is  between  ^^^"^  ^'*^' 
the   object   and   the   front  lens.     With   high-power  lenses,    the 
thickness  of  this  cover  glass  has  a  far  greater  efiect  on  the  quality 
of  the  image  than  the  length  of  tube. 

If  a  cover  glass  be  used  which  is  incorrect  it  can  be  corrected  correction  of 
to  some  extent  by  making  an  alteration  in  the  length  of  tube,  thidLl^y 
and  the  following  table  gives  the  approximate  amount  of  altera-  drawtube. 
tion  in  the  tube  length  required  to  correct  a  variation  in  the 
thickness  of  the  cover  glass  with  different  powers.  An  oil- 
immersion  lens  is  not  subject  to  this  variation  because  it  has 
nothing  but  glass,  or  its  optical  equivalent,  between  the  object 
and  the  front  lens.  If  the  cover  glass  is  thicker,  the  cedar-wood 
oil  is  proportionately  thinner.  For  this  reason  the  table  only 
refers  to  dry  lenses.  It  will  only  enable  corrections  to  be  made 
for  cover  glasses  which  vary  between  certain  limits.  For  in- 
stance, a  1/6-inch  cannot  be  corrected  for  an  uncovered  object 
by  its  means.  The  low-power  lenses  are  so  insensitive  to  small 
amounts  of  variation  that  the  table  is  chiefly  of  use  in  indicating 
that  the  thickness  of  cover  glass  is  unimportant  except  when 
working  through  thick  troughs  of  water.  It  should  be  remembered 
that  water  has  only  about  two-thirds  the  effect  of  glass,  and, 
therefore,  where  a  trough  is  used,  one-third  may  be  added 
to  the  figures  given. 


Table  of 

drawtube 

corrections. 


80 


THE  MICROSCOPE 


Length  of 

140  mm 

150  mm. 

160  mm. 

180  mm. 

200  mm. 

240  mm. 

Drawtube. 

Corresponding  Cover  Glass  Thicknesses. 

in. 

mm. 

in. 

mm. 

in. 

mm. 

in. 

mm. 

in. 

mm. 

in. 

mm. 

IJ  in.  (32  mm.)i  . 
2/3  in.  (16  mm.)i. 
1/3  in.  (8  mm.)i    . 
1/6  in.   (4  mm.)     . 

•200 
•100 
•018 
•008 

5 
2^5 

•46 

•2 

•140 
•085 
•012 
•0074 

35 

21 
•31 
•19 

•100 
•070 
•007 
•007 

2^5 

1-8 
•17 
•17 

•070 
•050 
•004 
•006 

b8 

1-3 

•1 

•15 

•040 
•030 

•0053 

•1 

•08 

•135 

•004 

•1 

1  The  lower  power  lenses  selected  for  the  purpose  of  compiling  this 
table  are  not  as  usually  supplied,  but  are  specially  corrected  for  longer 
tubes  to  show  the  variation  better. 


Canada  balsam  acts  in  a  similar  manner  to  glass,  and  a  layer 
of  Canada  balsam  between  the  cover  glass  and  the  object  has  the 
effect  of  increasing  the  thickness  of  the  cover  glass.  It  must 
be  allowed  for  unless  the  object  is  mounted  in  contact  with  the 
under-surface  of  the  cover  glass. 
Correction  of  It  will  occur  to  the  reader  that  the  correct  tube  length  can 
tS^nes^by  ^^  ascertained  by  observing  the  image  through  the  microscope. 
obserration.  ^]^jg  jg  very  readily  done  under  dark-ground  illumination,  and 
with  more  difficulty  with  transmitted  light.  Under  dark- 
ground  illumination  there  will  always  be  fine  specks  of  dust 
illuminated  as  brilliant  points ;  one  which  is  so  small  as  to  show 
no  apparent  outline  or  shape  should  be  selected  and  be  placed 
near  the  centre  of  the  field  of  view.  It  should  then  he  focussed 
backwards  and  forwards,  and  the  image  examined  on  each  side 
of  the  sharpest  focus.  If  the  light  is  all  coming  to  an  exact  focus 
at  any  one  point,  the  appearance  on  each  side  of  the  focus  will  be 
the  same  :  it  will  be  a  small  disc  or  patch  of  light  equally  intense 


Fig.  89. 


on  either  side.     If  it  is  out  of  adjustment,  one  side  will  be  clear 
and  the  other  side  hazy.     Fig.  89  shows  that  if   all  the  light 


OBJECT  GLASSES  AND  EYEPIECES  81 

from  the  object  glass  is  not  coming  to  the  same  point,  the  illu- 
mination will  not  be  equally  distributed  at  the  two  positions 
(A  and  B),  It  is  important  to  select  a  very  small  point  for  the 
purpose,  because  many  of  the  objects  seen  through  the  micro- 
scope are  partially  or  completely  transparent,  and  often  globular, 
and  act  as  small  lenses  themselves,  which  interferes  with  the 
phenomenon  unless  they  are  extremely  minute.  With  trans- 
mitted light  on  a  bright  field  the  same  plan  may  be  adopted, 
but  it  requires  much  more  careful  observation  because  the  image 
of  a  fine  speck  of  dust  when  out  of  focus  appears  as  a  faint  patch 
on  a  bright  field,  and  is  not  so  easily  observed  as  a  bright 
patch  on  a  dark  background.  High- power  eyepieces  should 
always  be  used  for  making  these  observations.  By  the  examina- 
tion of  insects'  scales  and  diatoms  much  greater  accuracy  can  be 
obtained  in  this  adjustment,  but  the  explanation  of  their  use 
requires  further  discussion  of  the  theory  of  the  microscope 
and  is  not  attempted  in  this  book. 

All  high- power  dry  object  glasses  are  made  for  use  with  a 
cover  glass  -006  inch  (-15  mm.)  or  -007  inch  (-17  mm.)  thick 
unless  specially  ordered  to  be  made  for  use  without  cover 
glass  for  polished  metal  specimens.  The  l/6th-inch  (4-mm.)  is  correction 
sometimes  made  with  a  correction  collar,  which  is  an  adjustment  ^^^- 
which,  by  altering  the  distance  between  the  component  lenses, 
enables  the  object  glass  to  be  corrected  for  any  thickness  of  cover 
glass  between  0  and  -01  inch  (-25  mm.).  Such  a  lens  in  the  hands 
of  a  beginner  should  always  be  used  with  its  correction  collar 
set  at  about  -007  inch  (-17  mm.)  unless  no  cover  glass  is  being 
used.  It  may  be  a  positive  disadvantage  to  use  such  an  object 
glass  unless  the  microscopist  is  practised  in  the  method  of  making 
the  adjustment. 

The  colour  correction  of  an  object  glass  is  referred  to 
later,  but  it  should  be  remembered  that  an  achromatic  object 
glass  can  sometimes  be  slightly  improved  by  the  use  of  a 
colour  screen,  as  such  lenses  always  give  a  slight  indication 
of  faint  colour.  The  faint  colour  efiects  often  seen  in  trans- 
parent objects  are,  however,  frequently  due  to  the  objects  acting 
as  small  uncorrected  lenses  themselves,  or  to  diffraction  effects. 

The  flatness  of  field  of  a  microscope  depends  on  the  object 
glass  and  the  eyepiece  combined.  The  edge  is  generally  not 
in  focus  at  exactly  the  same  position  as  the  centre  of  the  field. 
Such  a  defect  cannot  be  entirely  cured  in  the  best  lenses, 
because  to  do  so  would  sacrifice  the  finest  definition  in  the  centre  ; 
consequently,  for  the  most  exact  examination  the  object  must 
always  be  brought  near  to  the  centre  of  the  field.  With  low- 
power  lenses  the  defect  is  not  so  apparent,  but  with  high  powers 
there  is  always  a  marked  superiority  in  the  performance  near 

the  centre. 

The  following  table  gives  the  approximate  sizes  of  the  field 


82 


THE  MICROSCOPE 


Sizes  of 
field. 


of  view  with  various  object  glasses  at  the  standard  tube  length, 
and  the  working  distance  of  each  lens. 


standard 
screw  of 
object 
glasses. 


Variety  of 
object 
glasses 
required. 


Eyepieces, 

Working  Dis- 

Object Glasses. 

X  6. 

X  10. 

X  15  Field 
of  View. 

X  17. 

X  25. 

tance,  X  10 
Eyepiece. 

in. 

in. 

in. 

in. 

in. 

in. 

l^in.  (40mm.)  1  . 

•25 

•16 

•115 

[•11 

•1 

2^05 

U  in.  (32  mm.)  . 

•2 

•11 

•09 

•085 

•075 

•88 

2/3  in.  (16  mm.). 
2/3  in.  (16mm.)i 

}-08 

•045 

•035 

•032 

•03    1 

•25 
•11 

2/3  in.  (14  mm.)i 

•07 

•04 

•03 

•028 

•025 

•07 

1/3  in.  (8  mm.)    . 
1/3  in.  (8mm.)i. 

}^045 

•025 

•02 

•019 

•015 1 

•06 
•037 

1/6  in.  (4  mm.)    . 

•02 

•01 

•0085 

•0083 

•007 

•024 

1/8  in.  (3  mm.)    . 

•015 

•008 

•0075 

•0065 

•006 

•016 

1/12  in.  (2  mm.). 

•0085 

•005 

•004 

•0038 

•0035 

•01 

1  Those  marked  ^  are  apochromatic.     The  other  object  glasses  are 
achromatic. 

All  object  glasses  are  now  made  with  a  screw  standardised 
by  the  Royal  Microscopical  Society ;  its  specification  is  as  follows  : 

Thread,  Whitworth  Screw,  36  to  the  inch,  length  ^125  inch. 

Plain  fitting  on  object  glass  above  screw,  •!  inch  long,  ^759 
inch  diameter. 

Diameter  of  thread  of  object  glass  at  top  of  thread,  -7952  to 
•7982  inch. 

Diameter  of  thread  of  object  glass  at  bottom  of  thread,  -7596 
to  -7626  inch. 

Diameter  of  thread  of  nosepiece  at  top  of  thread,  '7644:  to 
•7674  inch. 

Diameter  of  thread  of  nosepiece  at  bottom  of  thread,  -8  to 
•803  inch. 

The  stem  of  all  the  Beck  object  glasses  are  of  a  standard 
diameter,  '65  inch. 

The  series  of  object  glasses  mentioned  in  this  book  is  sufficiently 
large  to  cover  all  the  requirements  of  microscopy,  with  the  possible 
exception  of  a  very  low  power  for  examining  unusually  large 
objects.  The  highest  power  is  a  l/12th  inch.  It  has  sufficient 
magnifying  power  to  show  all  the  detail  that  the  maximum  aperture 
will  resolve;  and  although  object  glasses  of  higher  power  can  be 
made,  they  cannot  be  made  with  this  maximum  aperture  because 
the  lenses  must  be  much  smaller,  and  as  a  fixed  distance  must  be 
allowed  for  a  cover  glass  they  cannot  collect  as  large  an  angle. 

Object  glasses  of  imtermediate  sizes  have  also  been  made 
from  time  to  time,  but  the  magnifying  power  given  by  an  inter- 
mediate size  can  be  so  readily  obtained  by  a  change  in  the  eye- 
piece that  they  are  of  little   advantage.     The  manufacture   of 


OBJECT  GLASSES  AND  EYEPIECES  83 

object  glasses  has,  therefore,  been  limited  to  a  somewhat  more 
restricted  number  of  sizes  than  was  customary  some  years  ago, 
with  considerable  advantage,  as  concentration  on  the  smaller 
number  of  lenses  has  tended  to  improve  their  quality. 

There  are  two  types  of  object  glasses,  achromatic  and  Achromatic 
apochromatic.  Both  types  are  excellent,  and  although  there  gfaS/ 
is  no  doubt  that  the  apochromatic  series  possess  qualities  which 
render  them  of  greater  service  where  the  most  exacting  scientific 
investigation  is  being  carried  out,  for  the  more  general 
work  this  extremely  high  quahty  of  optical  construction  is  not 
required.  Hence  the  achromatic  series  fill  the  requirement  for 
most  purposes,  and  are  in  more  universal  use,  on  account  of  the 
fact  that  they  can  be  made  to  a  simpler  formula  and  with  a  less 
number  of  component  lenses  and  less  expensive  materials.  The 
resolution  of  the  achromatic  lenses  is  of  a  very  high  order.  As 
an  example  of  their  good  performance,  the  diatom  "  Pleuro- 
sigma  Angulatum  "  has  dots  in  its  structure  which  are  approxi- 
mately 1/48000  of  an  inch  apart.  The  theoretical  aperture  which 
will  show  these  as  separate  dots  is  -5  N.A.  This  can  be  done  with 
a  Beck  8-mm.  achromatic  object  glass  which  has  this  aperture 
(•5  N.A.),  showing  that  an  ordinary  achromatic  object  glass,  if 
properly  constructed  and  adjusted,  is  so  perfectly  corrected  for  its 
zonal  and  other  aberrations  that  it  will  resolve  up  to  its  theoreti- 
cal limit.  On  a  Grayson's  ruling  this  lens  will  resolve  45,000 
lines  to  the  inch  with  a  green  screen  and  50,000  with  a  blue  screen. 
For  visual  purposes  with  a  colour  screen,  achromatic  lenses  can 
be  made  almost  optically  perfect,  but  the  apochromatic  series 
described  later  are  more  perfect  as  regards  their  colour  correc- 
tion, are  better  for  photography,  have  somewhat  larger  aperture, 
and  will,  therefore,  stand  the  use  of  higher  eyepieces,  giving  a 
slightly  better  defined  image,  especially  when  a  colour  screen  is 
not  employed. 

The  chief  feature  of  the  apochromatic  series  is  that  different  Apochro- 
glasses  are  employed  and  other  materials  substituted,  and  that  this  passes!  ^^'^ 
combined  with  a  different  formula,  involving  the  use  of  a  large 
number  of  component  lenses,  produces  an  object  glass  in  which 
there  is  more  perfect  correction  for  chromatic  aberration.  In  the 
achromatic  object  glasses  the  correction  is  made  for  two  colours 
of  the  spectrum,  but  in  apochromatic  lenses  this  correction  is 
made  for  three  colours.  For  very  fine  markings  undoubtedly 
apochromatic  object  glasses  give  superior  results ;  the  difference  is 
slight,  but  the  perfection  of  the  colour  correction  enables  certain 
objects  to  be  seen  with  a  greater  crispness  than  is  possible  with 
achromatic  lenses.  Although  the  achromatic  are  suitable  for 
photographic  work,  the  apochromatic  series  has  an  advantage. 
For  those  interested  in  the  optical  construction  of  these  lenses, 
we  append  a  somewhat  technical  note  on  the  theory  of  their 
construction. 


84 


THE  MICROSCOPE 


It  is  a  well-known  fact  that  glass  refracts*  the  various  colours 
by  a  different  amount,  and  consequently  a  single  lens  will  not  give 
an  image  free  from  colour  because  it  has  different  foci  for  different 
colours,  the  focus  for  red  in  a  positive  lens  being  further  from  the 
lens  than  the  focus  for  blue.  This  property  of  refracting  colours 
by  a  different  amount  is  called  the  dispersion  of  the  glass.  From 
the  time  of  Newton  to  that  of  Dolland  it  was  supposed  that  the 
dispersion  of  different  glasses  was  proportionate  to  their  refrac- 
tive powers  (/W.-1),  and  therefore  proportionate  to  their  foci. 
In  other  words,  it  was  thought  that  any  positive  and  negative 
lenses  which  had  the  same  effect  on  the  colour  must  have  the 
same  effect  on  the  focus  whatever  glass  they  were  made  from, 
and  that  combining  two  such  lenses  would  give  the  effect  of  a 
plain  piece  of  glass  without  any  focus.  Converselyj  they  supposed 
that  any  combination  of  lenses  which  had  a  focus  must  have  a 
colour  aberration  equal  to  a  single  lens  of  that  focus. 

In  the  middle  of  the  eighteenth  century  Dolland  discovered 
that  this  was  not  so,  and  that  whilst  the  flint  then  is  use  had  as 
compared  with  the  crown  glass  a  refractive  power  of  60  to  50, 
it  had  a  dispersive  power  of  60  to  36. 

If  we  take  a  crown  glass  of  1-5  refractive  index,  the  difference 
in  focus  between  the  pale  yellow  or  C-rays,  and  the  green  or  F- 
rays,  is  about  1/60  of  the  focus  ;  but  if  we  take  a  flint  glass  of  about 
1'6  refractive  index,  the  difference  between  the  corresponding 
rays  is  about  1/36  of  the  focus.  Therefore,  if  we  take  a  positive 
lens  of  crown  glass  which  is  36  inches  focus,  and  a  negative 
lens  of  flint  glass  which  is  60  inches  focus,  the  colour  will 
be  corrected  for  these  two  rays  when  the  two  lenses  are  put 
together,  and  the  result  will  be  an  achromatic  lens  of  90  inches 
focus. 

Although  in  this  combination  the  two  rays  C  and  F  would 
be  correct,  it  would  not  give  a  perfect  correction  for  the  other 
parts  of  the  spectrum,  because  the  refractive  power  of  the  two 
glasses  is  not  quite  regular  for  the  different  colours. 

For  instance,  the  four  coloured  rays  known  by  the 
spectrum  lines  C,  D,  F,  and  G  do  not  have  proportional  disper- 
sions ;  if  we  caU  the  dispersion  from  C  to  F  1,000,  we  find  that  in 
a  hard  crown  the  distance  from  C  to  D  is  295,  and  from  F  to  G 
568,  whilst  in  a  medium  flint  the  distances  are  respectively  285 
and  608.     This  may  be  expressed  in  the  following  manner. 


-to- 


-D— to- 


-F- 


-to — G 


Hard  crown 


Medium  flint  . 


295 


705 


285 


715 


568 


680 


OBJECT  GLASSES  AND  EYEPIECES 


85 


In  this  figure  the  lines  at  C  and  F  coincide,  but  those  at 
D  and  G  do  not,  and  the  want  of  coincidence  at  D  and  G  gives  an 
idea  of  the  secondary  error. 

If  we  make  a  pair  of  lenses  out  of  these  two  glasses  which 

when  combined  together  give  an  achromatic  lens  of  1  inch  focus, 

and  correct  for  C  and  F,  we  find  that  the  foci  for  the  different 

rays  are : 

C 1-00000 

D -99963 

E 1-00000 

G 1-00165 


In  all  kinds  of  optical  glass  with  high  dispersion  the  relative 
dispersion  from  F  to  G  is  higher  than  those  of  low  dispersion, 
but  in  some  the  difference  is  slight,  and  telescope  lenses  with 
reduced  secondary  spectrum  can  be  made  from  these.  By  com- 
bining three  glasses  together  apochromatic  telescope  object  glasses 
can  be  made,  but  in  these  the  lenses  have  to  be  of  relatively 
short  focus,  and  consequently  only  small  apertures  compared  to  a 
microscope  object  glass  can  be  obtained;  //6  is  a  very  large  aperture 
for  a  telescope  object  glass,  but  only  corresponds  to  '07  N.A. 
in  a  microscope  object  glass.  No  apochromatic  microscope 
object  glasses  have  yet  been  made  satisfactorily  by  this  means. 

The  peculiar  mineral  fluorspar,  however,  has  totally  different 
properties.  It  has  a  low  refractive  index  and  an  extremely  low 
dispersion  about  1/95  on  the  focus.  The  great  peculiarity, 
however,  is  that  it  has  a  larger  proportional  dispersion  from 
F  to  G  than  many  of  the  glasses  with  higher  dispersion ;  the 
corresponding  figure  being  583.  Now  if  we  make  a  similar  diagram 
to  the  last,  but  of  fluorspar  and  light  baryta  flint,  we  get : 


-to 


D 


-to- 


F- 


-to — G 


Fluorspar 


Light  baryta  flint 


296 

704 

583 

296 

704 

570 

A  combination  made  of  these  two  materials  and  achromatic,  of 
1-inch  focus,  gives  the  following  results  for  foci  of  different  colours  : 


C 
D 
F 
G 


1-00000 

1-00000 

1-00000 

-99947 


Now  it  is  quite  evident  that  combining  with  this  combination 
another  combination  such  as  the  first,  in  which  the  aberration  for 
F  to  G  is  in  the  opposite  direction,  it  is  possible  to  produce  a  lens 
in  which  all  the  foci  for  the  four  rays  are  the  same. 


86 


THE  MICKOSCOPE 


In  practice  the  matter  is  more  complicated  than  it  appears, 
because  the  thickness  of  the  lenses  not  only  alters  their  foci 
but  also  slightly  alters  the  ratio  of  their  partial  dispersions,  and 
this  has  to  be  allowed  for  to  get  the  corrections  accurate. 

In  achromatic  object  glasses  it  is  usual  to  correct  for  about 
C  to  F ;  this  means  that  when  correct  visually  there  is  a  very 
slight  error  for  D  ;  but  these  can  be  neglected  for  most  work,  but 
for  photographic  purposes  the  error  in  G  might  be  appreciable 
unless  a  coloured  screen  is  used. 

In  apochromatic  object  glasses  the  colour  is  corrected  for  at 
least  three  parts  of  the  spectrum,  and  also  the  spherical  aberration 
is  much  more  fully  corrected  for  aU  colours  ;  this  means  the  lower 
power  lenses  have  decidedly  larger  apertures  than  the  correspond- 
ing achromatic  lenses  and  that  with  all  apochromatic  object 
glasses  higher  eyepieces  can  be  used.  Also,  when  it  is  important 
to  distinguish  between  objects  with  slight  differences  of  colour, 
these  lenses  are  much  to  be  preferred. 


object  glass. 


2/3-inch 
object  glass. 


1/3-incli 
object  glass, 


Chaeacteristics  of  Object  Glasses  of  Different  Powers 

The  IJ-inch  (40-mm.  or  32-mm.)  object  glass  gives  a  maximum 
field  of  view  -2  inch  (5  mm.)  and  has  a  working  distance  of 
•88  inch  (22  mm.)  in  the  achromatic,  or  2*05  (50  mm.)  in  the 
apochromatic  series.  It  is,  therefore,  specially  useful  for  obtain- 
ing a  general  view  of  large  entomological  and  botanical  specimens. 
It  is  the  only  object  glass  with  which  opaque  vertical  reflectors  of 
the  type  of  the  Sorby  flat  silvered  mirror,  or  the  parallel  flat 
glass  mirror,  can  be  used  between  the  front  of  the  object  glass 
and  the  specimen,  and  is,  therefore,  useful  for  low-power  metallur- 
gical specimens.  Its  aperture  (-16  or  -17  N.A.)  gives  a  theoretical 
resolution  of  15,000  to  18,000  lines  to  the  inch,  according  to  the 
colour  of  the  light  employed. 

The  2/3-inch  (16-mm.  or  14-mm.)  object  glass  gives  a  maximum 
field  of  view  of  '08  inch  (2  mm.).  The  achromatic  16  mm. 
has  a  working  distance  of  '25  inch  (6^  mm.).  The  apochromatic 
16  mm.  and  14  mm.  have  working  distances  of  -11  inch  (3  mm.) 
and  '07  inch  (If  mm.)  respectively.  This  is  the  power  that  is 
the  most  useful  all-round  low-power  lens  for  every  purpose.  It 
forms  a  useful  finder  for  searching  specimens  to  be  examined 
later  with  a  high  power.  It  can  be  used  with  the  parabolic 
reflector  for  opaque  objects,  and  is  probably  used  in  larger  numbers 
than  any  other  lens.  The  aperture  of  the  achromatic  ('28  N.A.) 
gives  a  theoretical  resolution  of  25,000  to  30,000  lines,  and  of  the 
apochromatic  -35  N.A.,  30,000  to  36,000  lines  to  the  inch. 

The  1/3-inch  (8-mm.)  object  glass  gives  a  maximum  field 
of^view  of  -045  inch  (IJ  mm.).  It  has  a  working  distance  of 
about  -06  inch  (1|  mm.).  It  is  a  medium- power  lens  of  the  greatest 
use  for  many  purposes,  and  is  not  sufficiently  appreciated.     For 


OBJECT  GLASSES  AND  EYEPIECES  87 

bacteriological  and  pathological  investigation  it  can  be  used  to 
do  a  great  deal  of  the  work  for  which  two  object  glasses  are 
usually  employed.  With  a  low-power  eyepiece  it  has  a  field  of 
view  large  enough  for  searching,  and  with  a  high- power  eyepiece 
it  can  be  used  for  blood  counts  and  recognising  micro-organisms 
such  as  trypanosomes,  malaria  parasites,  and  bacteria,  and  for 
pond  life  it  is  perhaps  the  most  useful  all-round  power.  For 
metallurgy  it  is  an  excellent  lens  for  photography,  and  with  this 
lens  and  an  oil-immersion  lens  a  microscopist  can  frequently  do 
all  his  work  in  certain  branches  of  research.  This  lens  is  par- 
ticularly useful  for  dark-ground  illumination  with  a  substage 
condenser  and  patch-stops.  The  achromatic  with  an  aperture 
of  5  N.A.  gives  a  theoretical  resolution  of  48,000  to  52,000  lines, 
the  apochromatic  of  '65  N.A.  about  62,000  to  67,000  lines  to 
the  inch. 

The  1/6-inch  (4-mm.)  object  glass  gives  a  maximum  field  of  i/e-inch 
view  of  -02  inch  (1/2  mm.)  and  has  a  working  distance  of  -024  inch  °  ^^^  ^  ^^' 
(1/2  mm.).  It  is  the  universal  high-power,  and  when  only  two  lenses 
are  supplied,  for  cell- structure,  histology,  and  all  general  high- 
power  purposes,  it  is  more  popular  than  any  other  lens,  and  being 
made  in  large  quantities  is  moderate  in  price.  The  achromatic 
has  an  aperture  of  -85  N.A.,  the  apochromatic  -95  N.A.,  and 
theoretical  resolving  powers  of  about  81,000  to  88,000  lines  and 
90,000  to  100,000  lines  to  the  inch  respectively.  The  very 
large  aperture  of  the  apochromatic  lens  of  this  focus  and  its  perfect 
corrections  renders  it  specially  valuable  for  use  with  high  eyepieces 
when  an  oil-immersion  cannot  be  used.  In  this  case  the  1/6-inch 
with  a  correction  collar  should  be  selected,  and  the  microscopist 
should  become  familiar  with  the  correct  adjustment  of  this 
collar  (see  page  80). 

The  1/8-inch  (3-mm.)  oil-immersion  object  glass  has  a  maxi-  i/8-inch 
mum  field  of  view  of  -015  inch  (-STS-mm.)  and  a  working  distance  of  °^J^^*s  ass. 
•016  inch  (•4-mm.).  Being  an  oil-immersion  lens,  it  is  not  affected 
by  the  thickness  of  the  cover  glass  used,  and  is  thus  always 
working  at  its  best.  It  is  introduced  not  because  a  power  between 
a  1/6-inch  (4-mm.)  and  a  1/12-inch  (2-mm.)  is  often  required,  but 
because  a  special  lens  with  a  maximum  aperture  that  can  be 
used  with  dark-ground  illumination  is  urgently  required  for  this 
work.  As  explained  in  the  description  of  the  use  of  the  dark- 
ground  illuminator,  a  large  aperture  oil-immersion  lens  such  as  a 
1/12-inch  (2-mm.)  1^3  N.A.  must  be  stopped  down  to  the  aperture 
of  a  1/6-inch  (4-mm.)  dry  lens  to  enable  it  to  be  used  with  dark- 
ground  illumination,  and  resolving  power  is  thus  lost.  The 
1/8-inch  (3-mm.)  object  glass  can  also  be  used  to  do  most  of  the 
work  with  one  object  glass  that  is  generally  done  with  the  1/6-inch 
(4-mm.)  and  the  1/12-inch  (2-mm.).  It  has  an  aperture  of  -95 
N.A.  and  a  theoretical  resolving  power  of  90,000  to  100,000  lines 
to  the  inch. 


88 


THE  MICROSCOPE 


1/12-inch 
object  glass. 


The  1/12-incli  (2-inin.)  oil-immersion  object  glass  has  a  maxi- 
mum field  of  view  of  -OOSS  inch  (-2  mm.)  and  a  working  distance 
of  -01  inch  (1/4  mm.).  This  is  the  high-power  lens  which  must  be 
used,  if  it  is  necessary  to  see  the  finest  detail  which  can  be  observed 
with  any  microscope.  It  has  an  aperture  of  1-3  N.A.  and  a 
theoretical  resolving  power  of  125,000  to  135,000  lines  to  the 
inch.  It  is  the  object  glass  that  reaches  the  highest  limit  yet 
obtained  in  microscopic  vision,  and  is  a  necessary  portion  of  a 
complete  outfit.  The  apochromatic,  being  slightly  better  than 
the  achromatic,  is  worth  the  extra  cost  even  if  all  other  lenses 
are  of  the  achromatic  series.  If  structure  of  an  object  is  just 
beyond  the  limit  of  vision  of  a  low-power,  a  higher  power  object 
glass  can  be  used ;  but  this  does  not  apply  to  a  1/12-inch,  as  no 
higher  power  will  show  more  ;  and  if  the  quality  of  the  highest 
power  lens  is  such  that  even  slightly  higher  power  eyepieces  can 
be  employed,  the  scope  of  the  instrument  is  extended. 


Eyepieces 

Huyghenian  Eyepieces 

No. 

Focal  Length.    Magnifying 

3260 

42  mm. 

X    6 

3261 

25  mm. 

XlO 

3262 

17  mm. 

Compensating  Eyepieces 

X15 

3266 

45  mm. 

X    6 

3267 

30  mm. 

X    8 

3268 

22  mm. 

Xll 

3269 

15  mm. 

X17 

3270 

10  mm. 

X25 

Standard 
size. 


Best 

eyepieces 
for  general 
work. 


All  eyepieces  are  made  to  the  diameter  of  the  Royal  Micro- 
scopical Society's  No.  1  Standard  Drawtube,  'Ql?  inch  diameter. 
They  are  made  to  drop  in  loosely,  so  that  they  may  be  changed 
without  any  tendency  to  alter  the  adjustments  of  the  microscope. 
They  are  designated  by  their  focal  length,  and  their  magnifying 
power  is  given  for  the  distance  of  distinct  vision — 10  inches 
(250  mm.) — and  is  engraved  on  each  eyepiece. 

The  best  eyepieces  to  use  for  general  work  are  those  of  the 
lowest  powers,  42  or  45  mm.  and  25  or  30  mm.  The  eyepoint 
(T,  Fig.  1,  page  9)  is  large  with  low-power  eyepieces,  and  fine 
specks  of  dust  on  the  surface  of  the  eye  or  in  any  part  of  the 
instrument  do  not  readily  show.  The  higher  the  power  of  the 
eyepiece  used,  the  smaller  is  the  diameter  of  the  eyepoint,  and  any 
such  minute  obstacles  to  the  passage  of  the  light  become  more 
apparent.  It  is,  however,  of  the  utmost  value  to  be  able  to 
slip  in  a  high- power  eyepiece  for  occasional  examinations,  in 
order  to  increase  the  power  without  altering  the  adjustment  of  the 


OBJECT  GLASSES  AND  EYEPIECES 


89 


eces 


instrument,  and  for  this  purpose  a  17-  or  15-mm.  or  a  10-mm. 
eyepiece  is  required.  A  5-mm.  eyepiece  magnifying  50,  and  a 
2-5-mm.  magnifying  100,  are  made  to  order  for  special  testing 
purposes,  and  have  their  uses. 

Compensating  eyepieces  are  specially  corrected  to  work  with  Compensat 
apochromatic  object  glasses,  and  when  of  a  higher  power  than  ^'^'^''P' 
15  mm.  are  the  best  for  use  with  achromatic  object  glasses.     The 
difierence  in  performance  of  the  Huyghenian  and  the  compensating 
eyepieces  is  not  very  marked. 

Huyghenian  eyepieces  consist  of  two  plano-convex  lenses,  Huyghenian 
one  at  each  end  of  a  tube,  with  a  diaphragm  between  them,  ^^^p'^'^®* 
It  is  an  eyepiece  that  has  many  advantages  for  visual  work,  but 
it  is  not  the  best  for  photography.  It  is  also  not  quite  so  perfectly 
corrected  as  the  compensating  eyepieces  which  are  specially 
made  for  work  with  apochromatic  object  glasses.  The  lower  of 
the  two  lenses  is  called  the  field  lens,  because  it  increases  the  size 
of  field  while  the  upper  one  does  the 
magnifying.  The  two  lenses  are  of  such 
powers  and  placed  in  such  positions  that 
they  are  achromatic  and  give  a  fairly  flat 
field  for  visual  purposes,  but  do  not  do 
so  for  photography,  where  the  microscope 
has  to  be  re-focussed  in  such  a  manner 
that  an  actual  image  is  formed  behind 
the  eyepiece  instead  of  a  virtual  image 
projected  in  front  of  the  observer's  eye. 

The  corrections  of  an  eyepiece  need 
not  be  of  so  perfect  a  character  as  those 
of  an  object  glass,  because  the  individual 

bundles  of  rays  from  each  point  of  the  object  are  very  narrow 
beams  of  light  as  they  emerge  from  the  eyepiece,  and  the  defects 
of  an  eyepiece  are  reduced  in  a  similar  manner  to  those  of  an 
object  glass  when  it  is  stopped  down  by  a  pinhole  aperture. 
The  aperture  which  limits  the  size  of  the  beams  of  light  is  not  a 
pinhole,  but  the  same  effect  is  produced  by  the  narrow  angled 
cones  of  light  which  come  from  the  object  glass. 

The  magnifying  power  of  the  eyepiece  is  never  very  great 
compared  with  that  of  the  object  glass,  and  it  is  only  in  those  of 
high  power  that  the  corrections  are  of  such  importance  that  the 
extra  quality  of  the  compensating  series  are  very  noticeable, 
except  for  photography  or  where  entire  freedom  from  colour  is 
essential. 

The  so-called  projection  eyepieces  are  no  better  for  projection 
and  photography  than  the  compensating,  and  are  far  more  difficult 
to  adjust. 

Eyepiece  micrometers  or  plates  of  glass  ruled  with  squares 
or  cross  lines  may  be  dropped  upon  the  diaphragm  in  the  tube 
of  the  eyepiece  by  removing  the  cell  holding  the  upper  lens.     By 


Fig.  90. 


90 


THE  MICROSCOPE 


screwing  the  upper  lens  cell  up  and  down  into  tlie  tube  they  may 
be  sharply  focussed  (see  page  67). 

An  eyepiece  with  a  movable  pointer  in  the  field  of  view,  an 
erecting  eyepiece  and  a  polarising  eyepiece  are  described  in 
pages  72  and  73. 


Histology 

and 

pathology 


The  Selection  or  Object  Glasses  and  Eyepieces 

Advantage  Where  pricc  is  not  an  object,  it  is  advisable  to  have  a  complete 
of  complete  ggj^  Qf  either  achromatic  or  apochromatic  object  glasses,  including 
only  one  of  the  2/3-inch  apochromatic  and  one  of  the  1/6-inch 
apochromatic.  Each  size  has  its  special  uses  as  previously  described, 
and  they  do  not  overlap.  It  is  best  to  have  apochromatic 
object  glasses  and  a  complete  set  of  compensating  eyepieces. 
At  a  time  when  many  intermediate  sizes  of  object  glasses  were 
made,  a  selection  was  always  necessary ;  now  that  they  have  been 
reduced  to  a  smaller  number  of  standard  sizes,  they  are  all  of 
great  assistance  to  any  observer.  It  is  not,  however,  every 
microscopist  who  can  afford  to  buy  a  complete  set,  and  in  this 
case  the  selection  becomes  a  matter  of  importance.  For  work 
on  all  subjects,  all  sizes  will  probably  be  eventually  required, 
though  they  need  not  be  purchased  at  once. 

For  histological  and  pathological  work,  the  student  is  advised 
by  his  teacher  to  purchase  a  2/3-inch  (16-mm.)  and  a  1/6-inch 
(4-mm.)  object  glass  and  two  eyepieces  magnifying  X6  and  XlO, 
adding  a  1/12-inch  oil-immersion  for  pathological  work  at  a  later 
date.  It  is  a  question  whether  in  some  cases  he  might  not  do 
better  to  start  with  a  1/3-inch  (8-mm.)  and  an  1/8-inch  (3-mm.) 
oil-iramersion,  adding  a  IJ-inch  (32 -mm.)  for  very  low- power 
work  if  required.  In  such  a  case  he  should  have  three  eyepieces, 
X6,  XlO,  and  Xl5,  as  the  highest  power  eyepiece  enables  a 
great  deal  of  work  to  be  done  with  the  1/3-inch  (8-mm.)  that  would 
generally  be  done  with  a  1/6-inch  (4-mm.).  The  1/8-inch  (3-mm.) 
oil-immersion  is  a  very  useful  power.  The  1/3-inch  (8-mm.)  is 
only  slightly  affected  by  the  thickness  of  the  cover  glass,  and  the 
1/8-inch  oil-immersion  is  not  affected  at  all,  so  that  the  unskilled 
observer  is  more  likely  to  get  the  best  out  of  his  instrument  with 
these  two  powers.  The  addition  at  a  later  date  of  a  1/12-inch 
apochromatic  object  glass  and  a  high-power  compensating  eye- 
piece makes  a  very  perfect  outfit.  Where  price  is  of  great 
importance,  the  cheapest  outfit  will  be  a  2/3-inch  (16-mm.)  and 
1/6 -inch  (4-mm.),  as  usually  recommended. 

For  biological  work  the  same  remarks  apply  to  a  great  extent, 
but  the  2/3-inch  (16-mm.)  has  the  great  advantage  that,  used  with 
an  erecting  eyepiece,  it  turns  the  instrument  at  once  into  a  dis- 
secting microscope.  It  is  also  sometimes  troublesome  to  use  an 
oil-immersion  with  an  unmounted  specimen  examined  in  water 
under  a  cover  glass,  and  a  high- power  dry  lens  is  often  preferred. 


Biology. 


OBJECT  GLASSES  AND  EYEPIECES  91 

The  difficulties  of  using  an  oil-immersion  lens,  however,  cliiefly 
apply  to  cases  where  it  is  necessary  to  search  with  a  low  power, 
change  to  a  high  power,  and  then  rapidly  search  again,  as  in  the 
latter  process  the  oil  must  be  wiped  off  the  cover  glass  with  a 
piece  of  filter  paper  dipped  in  benzol  or  xylol  before  the  low  power 
is  used.  The  1/6-inch  dry  apochromatic  with  a  cover  glass 
adjustment  is  a  very  useful  lens  for  such  work,  because  it  has  a 
very  large  aperture  and  resolving  power,  and  if  carefully  adjusted 
for  the  cover  glass  thickness,  owing  to  its  very  perfect  corrections, 
can  be  employed  with  very  high  eyepieces  to  do  much  of  the 
work  that  would  otherwise  be  done  with  a  1/12-inch. 

For  botanical  work  the  2/3-inch  (16-mm.)  and  1/6-inch  (4-mm.)  Botany, 
are  generally  used  by  the  student.     A  great  deal  of  the  work 
could  be  better  done  with  IJ-inch  (32-mm.)  and  1/3-inch  (8-mm.) 
with  a  higher  eyepiece.     A  1/12-inch  oil-immersion  may  be  added 
for  cell  structure,  such  as  Karyokinesis. 

For  metallurgical  work  the  best  three  lenses  are  the  IJ-inch  Metallurgy. 
(32-mm.),  1/3-inch  (8-mm.)  and  1/12-inch  (2-mm.)  oil-immersion. 
For  photography  the  apochromatic  series  have  a  marked  advantage, 
as  colour  screens  need  not  be  used  ;  compensating  eyepieces 
should  always  be  selected  for  photomicrography  as  the  Huyghenian 
eyepieces  are  not  satisfactory  for  this  purpose.  For  chemical 
and  industrial  purposes  it  is  difficult  to  make  any  recommendation. 
The  objects  examined  are  so  varied  and  the  conditions  so  different 
that  nothing  but  a  complete  series  will  meet  every  requirement. 
It  is  best  to  study  the  capabilities  of  each  object  glass  as  given 
on  page  86,  and  select  according  to  circumstances. 

For  petrology  the  best  two  lenses  are  the  2/3-inch  (16-mm.)  Petrology, 
and  the  1/6-inch  (4-mm.) ;  the  high-power  dry  lens  being  essential 
for  observing  interference  figures,  as  a  large  aperture  is  necessary 
for  this  work.  A  low-power  IJ-inch  (32-mm.)  is  very  useful. 
The  apochromatic  series  must  be  used  with  caution  for  this 
purpose,  the  fluorspar  of  which  they  are  made  may  render  them 
unsuitable  in  some  cases. 

For  general  recreation  the  whole  series  will  appeal  to  the  General 
microscopist  who  wishes  to  dip  into  a  large  number  of  subjects,  ^^^^eation. 
If  only  two  object  glasses  are  required  he  should  begin  with  a 
IJ-inch  (32-mm.)  and  a  1/3-inch  (8-mm.)  and  three  eyepieces. 


CHAPTER  VI 

THE  MICROSCOPE  STAND 

Essential  In  all  microscopes  certain  characteristics  are  important.  The 
qualities.  quality  of  the  optical  portions  is  the  essential,  but  the  stand 
requires  to  possess  good  adjustments  and  rigidity  of  construction 
to  enable  the  optical  qualities  to  be  made  full  use  of.  Those 
who  are  not  competent  to  judge  of  the  optical  performance  may 
be  sometimes  tempted  to  criticise  small  details  of  mechanical 
construction  which  are  of  no  importance,  but  certain  main  points 
are  worthy  of  consideration. 
Base  and  The  basc  and  pillar  of  the  microscope  have  been  a  subject  of 

piUar.  j^jjg    discussion    among    microscopists.     If    the    instrument    is 

supported  in  a  rigid  manner,  their  shape  and  construction  is  not 
of  great  importance.  Two  chief  types  have  been  made,  one  of 
which  has  three  projecting  legs  of  varying  shapes,  the  other 
consists  of  a  flat  slab  with  a  pillar  fixed  upon  its  upper  side. 
The  former  is  generally  known  as  the  English  model,  and  the  latter 
as  the  horseshoe  base.  It  was  originally  introduced  more  or  less 
of  the  shape  of  a  horseshoe,  but  has  since  been  altered  in  its 
outlines.  Both  stands  rest  upon  the  table  on  three  toes  and, 
provided  the  distance  apart  of  these  toes  is  the  same,  the  two 
models  are  equally  rigid.  The  horseshoe  pattern  relies  for  its 
stability  slightly  more  upon  its  weight  than  its  size.  It  has  the 
advantage  that  it  can  be  used  rather  nearer  the  edge  of  the  table 
when  the  microscope  is  in  a  vertical  position,  and  that  all  the 
adjustment  of  the  substage  can  be  more  readily  got  at  than  in 
the  English  model,  where  the  side  projecting  legs  are  more  or 
less  in  the  way  of  the  hands.  It  also  requires  a  rather  smaller 
case  or  bell-glass  cover.  Most  microscopes  are  now  made  with 
the  piUar  and  slab  form  of  base  known  as  the  horseshoe,  or  with 
one  piece  that  has  a  shape  which  approximates  to  a  horseshoe 
base  and  pillar  combined.  Far  too  much  time  has  been  wasted 
in  the  past  by  arguing  on  the  relative  merits  of  the  two  forms. 
The  microscope  should  stand  rigidly  and  be  free  from  any  tremor 
in  its  parts.  In  cities  or  near  machinery  where  constant  vibration 
is  present  it  is  sometimes  worth  while  to  take  special  measures 
to  overcome  this.  A  slab  of  slate  supported  on  a  layer  of  cotton 
wool  an  inch  thick  will  generally  damp  out  vibration.     In  a 

92 


THE  MICEOSCOPE  STAND  93 

factory  with  rapidly- moving  macliinery,  the  microscope  table 
may  be  placed  on  a  stone  which  rests  on  an  inflated  motor-car 
tyre.  Under  ordinary  circumstances  such  precautions  are  not 
necessary,  and  any  firm  table  is  satisfactory. 

The  stage  should  be  firm  and  its  upper  surface  should  not  be  stage. 
less  than  4|  inches  above  the  table.  An  ebonite  covering  makes 
a  better  surface  than  metal  for  giving  a  smooth  motion  to  the 
slide.  It  is  less  likely  to  be  damaged  by  reagents,  but  a  brass 
stage  with  a  surface  ground  flat  is  very  satisfactory.  There 
should  be  a  horizontal  distance  of  not  less  than  3  inches 
between  the  optic  axis  or  centre  of  the  stage  aperture  and  the  limb 
to  enable  Petri  dishes  and  large  culture  plates  to  be  examined. 

The  body  tube  must  be  of  a  variable  length.  The  early  Body  tube. 
microscopes  were  generally  made  with  a  9-  or  10-inch  tube,  but 
have  been  entirely  superseded  by  the  more  compact  type  which 
has  a  tube  length  of  140  mm.,  which,  by  means  of  a  drawtube, 
can  be  increased  to  200  mm.  The  shorter  tube  length  has  an 
advantage  in  addition  to  the  reduction  in  the  size  of  the  microscope 
of  which  it  admits.  In  the  previous  chapter  it  has  been  explained 
how  the  variation  in  the  thickness  of  the  cover  glass  can  be 
largely  compensated  by  a  variation  in  the  length  of  the  drawtube. 
A  body  tube  of  great  length  must  be  moved  to  a  great  extent  to 
produce  much  alteration,  while  a  short  body  is  far  more  sensitive 
in  this  respect,  and  a  much  greater  range  of  correction  can  be 
obtained.  It  is  also  possible  by  an  extra  tube  to  further  lengthen 
a  short  body,  while  it  is  not  feasible  to  shorten  a  long  tube. 

The  drawtube  should  always  be  graduated  in  millimetres,  Drawtube. 
which  give  the  length  at  every  position,  and  it  should  work  with 
great  smoothness,  so  that  when  the  microscope  is  in  use  the 
length  of  the  tube  may  be  altered  without  exerting  any  force 
which  is  likely  to  upset  the  adjustment  of  the  instrument.  The 
sliding  fitting  should  always  be  in  cloth  or  other  fabric  which  will 
ensure  a  smooth  motion.  A  metal-to-metal  slide  is  not  so  satis- 
factory ;  such  a  slide  may  be  perfect  when  it  leaves  the  makers' 
hands,  but  the  slightest  film  of  tarnish  or  oxidisation  ruins  its 
working  and  gives  a  jerky,  uneven  motion.  Due  to  the  elasticity 
of  a  thin  cloth  or  a  fabric  slide,  it  cannot  be  quite  as  stiff  and  rigid 
as  a  metal  slide,  but  this  is  a  matter  of  no  practical  consequence, 
as  a  slight  movement  of  the  eyepiece  out  of  the  optic  axis  has 
no  effect  on  the  quality  of  the  image.  The  drawtube  must  be 
provided  with  a  diaphragm  to  prevent  reflections  at  the  inner 
sides  of  the  tube,  and  the  upper  portion  of  the  drawtube  should 
be  slightly  smaller  in  diameter  than  the  lower  part,  so  that 
pushing  the  eyepieces  in  does  not  tend  to  polish  the  tube  below 
the  position  where  the  shortest  eyepiece  fits.  The  lower  end  of 
the  drawtube  should  have  a  screw  fitting  for  the  use  of  a  low-power 
object  glass. 

The  coarse  and  fine  focussing  adjustments  must  be  well  made  Adjustments. 


94 


THE  MICROSCOPE 


Joint. 


Limb. 


and  must  work  with  a  smooth,  even  motion  that  allows  of  the 
most  delicate  setting  for  focus.  The  coarse  adjustment  should 
be  capable  of  focussing  with  a  1/6-inch  object  glass,  although 
after  the  focus  has  been  found  the  slow  motion  will  generally  be 
used.  It  is  an  advantage  to  have  a  series  of  divisions  on  the  slow 
motion  by  which  the  thickness  of  a  cover  glass  or  a  section  can 
be  ascertained  (see  page  53).  The  value  of  the  divisions  is  given 
under  the  description  of  different  microscopes.  The  two  adjust- 
ments should  work  in  fittings  which  are  made  with  the  utmost 
precision.  These  fittings  should  be  solid  metal  slides  without 
any  adjusting  screws.  The  wear  in  the  fittings  of  a  microscope 
is  infinitesimal  compared  with  those  of  running  machinery,  and 
slides  well  fitted  in  the  first  instance  will  wear  for  a  lifetime 
without  adjustment  if  properly  used.  All  kinds  of  adjustable 
fittings  have  been  tried,  but  have  been  abandoned.  The  adjusting 
screws  work  loose,  the  slides  do  n6t  have  to  be  so  well  fitted 
originally,  and  nothing  is  so  good  as  a  solid  slide  well  fitted  in 
the  first  instance. 

The  milled  heads  of  both  the  adjustments  should  move  in  the 
same  direction,  so  that  the  upper  portion  of  the  milled  heads  is 
going  away  from  the  observer  when  the  body  tube  is  going  down 
or  approaching  the  object.  Mistakes  made  by  turning  the  milled 
heads  in  the  wrong  direction  may  result  in  breaking  the  slide  or 
damaging  the  object  glass.  If  all  the  milled  heads  in  a  microscope 
move  in  one  direction,  such  mistakes  need  not  be  made. 

A  microscope  should  have  a 
joint  for  inclination.  The  instru- 
ment may  have  to  be  used  occa- 
sionally in  a  vertical  position,  but 
it  is  so  much  more  convenient  in 
any  other  position  that  an  in- 
clining joint  should  not  be  omitted 
from  the  stand.  The  writer  does 
all  his  most  difficult  tests  and  ex- 
aminations with  a  microscope  on 
an  optical  bench  in  an  almost  hori- 
zontal position,  the  axis  pointing 
down  only  about  15°.  The  eye- 
piece is  at  the  eye-height  of  the 
observer  when  in  a  sitting  position. 
Prolonged  observation  of  several 
hours  ceases  to  be  tiring  with  the 
microscope  thus  arranged.  An 
ordinary  microscope,  however,  in- 
cUned  to  about  45°  is  very  com- 
fortable for  prolonged  work. 
A  microscope  should  have  a  limb  that  can  be  readily  grasped 
by  the  hand  for  lifting.     It  must  never  be  lifted  by  its  body  or 


Fig.  91. 


THE  MICROSCOPE  STAND  95 

any  of  the  adjusting  milled  heads.  The  only  other  portion  of 
the  instrument  by  which  it  may  be  lifted  is  the  base  or  pillar. 
Valuable  instruments  may  be  badly  damaged  if  lifted  carelessly. 
Adjustment  slides  may  be  ruined,  pinions  and  screws  bent,  or  the 
entire  instrument  may  be  dropped  if  these  instructions  are  not 
followed. 

^A  suitable  size  for  the  mirror  of  a  microscope  depends  upon  Minor, 
how  far  it  is  placed  below  the  stage ;  a  large  mirror  close  to  the 
stage  is  of  no  advantage.  A  2-inch  diameter  mirror  3J  inches 
from  the  stage  will  converge  a  beam  of  light  at  approximately 
30°,  and  this  is  more  than  is  ever  required.  When  a  substage 
condenser  is  employed,  the  mirror  need  not  be  much  larger  than 
the  back  lens  of  the  condenser,  which  never  exceeds  IJ  inches. 
A  2-inch  mirror  is,  therefore,  more  than  sufficient  for  all  ordinary 
types  of  microscopes.  A  mirror  should  preferably  be  on  a  fitting 
by  which  its  distance  from  the  stage  may  be  varied.  It  is  not  only 
of  advantage  for  focussing  the  concave  mirror,  but  enables  very 
long  apparatus  to  be  used  in  the  substage  by  sliding  it  farther 
from  the  stage  than  its  usual  position.  It  is  convenient  that  it 
should  be  capable  of  swinging  to  one  side  for  inserting  substage 
apparatus  or  for  using  light  direct  from  a  source  of  illumination, 
but  in  use  it  must  always  be  placed  in  the  axis  of  the  instrument. 

All  the  microscopes  illustrated  in  this  book  possess  the  features 
here  described  as  being  of  importance  ;  the  following  brief  notes 
explain  their  special  characteristics. 

Except  the  special  metallurgical  and  petrological  microscopes, 
all  the  instruments  illustrated  are  suitable  for  every  branch  of 
work.  The  highest  class  of  research  work  calls  for  a  mechanical 
stage  and  the  best  substage  adjustments.  The  rack  and  pinion 
adjustment  to  the  drawtube  and  the  rotating  stage  are  of  con- 
siderable advantage,  but  are  not  essential. 

The  Standard  London  Microscope  is  illustrated  in  six  forms,  standard 
The  first  three  of  these  forms  are  the  same  except  as  regards  their  ^^'^oscopes. 
substages.     These   microscopes   fulfil    the    conditions    given    in 
previous  pages  as  to  the  essential  features  which  a  serviceable 
microscope  must  possess.   They  have  also  many  smaller  advantages 
and  refinements.     The  base  consists  of  an  iron  casting  of  suitable  The  base, 
weight  to  give  rigidity  to  the  instrument,  encased  in  a  covering 
of  vulcanite  which  gives  it  a  durable  finish.     It  is  of  such  a  spread 
as  to  prevent  the  instrument  from  tipping,  and  is  also  made  of 
such  a  shape  that  it  can  easily  be  fixed  down  to  a  bench  when  used 
for  photomicrography  when  the  instrument  is  used  horizontally. 
This  may  be  done  with  advantage,  as  it  prevents  the  microscope 
from  being  moved  during  the  process  of  attaching  the  camera. 
The  joint  of  the  instrument  is  stopped  at  the  exact  vertical  and 
horizontal  position.     The  stage  consists  of  a  brass  core  completely  The  stage. 
embedded  in  vulcanite.     This  method  is  more  satisfactory  than 
the  usual  method  of  fixing  on  thin  vulcanite  plate  on  the  top  of 


96 


THE  MICROSCOPE 


The  limb. 


The 

mechanical 
stage. 


The  body. 


The 


a  brass  stage,  as  it  is  less  subject  to  warping,  is  not  easily  chipped 
or  broken.  The  stage  has  four  holes  for  the  accommodation  of 
stage  clips.  The  limb  is  drilled  with  a  hole  by  means  of  which 
a  mechanical  stage  can  be  attached,  held  in  position  by  a  strong 
bolt  with  clamping  milled  head.  This  mechanical  stage  can  be 
fitted  by  the  user  of  the  microscope  without  returning  the 
instrument  to  the  maker,  although  it  is  best  to  do  so  if  possible, 
as  in  this  case  a  steady  pin  is  also  put  in  to  ensure  that  the  mechani- 
cal stage  is  in  its  exact  position,  and  thus  to  make  certain  that 
the  finder  divisions  read  correctly.  The  finder  divisions  read  from 
the  left-hand  side  and  the  bottom  of  the  3x1  slip.  The  standard 
body  of  the  microscope  is  of  rather  larger  diameter  than  is  usual 
with  the  ordinary  small  body  tube.  The  coarse  adjustment  is 
adjustmentsi  actuated  by  the  upper  milled  head,  and  the  fine  adjustment  by  the 
lower.  The  fine  adjustment  is  of  rigid  type  and  gives  a  very 
sensitive  and  smooth  motion.  It  has  two  speeds,  the  left-hand 
milled  head  travelling  the  body  at  half  the  speed  of  the  right- 
hand  milled  head.  The  substages  are  all  interchangeable,  and 
all  the  microscopes  are  supplied  with  the  holes  necessary  for  the 
fitting  of  any  of  the  various  forms  of  substage  which  can  be  attached 
by  the  user  with  the  aid  of  a  screw-driver.  Thus  a  microscope  with 
a  plain  tubular  substage  may  first  be  bought,  and  if  the  micro- 
scopist  at  a  later  date  feels  the  need  of  a  substage  with  focussing 
and  centring  adjustments,  this  substage  may  be  purchased 
separately,  and  he  is  able  to  fix  it  himself,  without  sending  the 
microscope  to  the  maker.  The  following  are  the  dimensions  of 
these  microscopes  : 


The 

substage 


Dimensions 
of  the 
microscope. 


4f 

>> 

m 

>> 

6i 

>> 

H 

»» 

7 

■§■ 

j» 

3i 

>> 

•01 

mm. 

6 

>> 

No.  3210. 


Size  of  base         .  .  .  .  .  .         6|  X  4  X  1         in. 

Distance  of  upper  surface  of  stage  from  table 

The  height  of  the  microscope  in  use  when  vertical  . 

The  height  of  its  optical  centre  when  horizontal 

Diameter  of  coarse  adjustment  milled  heads  . 

Diameter  of  fine  adjustment  milled  heads 

Travel  of  coarse  adjustment  .... 

Each  division  of  the  fine  adjustment  moves  the  body 
Travel  of  fine  adjustment     ..... 

Diameter  of  object  glass    screw,    Royal    Microscopical 

Society's  Standard  ...... 

Diameter   of    drawtube,    Royal   Microscopical   Society'; 

Standard  No.  1         .....  . 

Diameter    of    substage,    Royal    Microscopical    Society' 

Standard  ....... 

Diameter  of  mirror      ...... 

Focal  length  of  mirror  ..... 

Vertical  travel  of  mirror       ..... 

Tube  length 140 

Outside  diameter  of  upper  portion  of  drawtube 
Diameter  of  object  glass  stem        .... 
Distance  from  optic  axis  to  inside  of  limb 

No.  3210,  page  97,  is  the  simplest  form  of  the  microscope, 
and  has  a  plain  tubular  substage  into  which  slides  a  fitting  with 


•8      in. 


•917 


to 


1-527 

>> 

2 

»> 

3-5 

*» 

1-5 

>> 

)0 

mm 

1-05 

in. 

•65 

it 

3 

*t 

Fig.  92. — ^No.  3210,  Standard  London  Microscope,  with  plain  tubular 
substage  and  dust- tight  double  nosepiece. 

7  97 


Fig.  93. — No.  3211,  Standard  London  Microscoije,  with  screw 
focussing  swing-out  substage,  dust-tight  double  nosepiece. 


98 


Fig.  94. — No.  3213,  Standard  London  Microscope,  with  rack  and 
pinion  and  centring  swing-out  substage,  triple  nosepiece,  mechanical 
stage. 

99 


100  THE  MICROSCOPE 

an  iris  diaphragm.  In  the  upper  cell  of  this  a  small  Abbe  con- 
denser can  be  fitted,  and  also  a  series  of  patch-stops  or  a  coloured 
or  ground  glass.  The  fitting  can  be  moved  up  and  down  in  the 
tube  to  a  limited  extent  for  focussing. 

No.  3211.  No.  3211  (page  98)  has  a  substage  which  focusses  by  means  of 

a  screw  actuated  by  a  milled  knob.  When  this  screw  reaches  the 
end  of  its  travel  in  a  downward  direction  the  whole  fitting  carrying 
the  iris  diaphragm  and  condenser  swings  aside,  so  that  it  is  a 
very  simple  matter  to  entirely  dispense  with  the  condenser 
when  it  is  not  required.  The  substage  is  held  rigidly  in  the 
optic  axis  imtil  the  fitting  is  focussed  down  to  its  lowest  position, 
when  a  further  turn  of  the  milled  head  swings  it  out  of  position, 
thus  the  addition  of  this  motion  does  not  in  any  way  afiect  the 
rigidity  of  the  substage.  This  substage  is  suitable  for  use  with 
either  the  small  or  large  form  of  the  Abbe  condenser.  Centring 
motions  cannot  be  fitted,  and  it  is  consequently  not  suitable 
for  use  with  an  achromatic  condenser.  A  high-power  dark- 
ground  illuminator  can  be  used  with  it,  but  must  be  in  a  fitting 
that  is  provided  with  centring  screws. 

No.  3213.  No.  3213  (page  99).     This  microscope  has  a  substage  with  full 

adjustments — namely,  focussing  by  rack  and  pinion,  swing-out 
motion,  and  centring  motion.  The  focussing  is  actuated  by  a  large 
milled  head  on  the  right  of  the  instrument  travelling  in  the  same 
way  as  the  coarse  adjustment  milled  heads.  When  at  the  bottom 
of  its  travel  the  substage  may  be  completely  swung  aside.  Here 
again,  as  this  substage  is  held  in  position  by  a  guiding  pin,  until 
it  is  in  its  lowest  position,  there  is  no  tendency  to  lose  rigidity 
by  the  addition  of  the  swing- out  motion.  The  centring  is  actuated 
by  two  screws  with  milled  heads.  A  modified  form  of  this  stand. 
No.  3212,  is  made  which  is  the  same  as  No.  3213,  except  that  the 
substage  has  not  a  centring  adjustment.  This  is  suitable  when 
it  is  not  desired  to  use  a  higher  class  condenser  than  the  Abbe 
form,  but  for  all  more  exacting  work  the  No.  3213  is  preferable.  The 
substage  on  this  stand  No.  2313  enables  the  achromatic  condenser 
and  the  high-power  dark-ground  illuminator  to  work  to  their 
full  advantage.  The  illustration,  page  99,  shows  this  microscope 
with  a  detachable  mechanical  stage  attached  by  bolt  and  nut, 
as  mentioned  previously. 

The  Portable  Standard  London  Microscope 

Portable  This  microscope  has  been  designed  for  the  use  of  the  micro- 

No.  3221.  '  scopist  whose  work  requires  that  he  should  have  an  instrument 
of  the  usual  rigid  construction,  with  all  the  movements  neces- 
sary for  the  highest  forms  of  research  work,  but  to  whom  porta- 
bility is  also  an  advantage.  For  travellers  engaged  in  critical 
work,  and  bacteriologists  in  foreign  countries,  this  microscope  is 
especially  suitable.     The  stand  is  the  same   as  the  standard 


THE  MICROSCOPE  STAND 


101 


pattern  No.  3213  except  as  regards  the  base  and  stage.  The 
former  is  folding,  and  has  the  same  spread  as  that  of  the  standard 
model.  The  stage  with  the  substage  attached  removes  for 
packmg  into  the  case.  It  is  so  made  that  when  in  position  it 
IS  even  more  rigid  than  the  standard  form.  It  is  attached  on  a 
bracket  and  is  held  in  position  by  a  taper  bolt.  The  substage 
has  all  the_  adjustments  of  the  No.  3213,  including  focussing 
rack  and  pinion,  centring  by  screws  and  a  swing-out  motion^ 
The  instrument  is  packed  in  a  case  which  only  measures  1L\  x 
8  X  2J  inches.      It  is  as  perfect  an  instrument  in  every  way 


Fig.  95. — No.  3221,  Portable  Standard  London  Microscope 

in  case. 


as  jthe  ordinary  model.  The  incase  will  carry  two  eyepieces,  three 
object  glasses,  substage  condenser,  dark-ground  illuminator, 
a  detachable  mechanical  stage,  triple  nosepiece,  and  a  bottle 
of  oil,  together  with  a  supply  of  slips,  cover  glasses,  and  sundry 
small  apparatus.  It  does  not  weigh  much  less  than  the 
ordinary  model.  The  small  dimensions  of  its  case  render 
it  specially  suitable  for  travelling  where  a  bulky  instrument  is 
inadmissible.  It  has  no  disadvantages  due  to  its  portability, 
and  most  standard  apparatus  can  be  fitted  to  it.  For  the  ordinary 
microscopist  who  takes  his  instrument^from  place  to  place  it  is 
very  convenient. 


Fig.  96. — No.  3221,  Portable  Standard  London  Microscope,  with 
rack  and  pinion,  and  centring  substage,  mechanical  stage,  triple  nose- 
piece,  and  condenser. 


102 


Fig.  97. 

No.  3216,  Standard  London  Microscope,  with  large  body,  circular 
stage,  and  complete  substage  adjustments. 

No.  3217,  Rack  and  pinion  focussing  and  double^extension  draw- 
tube, 

103 


104  THE  MICROSCOPE 

The  Standard  London  Microscope  with  Circular 
Rotating  Centring  Stage 

This  instrument  is  made  in  four  forms.  No.  3214  has  a  screw 
focussing  substage  and  is  the  same  as  No.  3211,  page  98,  with 
the  addition  of  the  circular  rotating  and  centring  stage. 

No.  3215  has  a  rack  and  pinion  focussing  and  centring  sub- 
stage  and  is  the  same  as  No.  3213,  page  99,  with  the  addition 
of  the  circular  rotating  and  centring  stage. 

No.  3216  is  the  same  as  No.  3215,  but  with  a  large  2-inch 
body  instead  of  the  standard  size.  Both  the  nosepiece  and  the 
drawtube  end  of  the  body  can  be  unscrewed,  and  a  photographic 
lens  can  be  slid  into  the  centre  of  the  tube  for  photographing 
large  specimens.  The  large  size  body  does  not  cut  off  the  angle 
of  view  given  by  such  a  photographic  lens.  Also,  if  the  drawtube 
end  of  the  tube  be  unscrewed  and  the  nosepiece  left  in  position, 
low- power  lenses  with  a  large  angle  of  view  may  be  used  in  the 
nosepiece  for  a  similar  purpose. 

No.  3217  is  the  same  as  No.  3216,  but  with  a  rack  and 
pinion  adjustment  to  the  drawtube,  and  is  provided  with  a 
second  drawtube,  enabling  the  length  of  the  tube  to  be  varied 
from  140  mm.  to  250  mm.  The  drawtube  is  very  large  in  diameter, 
and  can  be  provided  with  extra  large  eyepieces,  1*41  inch  diameter, 
of  the  No.  1  R.M.S.  standard  size. 

The  circular  mechanical  stage  illustrated  on  page  52  fits 
any  of  these  four  models. 

This  microscope,  with  an  interchangeable  binocular  body 
described  later,  makes  a  very  perfect  research  microscope. 

The  Massive  Model  Microscope 

No.  3201.  There  are  certain  cases  in  which  most  small  microscopes  give 

dissatisfaction  for  very  delicate  work,  and  this  model  was  first 
made  for  The  National  Institute  for  Medical  Research,  who  gave 
valuable  assistance  in  the  design  and  construction.  It  is  intended 
for  those  who  feel  the  want  of  a  very  perfect  instrument.  It  has 
been  made  throughout  on  a  very  heavy  and  stiS  design.  It  does 
not  stand  much  higher  than  the  standard  model,  but  it  is  unusually 
strong  and  stiff,  so  that  no  vibration  or  flexure  can  take  place. 
The  limb  consists  of  a  massive  brass  casting  which  extends  in 
one  piece  from  the  body  to  the  mirror.  The  tail-piece  and  fine 
adjustment  slide  are  planed  out  in  one  continuous  cut  so  as  to 
ensure  the  perfect  alignment  of  the  substage  with  the  focussing 
adjustment.  The  stage,  which  is  strengthened  below  by  side 
ribs,  is  rigidly  fixed  on  to  the  limb,  so  that  it  is  as  strong  as  a  solid 
piece.  It  is  of  great  advantage  to  have  a  stage  so  solid  that  it 
does  not  show  movement  under  the  highest  powers  by  the  weight 


Fig.  98. — No.  3201,  Massive  Model  Microscope. 


105 


106  THE  MICROSCOPE 

of  the  hands  placed  even  heavily  upon  it.  The  pillar  and  base 
are  equally  heavy  and  free  from  any  spring.  The  fine  adjustment 
is  exceptionally  delicate — one  revolution  of  the  milled  head  moves 
the  body  only  '1  mm.  Each  division  on  the  milled  head  is  equal 
to  "OOl  mm.  The  coarse  adjustment  milled  heads  are  very  large, 
enabling  a  finer  adjustment  to  be  made.  The  stage  is  square, 
measuring  4  J  X  4f  inches.  In  the  simple  model  it  is  flat,  with  a 
gap  cut  out  in  front,  and  the  standard  mechanical  stage  can  be 
attached  to  it  at  will  in  a  similar  manner  to  that  of  the  standard 
model,  being  clamped  to  the  limb  through  an  aperture  left  for 
the  purpose.  "When  a  mechanical  stage  is  supplied  at  the  same 
time  as  the  microscope,  it  is  fitted  with  a  steady  pin  entering  a 
second  hole  in  the  limb,  so  that  it  cannot  be  attached  in  an  in- 
correct position.  In  the  best  form  of  instrument,  as  illustrated 
on  page  105,  the  square  stage  has  two  dovetailed  grooves  planed 
in  its  surface,  and  the  mechanical  stage  racks  up  and  down 
these  grooves  or  can  be  removed  at  will.  This  mechanical  stage 
has  its  actuating  milled  heads  projecting  laterally  on  the  right- 
hand  side.  The  upper  one  moves  the  slide  laterally  and  has 
3  inches  (75  mm.)  travel,  the  lower  one  moves  the  slide  vertically 
and  has  a  travel  of  IJ  inches  (30  mm.).  The  latter  motion  is 
provided  with  a  clamp  screw,  so  that  it  can  be  locked  to  prevent 
any  chance  of  the  slide  moving  when  the  instrument  is  in  a 
horizontal  position.  This  prevents  any  settling  down  of  the 
object  during  photomicrography.  Verniers  reading  to  1/10  mm. 
are  provided  to  both  movements  in  convenient  positions  for 
reading. 

The  substage  racks  up  and  down  on  the  lower  portion  of 
the  limb,  which  is  accurately  in  the  optic  axis  of  the  microscope, 
and  the  mirror  fits  by  means  of  a  sliding  fitting  on  the  same  slide. 
The  substage  has  centring  adjustments  and  is  of  the  standard 
size,  but  at  its  upper  end  is  fitted  with  a  dovetailed  fitting  to 
receive  the  condensers  or  dark- ground  illuminators.  All  the 
illuminators  are  mounted  on  dovetailed  slides  which  slide  easily 
into  the  dovetailed  fitting,  and  are  held  accurately  in  position 
by  a  clamping  milled  head.  Each  illuminator  is  accurately 
centred  and  of  the  same  length,  so  that  they  can  be  rapidly 
interchanged  while  the  object  is  under  observation.  The  front 
portion  of  the  stage  is  cut  out  to  enable  this  to  be  done,  and  even 
an  oil-immersion  condenser  can  be  changed  for  a  dark-ground 
illuminator  while  the  slide  is  under  observation.  While  these 
illuminators  are  in  use,  the  tubular  portion  of  the  substage  is 
free  to  receive  apparatus  which  can  be  used  in  conjunction  with 
them.  The  back  of  the  foot  of  the  microscope  carries  a  short 
vertical  post,  and  when  the  microscope  is  placed  in  a  horizontal 
position  for  photomicrography  this  takes  the  weight  of  the  limb 
and  makes  a  rigid  support  under  conditions  where  a  slight  tremor 
might  ruin  the  sharpness  of  a  photograph.     The  body  is  of  the 


THE  MICROSCOPE  STAND  107 

large  2-inch  diameter,  with  a  drawtube  giving  a  variation  in 
length  from  140  mm.  to  200  mm.  A  rack  and  pinion  double 
extension  drawtube,  as  illustrated  on  page  103,  can  be  fitted  if 
desired.  An  interchangeable  binocular  body  can  be  fitted  to 
the  instrument,  and  a  circular  rotating  stage  can  be  made  in 
place  of  the  square  stage  ;  but  in  this  case  a  gap  cannot  be  cut 
out  in  front,  and  some  of  the  advantages  of  the  interchangeable 
substage  apparatus  are  lost.  The  apparatus  can  be  interchanged, 
but  only  after  racking  down  the  substage  by  the  amount  of  the 
thickness  of  the  stage. 

With,  this  massively  made  microscope,  the  body  and  apparatus 
can  be  relied  upon  to  be  always  truly  in  the  optic  axis,  the 
manipulation  of  one  part  of  the  instrument  does  not  tend  to 
upset  the  adjustment  of  the  other  parts,  and  when  using  the  very 
highest  power  lenses  it  is  a  pleasure  to  work  on  account  of  its 
stability  and  the  delicacy  of  all  its  adjustments. 

The  Binocular  Microscope 

Hitherto  binocular  microscopes  have  not  been  used  for 
research  work,  except  in  special  cases.  For  practical  purposes 
the  monocular  has  for  many  years  held  the  field,  and  the  use  of 
binoculars  has  practically  been  restricted  to  workers  who  only 
use  low  powers,  or  for  exhibition  purposes.  The  reason  of  this 
is  simply  explained  :  the  one  advantage  of  using  two  eyes  did  not 
outweigh  the  loss  of  the  many  advantages  possessed  only  by  the 
monocular  stands. 

Binocular  microscopes  may  be  divided  into  three  types,  and 
a  brief   description  of  each  type  will  set  forth  their 
respective  merits  and  demerits. 

Type  1,  best  represented  by  the  "  Wenham," 
bisected  the  beam  of  light  that  emerged  from  the 
object  glass  (0,  Fig.  99)  and  directed  the  right-hand 
half   into  the   one  eye,  the  left-hand  half   into  the 

other. 

Binocular  vision  with  this  was  not  equal  to 
monocular,  because  by  reducing  the  size  of  the 
beam  of  light  which  formed  each  image  it  reduced 
the  resolving  power  of  the  microscope.  It  could  not  be  used 
with  high-power  object  glasses,  because  the  prism  could  not  be 
placed  sufficiently  close  to  the  back  lens  of  the  object  glass  to 
properly  bisect  the  beam  of  light  into  two  separate  halves 
before  the  rays  had  intermingled.  Efforts  to  accomplish  this 
by  mounting  high- power  object  glasses  in  special  short  mounts 
only  partly  overcame  the  difficulty,  and  rendered  the  use  of 
revolving  nosepieces  impossible. 

This    type    of    instrument   involved   long    tubes    and    con- 
sequently bulky  instruments,  and  could  only  be  satisfactorily 


108 


THE  MICEOSCOPE 


Fig.  100. 


employed  when  the  illumination  was  specially  arranged  so  that 
the  whole  of  the  object  glass  was  equally  illuminated.  If,  for 
instance,  a  fine  oblique  feather  of  light  was  employed,  it  would 
only  enter  one  side  of  the  object  glass  and  consequently  only  one 
eye  would  receive  the  light,  the  other  eye  seeing  no  image.  In 
like  manner,  if  more  light  happened  to  be  entering  one  half 
of  the  object  glass  than  the  other,  the  illumination  of  the  two 

eyes   was    different,    often    to   the    extent    of 
making  binocular  vision  inoperative. 

It  will  be  seen  that  none  of  these  dis- 
advantages exists  in  the  new  instrument  here 
described. 

The  second  type  of  binocular  was  in  one 
respect  on  the  right  principle.  In  all  models 
of  this  type  the  beam  of  light  is  not  bisected 
into  two  halves,  but  the  entire  beam  is  filtered 
into  two  portions,  so  that  some  light  from 
every  part  of  the  object  glass  goes  to  each  eye. 
The  Powell  and  Lealand  (Fig.  100)  shows  the 
earliest  form,  the  whole  light  from  the  object 
glass  (0)  impinges  on  a  glass  plate  (1)  and  the 
major  part  passes  through  this  thick  glass  plate, 
emerging  in  a  direction  parallel  to  and  almost  continuous  with  its 
original  direction  ;  but  a  percentage  is  reflected  at  the  first  surface 
and  proceeds  to  the  prism  (2),  which  reflects  it  up  a  second  tube, 
placed  at  an  angle  with  the  optic  axis  of  the  direct  beam. 

This  type  of  instrument  gives  equal  resolution  to  that  of  a 
monocular  microscope  because  the  size  of  the  beam  which  forms 
each  image  is  not  reduced. 

With  the  Powell  and  Lealand  form,  however,  the  tubes  of 
the  microscope  must  be  long  and  the  instrument  bulky,  and  it 
suffers  from  the  very  grave  defect  that 
the  light  that  is  reflected  is  so  feeble  as 
to  be  insufficient  for  satisfactory  vision. 
The  light  in  one  eye  is  only  of  about 
one-sixth  the  intensity  of  that  of  the 
other. 

The  Abbe  binocular  eyepiece,  which 
acted  optically  on  the  same  principle  in 
not  bisecting  the  beam  into  two  halves, 
but  in  filtering  the  light  by  a  reflected 
and  a  refracted  beam,  improved  the  light  distribution,  but  only 
made  the  relative  illumination  in  the  two  eyes  about  1  to  2|, 
and,  while  not  curing  this  defect,  introduced  a  farther  dis- 
advantage. The  general  plan  of  this  eyepiece  is  shown  in  Fig.  101, 
and  it  will  be  seen  that  the  light  which  is  split  up  into  two  by 
reflection  and  transmission  at  the  surface  is  resolved  into  two 
beams,  one  (A  D)  which  is  transmitted,  the  other  (A  B  C)  which 


/ 


Fig.  101. 


THE  MICROSCOPE    STAND 


109 


is  reflected,  and  the  reflected  light  at  the  time  it  emerges  has 
travelled  a  path  that  is  longer  than  the  direct  light  by  the 
amount  A  B.  This  difficulty  was  overcome  by  making  a  special 
pair  of  eyepieces  whose  focal  points  were  different  in  position, 
but  it  limits  the  use  of  the  instrument  to  the  use  of  special 
eyepieces. 


Beck 


Fig.   102. — Diagram  showing 


(A) 

Eyepiece. 

(B) 

Drawtube. 

(D) 

Body. 

(E) 

Coarse  focussing  adjustment 

(F) 

Prism  box  knob. 

(&) 

Sliding  prism  box. 

(H) 

Object  glass. 

(I) 

Stage. 

(K) 

Substage  condenser. 

W 

Iris  diaphragm. 

paths  of  light  tlirough  the  Beck  Binocular 
Microscope. 

(M)      Substage  focussing  adjustment. 

(N)       Mirror. 

(0)       Pillar. 

(P)       Base. 

(x  y)    Object. 

(a/  y')  Image  formed  by  object  glass. 

(x'  y")  Virtual  image  formed  by  eyepiece. 

(«  «')  Ramsden  discs — conjugate  images  of 
back  equivalent  plane  of  object 
glass. 


no  THE  MICROSCOPE 

The  second  type  of  instrument  therefore,  while  giving  good 
resolution,  was  not  free  from  other  defects,  and  was  not  there- 
fore equal  to  the  monocular. 

The  third  type  of  binocular,  which  consists  of  two 
microscopes  set  at  an  angle  to  one  another,  both  pointing  at 
the  focal  point,  while  quite  satisfactory  in  its  performance,  is 
limited  to  the  use  of  low  powers  and  requires  specially  mounted 
and  accurately  adjusted  pairs  of  object  glasses. 

It  will  be  gathered  from  this  description  of  the  properties  of 
previous  instruments  what  were  the  difficulties  that  had  to  be 
overcome  in  making  a  really  satisfactory  binocular,  and  in  the 
following  description  of  the  properties  of  the  Beck  binocular  we 
treat  each  point  separately,  explaining  how  the  objections  have 
been  removed. 
Eeaoiation.  The  resolving  power  of  a  microscope  is  a  measure  of  the 

fineness  of  detail  that  it  will  depict  in  the  image  which  it  forms, 
quite  apart  from  the  magnifying  power.  The  microscope  must 
have  sufficient  magnifying  power  to  render  such  detail  visible  to 
the  eye,  but  no  amount  of  extra  magnifying  power  is  of  use 

unless  the  resolving  power  is 
sufficient  to  produce  an  image 
containing  the  requisite  detail. 
Resolving  power  depends  upon 
the  size  of  the  cone  of  light 
Fig.  103.     -  which  forms  each  point   of  the 

image.  Suppose  the  lens  0 
(Fig.  103)  represents  the  object  glass  forming  an  image  of  the 
central  point  of  the  object  C  at  a  point  D  in  the  centre  of 
the  image ;  the  resolution  for  a  given  magnifying  power  will 
depend  on  the  diameter  A  B  of  the  cone  of  light  A  D  B  which 
forms  the  image ;  this  cone  of  light  has  an  exact  ratio  to  the 
angle  A  C  B  of  the  light  which  enters  the  lens  from  each  point 
of  the  object,  and  it  is  by  means  of  the  angle  A  C  B  that 
the  resolving  power  is  generally  and  more  conveniently  expressed 
as  numerical  aperture  (N.A),  but  it  might  be  expressed  with 
reference  to  the  angle  A  D  B.  It  is  evident  that  if  the  cone 
of  light  A  D  B  be  bisected  and  the  complete  half  0  D  A  be 
used  to  form  the  image  received  by  one  eye  and  the  complete 
half  0  D  B  used  to  form  the  image  received  by  the  other 
eye,  the  cone  of  light  forming  each  image  is  only  half  the  size, 
and  the  resolution  or  power  of  depicting  fine  detail  is  reduced 
thereby ;  thus  this  method  of  making  a  binocular  microscope 
reduces  its  power  of  resolving  fine  detail. 

The  new  Beck  binocular  acts  on  a  different  principle.  Above 
the  object  glass  is  a  prism  shaped  as  shown  in  Fig.  104.  The 
whole  of  the  light  from  the  object  glass  0  passes  through  the 
surface  of  the  glass  B  A  to  a  surface  E  A,  which  is  coated  with  a 
semi-transparent  surface  of  silver.     This  allows  part  of  the  light  to 


THE  MICROSCOPE  STAND 


111 


Fig.  104. 


pass  througli  and  part  to  be  reflected  into  tlie  second  tube  of  the 
microscope  as  shown  by  the  dotted  lines,  thus  the  full  size  beam 
goes  to  form  each  image  and  no  lack  of  resolution  occurs  ;    two 
perfect  pictures  are  produced  with  maxi- 
mum detail,  one  in  each  eye. 

It  may  be  expected  by  those  who 
have  not  followed  the  vast  improvement 
that  has  recently  taken  place  in  optical 
manufacture  that  the  effect  of  light 
passing  through  the  prisms  woidd  injure 
the  quality  of  the  image.  This  is  not 
the  case ;  the  flat  surfaces  can  be  polished 
without  an  error  of  one-millionth  of  an 
inch,  and  no  optical  designer  now  hesi- 
tates to  make  use  of  prisms  in  optical 
instruments  even  of  the  most  exacting 
requirements. 

As  the  transparency  and  reflecting  power  of  the  surface  E  A  Equal 
(Fig.  104)  can  be  regulated  according  to  the  amount  of  silver  that  tio™ 
is  deposited,  the  relative  intensity  of  each  image  can  be  made 
identical,  and  the  right-  and  left-hand  images  are  equal  in  brilliancy. 
As  to  the  intensity  of  the  mental  impression,  it  has  been  urged 
that  when  an  initial  body  of  light  is  divided  into  two  brilliant 
parts  and  one  part  is  sent  into  each  eye  of  the  observer,  the  effect 
of  brilliancy  is  the  same  as  if  the  whole  light  be  directed  into  one 
eye  only.  Certainly  there  is  some  reason  for  this  argument, 
though  it  may  be  an  over-statement  of  the  case.  It  is,  however, 
no  disadvantage  if  a  slightly  stronger  light  is  required  with  a 
binocular  than  a  monocular  microscope.  The  monocular  ob- 
server, in  order  to  more  readily  concentrate  his  attention  on  the 
employed  eye,  is  apt  to  use  an  illumination  that  is  far  too  brilliant, 
to  the  detriment  of  his  eyesight.  In  the  use  of  the  binocular, 
both  eyes  are  equally  stimulated,  and  there  is  no  temptation  to 
use  excessive  illumination,  and  theory  goes  to  show  that  a  low 
illumination  is  more  efficient  for  displaying  fine  detail. 

The  diagram  of  the  binocular  prism  (Fig.  104)  shows  that  the  Equal 
distance  from  the  surface  E  A,  where  the  beam  of  light  is  divided  p^thTfor 
into  two  portions,  to  the  two  eyepieces  is  not  of  equal  length,  both 

the  light  on  the   right-hand  side   has    to  ^°^^^ 
travel  a  distance    G  H  farther   than    the 
light    that   passes    directly   through.      It 
would,  therefore,  not  be  possible  to  focus 
both  beams  of  light  to  the  same  points  in 
the  two  eyepieces ;  if  this  were  not  com- 
pensated,   one   image    would    be    out   of 
focus  when  the  other  was  sharp.     Fig.  105  shows  how  placing 
a  plate  of  glass  in  the  path  of  a  beam  of  light  converging  to 
a  focus  at  A  has  the  effect  of  extending  the  focus  to  B,  and 


A  B 


Fig.  105. 


112  THE  MICEOSCOPE 

is  a  means  of  overcoming  what  would  otherwise  be  a  serious 
error.  It  is  corrected  in  the  Beck  binocular  by  combining 
a  parallel  plate  of  glass  of  the  required  thickness  with  the 
right-hand  prism,  thus  equality  in  the  focus  and  in  the  magnify- 
ing power  of  the  two  images  is  ensured.  The  binocular  prism  is 
carried  in  a  sliding  box  in  the  body  of  the  microscope  (Fig.  102). 
By  sliding  it  out  of  the  optic  axis  the  microscope  is  converted 
into  a  monocular  instrument,  or  by  unscrewing  the  knob  (F, 
Fig.  102)  it  can  be  slid  completely  out  of  the  microscope  for 
cleaning  or  dusting.  It  is  quite  safe  to  remove  the  prism  complete 
in  its  box,  as  it  returns  with  accuracy  to  its  exact  position,  and 
the  adjustment  will  not  be  interfered  with.  Dust  may  be  removed 
from  the  prism  with  a  camel's-hair  brush  or  it  may  be  carefully 
wiped  with  a  silk  handkerchief  or  leather  ;  but  glass  should  never 
be  touched  with  the  fingers,  a  greasy  smear  damages  the  defini- 
tion more  than  a  considerable  amount  of  dust. 

The  fact  that  when  the  prism  moves  to  one  side  the  instrument 
becomes  absolutely  the  same  as  a  monocular  microscope  renders 
this  microscope  equally  useful  for  photography,  drawing,,  mi- 
crometry, or  any  other  purpose. 

Many  believe  that  eventually  the  binocular  will  be  almost 
universally  used,  but  we  recognise  that  at  present  this  opinion 
may  not  be   shared  by  all,  and  that  an  opportunity  of  using 
either  monocular  or  binocular  should  be  provided. 
Short  The  construction  of  this  binocular  renders  it  possible  to  retain 

le^th.  ^^^  short  tube  of  the  compact  monocular  microscope.  This 
binocular  body,  indeed,  can  be  fitted  to  most  of  the  various 
recent  models  of  monocular  microscopes.  When  the  drawtubes 
are  partially  extended,  the  tube  is  of  the  standard  160  mm. 
length,  the  binocular  microscope  is  thus  rendered  as  compact 
and  serviceable  as  the  monocular  type.  In  the  older  types  of 
binocular  microscopes  a  tube  of  about  9  to  10  inches  in  length 
was  required  in  order  to  extend  the  eyepieces  to  the  necessary 
interocular  distance,  but  examination  of  the  diagram  (Fig.  102) 
shows  that,  owing  to  the  peculiar  construction  of  the  prism,  the 
tubes,  instead  of  converging  towards  the  prism,  converge  to 
an  apex  about  3J  inches  below  it ;  thus,  although  the  standard 
angle  of  normal  convergence  is  retained,  the  tubes  need  not 
be  long  to  give  the  required  separation  for  the  eyes.  The 
tubes  converge  at  an  angle  of  about  14°.  This  will  be  found 
in  practice  to  give  absolute  comfort  for  either  long  or  short 
periods  of  working.  The  eyes  are  in  exactly  the  condition 
required  for  reading  a  book. 

Binocular  telescopes  which  are  used  with  the  eyes  looking  out 
horizontally  at  distant  objects  generally  and  correctly  have  their 
two  tubes  parallel,  but  this  is  unsuitable  for  a  microscope.  The 
microscopist  who  uses  his  instrument  alternately  with  examining 
objects  on  the  table  on  which  it  stands  would  find  it  difficult 


THE  MICROSCOPE  STAND  113 

and  tiring  to  constantly  change  the  direction  of  his  convergence, 
such  is  the  force  of  habit  that  the  mere  action  of  bending  the  head 
downwards  induces  the  convergence  of  the  eyes  necessary  for 
examining  near  objects. 

Any  make  of  object  glass  or  eyepiece  of  the  standard  size  Any  make 
can  be  used.     There  are  absolutely  no  special  requirements— a  °^,  ^^^^^* 
revolving  nosepiece,  an  objective  changer,  or  any  form  of  apparatus  eyepiece 
can  be  employed. 

The  interocular  distance  is  varied  by  turning  the  milled  head  The 
on  the  direct  tube  of  the  microscope  (Fig.  108),  this  causes  both  ji'j^^e'*' 
drawtubes  to  move  in  or  out  and  alters  the  distance  between  the 
oculars  from  2  inches  to  2J  inches,  which,  as  the  observer's  eyes 
cannot  be  in  contact  with  the  eyepieces,  represents  interocular 
distances  of  about  2^  inches  to  2f  inches.  The  tube  length  is 
the  standard  160  mm.  at  an  intermediate  position.  For  those 
whose  eyes  are  farther  apart  than  this,  tubes  can  be  so  con- 
structed that  they  give  extra  separation. 

If  the  two  eyes  of  an  observer  are  dissimilar,  the  necessary  lens 
to  render  them  equal  can  be  supplied  in  a  cap  to  fit  over  the 
eyepiece.  This  is  a  better  plan  than  the  separate  focussing 
adjustment  provided  in  a  binocular  telescope,  because  to  effect 
an  alteration  in  focus  by  means  of  the  microscope  eyepiece 
requires  such  a  large  amount  of  motion. 

The  advantages  of  binocular  vision  are  not  only  that  a  Binocular 
stereoscopic  relief  can  be  obtained:  the  rest  to  the  eyes  prevents 
fatigue  and  improves  the  quality  of  the  vision  ;  not  only  is  more 
seen,  but  the  perceptive  faculties  are  much  more  constant.  It 
is  frequently  found  that  after  a  quarter  of  an  hour's  examina- 
tion with  a  monocular  microscope,  the  perception  of  fine  detail 
goes  and  does  not  return  till  after  a  pause.  This  does  not  seem 
to  occur  with  binocular  vision,  or  at  least  to  only  a  slight  degree. 

A  further  and  somewhat  more  serious  consequence  of  mon- 
ocular vision  is  that  the  employed  eye  generally  loses  its  visual 
intensity  of  light.  In  order  to  concentrate  the  attention 
upon  the  employed  eye,  a  stronger  light  than  is  wise  is  often 
used,  and  by  degrees  an  illumination  that  appears  white  to  the 
unemployed  eye  is  only  grey  to  the  other.  Most  microscopists 
who  do  not  force  themselves  to  use  the  two  eyes  alternately  will 
find  that  the  perception  of  light  is  less  with  the  eye  which  has 
been  most  used. 

Doubt  has  been  at  times  expressed  as  to  whether  a  microscope  stereo?  cop ic 
looking  at  an  object  with  a  single  object  glass  can  under  any 
circumstances  give  a  really  stereoscopic  relief.  Those  who  have 
worked  with  a  binocular  microscope  do  not  retain  such  a  doubt,  and 
the  explanation  of  the  phenomenon  is  quite  satisfactory.  Suppose 
that  0  (Fig.  106)  represents  the  objective  and  that  an  object 
at  X  consists  of  a  fine  blade  of  material  placed  on  end,  all  the  light 
from  the  left-hand  of  this  blade  which  enters  the  object  glass  at  all 

8 


vision. 


vision. 


114  THE  MICROSCOPE 

readies  the  left-hand  of  the  lens  only,  and  from  the  right-hand  side 
of  X  reaches  the  right-hand  side  only.  If  the  light  from  the 
lens  0  is  geometrically  divided  and  passed  to  one  eye  at  A,  and 
the  other  at  B,  a  perfect  stereoscopic  picture  will  result,  as  though 
the  eyes  were  looking  on  both  sides  of  a  card  held 
in  front  of  them  in  the  well-known  experiment  on 
binocular  vision.  A  microscope  inverts  the  image,  and 
consequently  to  pass  the  correct  image  to  the  eyes  to 
obtain  the  stereoscopic  relief,  the  light  from  the  right- 
hand  side  of  the  object  glass  must  be  passed  to  the 
left  eye,  and  vice  versa. 

The  first  kind  of  binocular  microscope  described 
(Fig.  99)  bisected  the  beam  of  light  at  the  back  of  the  object  glass 
and  passed  one  beam  to  each  eye,  and  for  long  it  was  supposed 
that  unless  the  beam  were  thus  divided  immediately  behind  the 
back  lens  of  the  object  glass,  no  microscope  could  be  made  which 
would  give  stereoscopic  relief.  By  examining  the  diagram  of  the 
rays  passing  through  a  microscope  as  indicated  in  Fig.  102,  it  will 
be  seen  that  the  rays  of  light  intermingle  after  they  leave 
the  object  glass,  and  at  no  other  place  between  the  lenses  could 
the  right-hand  half  of  the  rays  entering  the  object  glass  be 
separated  from  the  left  half.  It  might  be  done  for  any  particular 
bundle  like  that  indicated  by  the  shaded  portion,  but  not  for  all 
such  bundles ;  a  diaphragm  placed,  for  instance,  over  half  the 
field  half-way  up  the  tube  would  obliterate  almost  all  the  light 
from  one  side  of  the  object,  and  allow  all  to  pass  from  the  other 
side  of  the  object.  It  would  not  obliterate  all  the  rays  that 
enter  from  one  side  of  the  object  glass,  but  would  obscure  half 
the  object. 

It  will,  however,  be  noticed  in  Fig.  102  that  all  the  rays  of 
light,  after  passing  through  the  microscope,  pass  through  a  small 
area  called  the  Kamsden  circle  [zz')  just  above  the  eyepiece. 
This  circular  disc  is  a  picture  formed  by  the  eyepiece  of  the  aper- 
ture of  the  object  glass.  At  this  place  the  light  may  be  divided 
just  as  if  it  were  the  back  of  the  object  glass,  and  if  in  this  place 
a  complete  circular  bundle  of  light  is  received  from  each  eyepiece 
of  a  binocular  microscope  it  is  possible,  by  placing  suitable  dia- 
phragms at  these  points,  to  exclude  from  the  right  eye  all  light 
that  enters  the  object  glass  from  the  right-hand  side  of  each 
point  on  the  object,  and  from  the  left-hand  eye  all  light  that 
enters  the  object  glass  from  the  left-hand  side  of  the  object 
points.  Thus,  two  D-shaped  diaphragms  placed  at  the  positions 
of  the  Ramsden  circles  exclude  from  each  eye  the  correct  portions 
of  light  and  give  the  stereoscope  relief  with  the  same  efficiency  as 
the  first  kind  of  binocular  microscope,  except  for  the  loss  of  light. 
There  is,  however,  a  practical  objection  to  this  procedure.  The 
proper  use  of  the  microscope  is  dependent  on  the  eyes  being  so 
placed  that  these  discs  are  within  the  eye  very  near  to  the  pupils, 


THE  MICROSCOPE  STAND 


116 


Fig.   107. 


and  therefore  such  suggested  diaphragms  cannot  be  placed  in  the 
correct  positions— in  fact,  due  to  the  eyelids  and  eyelashes  of  the 
observer,  they  cannot  even  be  placed  near  the  correct  position. 

But  there  is  another  method  of  stopping  out  the  portions 
required  to  give  a  stereoscopic  picture. 
If  the  eyepieces  be  placed  at  a  slightly 
incorrect  interocular  distance,  the  pupils 
of  the  observer's  eyes  cut  ofi  the  edges  of 
the  two  Ramsden  discs  (Fig.  107),  and 
as  the  stereoscopic  effect  with  a  high- 
power  object  glass  is  generally  exag- 
gerated, a  very  small  movement  is 
sufficient  to  give  perfect  depth  of  vision. 
The  tubes  of  the  new  microscope 
are,  however,  inclined,  and  there  is  no 
necessity  to  vary  the  interocular  dis- 
tance. The  observer  naturally  *  places 
his  eyes  so  that  the  whole  of  the  Ramsden 
discs  (Fig.  107)  enter  the  pupils  of  the  eyes, 

and  obtains  all  the  advantages  as  to  aperture,  resolution,  and  illu- 
mination of  a  monocular  microscope.  Then,  by  moving  his  head 
either  forward  or  backward,  he  cuts  off  with  his  pupils  the  one 
or  other  side  of  the  Ramsden  discs  and  obtains  either  stereoscopic 
or  pseudoscopic  relief  instantly.  The  movement  required  is  scarcely 
over  an  eighth  of  an  inch,  and  the  result  is  that  all  the  advantages 
of  stereoscopic  relief  are  obtained  without  sacrificing  anything. 

The  result  of  the  movement  of  the  head  is  very  astonishing  : 
if  objects  are  being  examined  which  lie  on  different  levels,  one 
point  appears  either  in  front  of  or  behind  another  at  will,  and  the 
position  of  the  observer's  head  indicates  which  is  the  stereoscopic 
or  pseudoscopic  picture. 

The  Beck  high-power  binocular  body  can  be  suppHed  on  any 
of  the  microscopes  illustrated,  either  in  place  of  the  ordinary 
body  or  as  an  extra  interchangeable  body. 

Metallurgical  microscopes  require  certain  special  features  Metai- 
because  almost  all  objects  for  which  they  are  used  require  mfSoscopes. 
illumination  from  above.  A  great  deal  of  their  examination 
is  done  with  high  powers  with  one  or  other  of  the  vertical  illu- 
minators mentioned  on  page  41.  It  is,  therefore,  important  that 
the  beam  of  light  for  the  use  of  these  illuminators,  having  once 
been  adjusted,  should  be  allowed  to  remain  in  a  fixed  position. 
If  the  body  tube  of  the  microscope  to  which  these  illuminators 
are  attached  is  focussed  up  and  down  to  examine  specimens  of 
different  thickness  or  to  enable  different  object  glasses  to  be  used, 
the  illuminator  cannot  be  kept  opposite  to  the  illuminating  beam 
of  light.  Metallurgical  microscopes  must,  therefore,  be  made  in 
a  manner  that  will  overcome  this  difficulty.  The  three  following 
forms  of  microscopes  show  three  methods  of  accomplishing  this. 


Fig.   108. — The  Binocular  High-  and  Low-power  Microscope  as 

appUed  to  Stand  No.  3213. 


116 


Fig.   109. No.  3227,  Metallurgical  Microscope,  with  rack  and  pioion 

focussing  stage,  object  glass,  and  vertical  illuminator. 

117 


118 


THE  MICEOSCOPE 


The  Beck-Kowley  Metalliirgical  Attacliment  (Fig.  110)converts  the 
Standard  London  Microscope  into  a  metallurgical  instrument  by  the 
use  of  prisms.  The  Standard  Metallurgical  Microscope  (Fig.  109) 
is  a  model  of  the  Standard  London  Microscope  in  which  the  stage  of 
the  microscope  is  not  fixed  to  the  limb  of  the  instrument,  but  is 
carried  in  a  strong  slide,  and  can  be  focussed  up  and  down  by 

means  of  a  rack  and 
pinion,  so  that  the 
focussing  can  be  done 
by  the  stage  and  not 
by  the  body.  It  does 
not  detract  from  the 
performance  of  the  in- 
strument for  other  pur- 
poses, and  when  racked 
up  to  the  correct  posi- 
tion will  work  with  the 
mechanical  stage  of  the 
standard  microscope. 
It  can  also  be  supplied 
with  any  of  the  standard 
substages,  although  for 
purely  metallurgical 
purposes  a  substage  is 
not  required. 

The  third  method 
(Fig.  Ill)  has  an  elec- 
tric light  fixed  to 
the  body  tube  of  the 
microscope  which  moves 
up  and  down  with  the 
illuminator  as  it  is 
focussed. 

The  Beck  -  Rowley 
Metallurgical  Attach- 
ment converts  an 
ordinary  microscope 
into  an  efficient  metal- 
lurgical instrument 
The  attachment  may  be  readily  attached  or  removed  without 
any  alteration  to  the  microscope. 

With  this  illuminator  the  light  is  projected  along  the  tilting 
axis  of  the  microscope,  and  from  thence  by  means  of  prisms  into 
the  vertical  illuminator ;  when  this  method  is  employed  the 
microscope  tube  can  be  racked  up  and  down  for  focussing  in  the 
ordinary  way,  and  the  inclination  of  the  microscope  can  be 
effected  without  in  any  way  interfering  with  the  original  accuracy 
of  iUumination. 


The  Beck- 
Rowley 
Metal- 

Khment.FlG.    llO.-No.    3225  ^^        .      ^    ^  ^ 

Microscope,  and  Metallurgical  Attach- 
ment. 


Standard    London 


THE  MICROSCOPE  STAND  119 

It  will  be  seen  from  the  illustration  tliat  the  attachment  con- 
sists of  two  pieces,  one  screwing  on  to  the  body-tube  of  the 
microscope,  and  including  the  vertical  illuminator  (D),  and  the 
other  fitting  into  the  standard  substage  or  understage  and  held 
in  place  by  the  screw  H. 

Light  from  any  desired  source  is  projected  into  the  prism  A 
and  reflected  into  the  prism  B,  and  from  thence  to  the  prism 
C  and  again  into  the  vertical  illuminator,  where  the  light  is 
reflected,  by  the  thin  glass  reflector  D,  downwards  through  the 
object  glass  to  the  metal  surface  to  be  examined. 

A  removable  lens  is  fitted  at  G  which  can  focus  the  iris  dia- 
phragm (F)  upon  the  object  and  enables  all  extraneous  light  to 
be  cut  ofi;  a  holder  (E)  for  light  filters  and  ground  glass  is 
placed  immediately  behind  the  iris  diaphragm  and  enables  the 
diaphragm  to  be  used  as  the  light  source.  The  iris  diaphragm  and 
ground  glass  can  be  moved  so  that  it  can  be  focussed  upon  the 
object,  thus  giving  so-called  "  critical"  illumination. 

The  reflector  in  the  vertical  illuminator  is  readily  removed  for 
cleaning  or  replacement.  A  thin  glass  and  a  thicker  parallel 
glass,  a  green  glass  and  a  ground  glass  are  supplied  with  each 
instrument. 

For  geology  and  mineralogy  the  illuminator  will  also  be  found 
of  value  in  the  examination  of  polished  specimens  of  ores  and  rocks. 
The  fact  that  objectives  of  different  powers  can  be  used  and 
focussed  without  interfering  with  the  adjustment  of  the  light  is 
of  special  importance  in  the  examination  of  opaque  metalliferous 
minerals. 

The  bench  metallurgical  microscope  (Fig.  Ill)  has  no  pillar  and  The  bench 
base.  It  has  a  limb  carrying  the  body  with  the  usual  coarse  and  fine  ^r^!^i 
adjustments  fixed  to  a  large  square  stage.  This  stage  is  carried  on  microscope. 
four  levelling  screws  one  at  each  corner  of  the  stage.  The  micro- 
scope can  be  stood  upon  a  table  or  bench  and  used  in  the  ordinary 
way  with  specimens  placed  on  the  stage,  or  it  may  be  placed  on  a 
large  metal  or  other  surface,  and  the  surface  examined  by  focussing 
the  object  glass  down  through  the  aperture  in  the  stage.  To  render 
this  microscope  convenient  for  metallurgical  work  a  metal  filament 
electric  lamp  is  attached  to  the  illuminator  and  is  provided  with  a 
pair  of  light-tight  tubular  covers.  It  moves  up  and  down  as  the 
microscope  is  focussed,  thus  allowing  the  instrument  to  be  focussed 
without  interfering  with  the  illumination.  The  prism  illuminator 
is  supplied  with  this  microscope  because  it  is  more  suitable  for 
low  powers  and  almost  equally  good  for  high  powers.  This  in- 
strument is  useful  for  many  other  purposes,  including  the  Brinnel 
test,  in  which  case  a  scale  is  fitted  into  the  eyepiece.  The  electric 
filament  lamp  can  be  used  on  any  voltage  from  100  to  250  volts, 
and  on  direct  or  alternating  currents.  It  is  troublesome  to  use  a 
lamp  of  low  voltage  which  requires  accumulators,  but  for  those 
who  have  6-  or  12-volt  accumulators  suitable  lamps  can  be  supplied. 


120 


THE  MICROSCOPE 


In  the  tubular  portion  connecting  the  lamp  to  the  illuminator  there 
are  two  slots  into  which  colour  screens,  ground  glass,  or  a  focussing 
lens  can  be  dropped,  and  a  ground  glass,  a  green  glass,  and  a 
lens  are  supplied  with  the  microscope  for  the  purpose. 


Fig.   111. — No.  3226,  Metallurgical  Bench  Microscope  with  prism 
illuminator  and  electric  lamp  and  fittings. 


Pefcroiogicai  A  pctrological  microscopc  is  essentially  an  ordinary  microscope 
microscopes.  pjQyj(jg(j  ^j^j^^  ^  number  of  special  adjustments  and  appliances 
for  the  study  of  rocks  and  crystals.  The  most  important  of  these 
additions  are  a  polarising  apparatus  and  a  rotating  stage.  A 
polarising  apparatus  consists  of  a  Nicol  prism  made  of  Iceland 
spar  which  must  be  placed  below  the  object  to  be  examined  and 
a  similar  but  somewhat  smaller  prism  which  must  be  placed 
above  the  object.  At  least  one  of  the  prisms  and  the  object 
must  be  capable  of  rotation,  and  the  amount  of  the  rotation 
determined  on  a  scale.    There  must  be  a  means  of  rapidly  throwing 


THE  MICROSCOPE  STAND  121 

out   of  the    axis  one,    or   preferably    both  prisms,  so   that   an 
immediate  change  from  polarised  to  ordinary  light  may  be  made. 

The  eyepiece  must  be  provided  with  cross  lines  for  measuring 
the  angles  of  crystals  by  setting  first  one  and  then  the  other 
edge  against  one  of  the  lines  and  measuring  the  angle  of  rotation 
of  the  stage  required  to  effect  such  setting.  As  the  various 
object  glasses  and  their  mountings  are  never  perfectly  inter- 
changeable, a  rotating  stage  must  be  provided  with  centring 
adjustments,  so  that  the  axis  of  rotation  can  be  made  to  exactly 
coincide  with  the  optic  axis.  The  Sloan  object  glass  changer 
described  on  page  20  is  a  very  useful  appliance  for  adjusting 
individual  object  glasses,  and  is  far  preferable  to  a  double  or 
triple  nosepiece  for  rapidly  changing  them.  The  prisms  should 
be  provided  with  spring  clips  so  that  as  they  are  rotated 
the  positions  when  the  prisms  are  "  crossed  "  may  be  felt.  The 
lower  prism,  called  the  polarising  prism,  should  be  large  enough 
to  enable  the  back  of  the  condenser  or  the  object  to  be  fully 
illuminated,  but  its  size  is  determined  to  some  extent  on  the  supply 
of  Iceland  spar,  which  cannot  always  be  obtained  in  large  crystals. 
The  upper  prism,  called  the  analysing  prism,  need  not  be  so  large, 
and  may  be  fitted  in  one  of  three  positions — either  immediately 
over  the  object  glass,  in  the  interior  between  the  two  lenses  of  the 
eyepiece,  or  over  the  top  of  the  eyepiece.  If  it  is  immediately 
over  the  object  glass  it  makes  a  slight  change  in  the  exact  focus 
of  the  microscope  when  pushed  in  and  out  unless  furnished  with 
a  compensating  lens  or  block  of  glass,  which  is  seldom  fitted,  as 
most  observers  object  to  the  introduction  of  an  extra  optical 
element,  when  there  is  no  real  necessity.  With  the  analyser  in 
this  position  the  use  of  a  quartz  wedge  is  less  convenient  than  in 
the  other  two  forms.  In  both  the  other  forms  the  quartz  wedge 
fits  through  a  slot  which  is  in  the  focus  of  the  upper  lens  of  the 
eyepiece.  The  analysing  prism  in  the  interior  of  the  eyepiece 
is  probably  the  most  convenient  form,  because  if  fitted  above  the 
eyepiece  its  considerable  thickness  prevents  the  eye  from  being 
placed  in  the  eyepoint  of  the  microscope  (see  Fig.  1,  page  9) 
and  seriously  restricts  the  field  of  view. 

The  interference  rings  and  brushes  of  crystals  are  formed  if 
a  wide-angle  cone  of  light  be  made  to  pass  through  the  object 
by  a  small,  specially  made  substage  condenser  and  if  this  wide- 
angle  cone  of  light  be  collected  by  a  wide-angle  object  glass.  A 
1/6-inch  (4-mm.)  is  generally  used  for  this  purpose.  The  image 
of  these  rings  and  brushes  is  formed  at  the  back  focus  of  the 
object  glass  very  close  to  the  back  lens  of  the  latter,  and  there  are 
three  methods  of  observing  them.  In  the  microscope  in  which 
the  analysing  prism  is  immediately  over  the  object  glass,  the  eye- 
piece may  be  taken  out  and  the  eye  placed  two  or  three  inches 
away  from  the  upper  end  of  the  tube  of  the  microscope.  The 
image  will  then  be  seen,  but  it  will  be  very  small.     If  the  analysing 


122  THE  MICROSCOPE 

prism  is  not  in  this  position  this  method  is  not  permissible,  because 
removing  the  eyepiece  removes  the  analysing  prism,  which  is  an 
essential  to  the  image  being  formed.  When  an  eyepiece  analysing 
prism  is  used,  a  lens  known  as  a  Bertrand  lens  may  be  screwed  into 
the  lower  end  of  the  drawtube  or  placed  into  the  body  through 
a  special  slot  made  for  the  purpose,  and  the  drawtube  pushed  up 
and  down  until  the  image  is  clearly  focussed.  The  Bertrand 
lens  converts  the  drawtube  into  a  low-power  microscope  which  is 
focussed  to  give  a  sharp  image  of  the  back  focal  plane  of  the  object 
glass,  and  a  magnified  image  is  obtained.  This  method  sufiers 
from  the  inconvenience  that  the  drawtube  must  generally  be 
removed  to  put  in  the  Bertrand  lens,  and  that  it  is  troublesome 
to  use  a  sliding  drawtube  in  a  petrological  microscope,  as  it  may 
interfere  with  the  accuracy  of  the  crossed  position  of  the  prisms. 
The  best  method  of  observing  the  rings  and  brushes  is  by  means 
of  a  small  microscope  called  a  Becke  lens,  which  fits  on  to  the 
top  of  the  eyepiece  and  gives  a  highly  magnified  image  of  the 
eyepoint  or  Ramsden  circle  of  the  microscope  (Fig.  1,  page  9). 
The  image  of  the  rings  and  brushes  is,  as  previously  mentioned, 
in  the  back  focal  plane  of  the  object  glass,  but  this  is  reproduced 
by  the  eyepiece  in  the  eyepoint,  and  it  may  be  examined  equally 
well  in  this  position.  The  use  of  the  Becke  lens  does  not  interfere 
with  any  of  the  adjustments  of  the  instrument,  and  is  to  be 
preferred  to  any  other  plan. 

Petrological  microscopes  cannot  be  thoroughly  explained 
without  considerable  discussion  of  the  theory  of  polarised  light, 
which  is  not  attempted  in  this  book.  There  are  excellent  books 
on  Petrology  to  which  the  student  is  referred,  and  to  whom 
the  following  technical  description  of  a  petrological  microscope 
will  then  appeal, 
standard  The  Standard  London  Petrological  Microscope  is  made  in  two 

m1cro3c?pe!  ^^rms  (Nos.  3222  and  3223).  Both  forms  have  the  rack  and 
pinion  spiral  coarse  adjustment  and  the  double- speed  fine  adjust- 
ment of  the  Standard  London  Microscopes  ;  both  have  a  circular 
rotating  stage  divided  in  degrees  and  cross-finder  divisions  on 
the  surface  with  centring  screws  to  set  the  axis  of  rotation  in  the 
optic  axis  ;  they  both  have  cross-wires  to  the  eyepieces,  a  polariser 
in  a  swing-out  fitting  below  the  stage,  and  a  wide-angle  series 
of  converging  lenses  in  a  sliding  fitting  in  the  stage.  This 
condenser  can  also  be  fitted  in  an  independent  swing-out  and 
focussing  arm,  which  enables  the  condenser  to  be  thrown  out  of 
the  optic  axis  in  a  manner  similar  to  the  polariser.  The  polariser 
is  provided  with  spring  clicks  at  positions  of  crossed  prisms  and 
lines  at  parallel  positions.  No.  3222  has  an  analyser  in  a  push- 
out  fitting  above  the  object  glass  at  the  lower  end  of  the  body. 
Below  this  is  a  slot  covered  by  a  revolving  tube  when  not  in  use, 
for  the  insertion  of  mica  or  quartz  plates.  A  Becke  lens  slides 
over  the  eyepiece  for  examining  the  rings  and  brushes  of  crystals. 


No.  3223. 


Fig.   112. 
No.   3222,  Petrological  Microscope  Nosepiece  Analyser. 
No.  3223,  Petrological  Microscope  Eyepiece  Analyser. 


123 


124  THE  MICROSCOPE 

No.  3223  has  the  analysing  prism  in  a  revolving  fitting  within  the 
eyepiece.  It  is  a  form  of  the  Abbe  prism,  devised  by  Mr.  E.  M. 
Nelson,  which  pushes  in  and  out  of  position.  Its  great  advantage 
is  that  it  gives  the  full  field  of  view.  Its  only  disadvantage  is 
that  in  certain  circumstances  a  faint  second  image  of  the  cross-lines 
can  be  observed,  but  this  is  of  no  practical  disadvantage.  It  is 
provided  with  spring  clicks  at  positions  of  crossed  Nicols  and  lines 
at  parallel  positions.  Below  the  analysing  prism  a  slot  is  pro- 
vided for  the  insertion  of  a  quartz  wedge  or  mica  plate.  The  top 
lens  of  the  eyepiece  is  provided  with  an  adjustment  for  focussing 
to  either  the  quartz  wedge  or  the  cross- wires,  and  a  Becke  lens  is 
provided,  fitting  over  the  eyepiece,  for  examining  the  rings  and 
brushes  of  crystals. 

The  whole  eyepiece  pushes  into  the  drawtube  with  a  pin 
fitting  into  a  slot  so  that  the  position  of  crossed  Nicols  may  be 
correct  when  the  prisms  are  set  in  their  clicked  position.  On 
either  side  of  the  clicked  position  a  line  is  marked  on  the  flange 
of  the  eyepiece  which  is  2J°  away  from  the  true  position  for  the 
total  extinction.  By  setting  the  analyser  to  these  positions  a 
better  determination  of  the  extinction  can  sometimes  be  obtained. 
A  shutter  with  a  series  of  apertures  is  provided  which  can  be 
introduced  into  the  field  of  view  to  cut  ofi  all  parts  of  the  field 
except  the  centre.  A  slot  is  provided  at  the  lower  end  of  the 
polariser  fitting  for  the  insertion  of  a  plate  with  a  fine  aperture 
and  a  slit,  for  the  testing  of  refractive  index  by  the  Becke  shadow 
test. 
Oircuiar  ^  A  circular  mechanical  stage  (page  52)  can  be  fitted  to  either  of 
the  above  instruments,  and  all  apparatus  of  standard  microscopes 
can  be  supplied,  but  the  substa.ge  apparatus  is  supplied  in  slightly 
longer  mounts  to  accommodate  for  the  extra  thickness  of  the 
mechanical  stage. 


mechanical 
soage. 


CHAPTER  VII 

THE  MICROSCOPE   AS  A  RECREATION 

Science  owes  more  to  the  discoveries  made  with  the  microscope 
than  to  those  made  with  any  other  instrument,  but  it  is  not 
always  appreciated  what  a  fund  of  enjoyment  is  available  to  all 
by  making  use  of  the  addition  to  one's  eyesight  that  the  microscope 
affords.  The  reader  may  have  met  an  enthusiast  who  devotes 
hours  at  a  time  to  gazing  down  the  tube  of  this  instrument,  and 
have  wondered  what  could  so  engross  his  attention.  If  questioned , 
such  an  enthusiast  might  have  explained  that  in  the  stagnant 
ponds  and  ditches  he  had  discovered  numbers  of  curious  and 
amazing  animals — creatures  that  had  been  unobserved  for  thou- 
sands of  years  because  they  were  small — creatures  more  varied 
than  the  inmates  of  the  Zoological  Gardens,  and  of  types  of 
astonishing  originality  and  beauty. 

A  visit  to  a  weedy  pond  with  a  few  bottles,  the  collection  of 
some  of  the  water,  weed,  and  mud,  and  their  examination  under 
the  microscope  will  be  convincing  proof  that  the  enthusiast  was 
correct. 

For  some  time  an  observer  may  be  content  to  watch  these 
new-found  animalcula  and  wonder  at  their  curious  diversity  of 
appearance,  but  the  time  will  probably  arrive  when  he  will 
desire  to  know  more  of  their  habits ;  he  will  then  discover  that 
during  the  last  sixty  or  seventy  years  books  have  been  written 
about  them.  The  first  glance  at  such  books  may  fill  him  with 
dismay ;  they  are  filled  with  long  words  and  terrible  names,  and 
it  would  almost  appear  that  a  new  language  has  been  evolved 
to  describe  these  minute  creatures. 

Further  examination,  however,  will  show  that  the  terminology 
is  but  a  thin  veneer  and  that  a  method  is  discernible  in  the 
apparent  madness  of  these  writers.  They  state  that  they  have 
discovered  a  history  of  existence,  which  they  call  development, 
which  shows  how  in  the  ages  that  have  gone,  great  and  complex 
animals— perhaps  man  himself — have  grown  from  simple  and 
minute  beginnings.  The  more  enterprising  of  these  simple  forms 
have,  they  say,  from  time  to  time,  altered  their  characteristics 
and  grown  through  gradual  stages  to  more  complex  forms.  Some 
have  advanced  while  others  have  remained  in  their  original  con- 

125 


126 


THE  MICROSCOPE 


Fig.      113.— 

Amoeba. 


dition.  These  steps  have  not  been  obliterated,  and  amongst 
the  denizens  of  our  ponds  and  ditches  are  to  be  found  specimens 
of  many  of  these  early  phases  of  life — specimens  so  nearly  alike 
that  it  is  quite  possible  to  follow  the  lines  along  which  one  form 

has  developed  into  another.  Such  a  concep- 
tion leads  one  to  examine  the  ponds  and 
ditches  with  a  connected  idea. 

The  class  of  creatures  which  represents  this 
least  complex  form  of  existence  is  called 
Protozoa,  quite  as  simple  a  name  as  kangaroo 
when  you  become  accustomed  to  it,  and  to 
those  who  remember  their  classics  a  much  more 
descriptive  one. 
In  almost  any  pond  with  weed,  a  careful  search  will  produce 
a  creature  called  an  Amoeba,  which  is  the  least  elaborate  piece 
of  living  animal  matter  known.  One  calls  it  a  piece  of  living 
matter,  for  it  is  nothing  more  than  a  morsel  of  jelly,  which  changes 
its  shape  every  minute.  This  jelly  has  no  case  or  skin,  but,  as 
it  does  not  dissolve,  it  remains  separate  from  the  water  like  a 
bubble  of  oil.  It  can  move  its  contents  to  one  end  of  itself, 
thus  increasing  for  the  time  being  that  end  and  diminishing  the 
other,  and  so  it  flows  about  in  any  direction,  altering  its  shape 
to  an  indefinite  extent,  forming  itself  either  into  a  long  projection 
as  a  tiny  trickling  stream,  swelling  out  into  circular  knobs,  or 
doing  both  at  the  same  time.  In  this  way  it  slowly  moves  about 
without,  so  far  as  can  be  seen,  any  fixed  intention ;  and,  as  the  jelly 
of  which  it  is  made  is  filled  with  fine  particles,  the  flowing  of  the 
fluid  creature  can  be  easily  watched.  Besides  these  tiny  particles 
there  are  much  larger  things  rolling  about  within  its  substance. 
These  are  often  recognisable  as  shells  of  diatoms  and  of  other 
tiny  creatures  that  are  to  be  met  with  alive  swimming  about 
in  the  neighbourhood  of  the  Amoeba.  If  the  Amoeba  be  care- 
fully watched,  it  will  be  seen  that  when  it  comes  across  something 
which  appears  suitable  it  begins  to  pour  itself  out  in  three  or 
four  streams  all  around  the  desired  object, 
and  these  streams,  as  they  meet  round  the 
victim,  join  together.  The  object  thus 
caught  and  enclosed  remains  in  the  jelly, 
where  it  is  slowlv  dissolved. 

The  Amoeba  feeds  by  literally  putting 
itself  outside  its  food.  When  the  victim 
has  been  dissolved,  the  hard  and  insoluble 
parts  are  allowed  to  escape  back  into  the 
water,  and  the  portion  that  is  assimi- 
lated goes  to  increase  the  size  of  the  jelly.  It  is  not,  how- 
ever, correct  to  say  that  this  creature  consists  of  nothing  but 
the  granular  jelly  filled  with  the  remains  of  the  things  it  has 
absorbed.     It  has  two  primitive  organs — one,  a  small  spot  of 


Fig.   114.— Villous 
Amceba. 


THE  MICROSCOPE  AS  A  RECREATION 


127 


Fig.   115.~Difflugia. 


somewhat  darker  and  harder  material,  is  always  present  and  is 
essential  to  life.  What  part  it  plays  is  unknown ;  it  appears 
to  be  a  kind  of  vital  spark,  and  is  called  the  Nucleus.  The  other 
organ  is  notliing  more  or  less  than  a  good-sized  bubble,  called 
the  Vacuole. 

The  Amoeba,  the  simplest  form 
of  animal  that  exists,  is  so  colour- 
less and  so  transparent  that  every- 
thing going  on  in  its  interior  is 
visible.  Its  structure  can  be 
understood  at  a  glance,  and  start- 
ing from  this  simple  form  we  can 
find  creatures  varying  from  each 
other  but  slightly,  which  show 
step  by  step  an  almost  com- 
plete series  of  stages  of  development  up  to  elaborate  organisms. 
For  instance,  there  is  one  species  of  Amoeba  which  has  one 
end  of  its  body  hardened  into  an  unchanging  shape — just  one 
corner  only  around  which  some  of  the  jelly  has  hardened  up  at  the 
edge,  showing  the  commencement  of  the  development  of  a 
covering,  while  the  rest  of  the  creature  is  exactly  like  its  simpler 
brother,  having,  with  the  exception  of  this  little  corner,  no  fixed 
shape,  but  pouring  about  as  before. 

The  next  shape  is  reached  in  the  Difflugia.  It  is  an  Amoeba 
and  possesses  the  same  curious  means  of  engulfing  food ;  but  when 
in  the  course  of  its  meals  it  gets  outside  pieces  of  sand  or  similar 
indigestible  material,  it  retains  them,  fixing  them  around  the  surface 
of  its  body  until  a  cap  is  formed  and  only  a  small  portion  of  the 
jelly  is  left  free.  These  particles  are  cemented  together  with 
some  of  the  hardened  jelly,  and  form  a  rough  shell  in  the  shape  of 
an  egg  with  one  end  broken  off.  From  this  open  end  the  creature 
flows  in  irregular  projections  of  jelly  to  catch  food,  and  crawls 

about  carrying  the  shell  on  its  back. 
It  seems  to  have  a  power  of  selection 
as  to  the  size  and  shape  of  the  grains 
that  will  form  a  satisfactory  shell, 
and,  although  there  is  not  a  perfect 
regularity  in  its  construction,  it  is 
evidently  not  left  entirely  to  chance. 
A  further  development  in  the 
direction  of  producing  a  protective 
covering  is  shown  in  the  beautiful 
Heliozoa.  In  this  case  a  spherical 
shell  is  deposited,  perforated  with 
tiny  holes,  through  which  fine  rays  of  jelly  exude  in  the 
form  of  delicate  filaments.  Here  the  shell  is  not  built  up 
of  pieces  of  sand,  but  is  probably  formed  of  the  products  of 
digestion. 


Fig.  116. — Heliozoa. 


128 


THE  MICROSCOPE 


Fig.   117. — ^Foraminifera. 


The  Foraminifera  are  from  a  structural  point  of  view  similar 
to  the  Heliozoa,  being  morsels  of  jelly  having  the  power  of  forming 

round  themselves  shells  of  chalk 
extracted  from  their  food  and  the 
water  in  which  they  live.  These 
shells  take  myriads  of  different 
forms,  but  have  one  thing  in  com- 
mon :  they  are  perforated  with 
multitudinous  holes  through  which 
slender  threads  of  jelly  exude.  To 
this  family  belong  the  shells  which 
form  chalk.  Innumerable  numbers 
of  these  tiny  creatures  fall,  as  they 
die,  to  the  bottom  of  the  ocean, 
forming  there,  in  the  course  of  ages, 
a  layer  of  chalk  which  may  later 
be  raised  by  volcanic  action  above  the  sea-level. 

Such  examples  illustrate  the  gradual  development  of  a  shell, 
the  creature  in  every  other  respect  retaining  its  original  simplicity. 
We  can  now  trace  development  in  a  different  direction 
leading  to  more  complex  creatures  endowed  with 
locomotion.  The  jelly  or  protoplasm  of  which  the 
living  animal  is  formed  appears  to  slightly  harden 
all  round  its  borders,  and  a  creature  of  a  more  or 
less  definite  shape  is  produced,  still  very  elastic 
and  capable  of  retracting  or  extending  itself  to  per- 
haps three  times  its  normal  length. 

It  has  a  somewhat  pointed  end,  and  the  margin    fig.  118. 

of  its  body  is  still  sufficiently  soft  to  enable  it  to  Trypano- 
feed  by  absorbing  into  its  substance  through  any  some, 
portion  of  the  surface  small  particles  of  food,  but 
it  cannot  get  outside  such  large  things  as  the  Amoeba.  This 
is  the  creature  which,  if  it  finds  its  way  into  the  blood  of 
animals  or  men,  causes  in  one  case  the  tzetze-fly  disease  and  in 
the  other  the  dread  sleeping-sickness,  and  it  is  known  as  the 

Trypanosome. 

A  further  stage  shows  the  de- 
velopment of  a  fiagellum,  or  whip, 
which  is  formed  by  the  drying  up 
and  hardening  of  the  pointed  end  of 
the  body.  The  fiagellum  vibrates,  and 
by  its  aid  the  creature  can  swim 
about  with  considerable  rapidity.  In- 
numerable forms  of  these  Flagellata 
are  found  in  all  decaying  matter,  and 
their  activity  is  surprising.  Some  of 
them  have  further  extended  their  cell  wall  into  a  sucker,  by 
which  they  attach  themselves  to  some  fixed  object,  and  whole 


Fig.  119. 


-Flagellata. 


THE  MICROSCOPE  AS  A  RECREATION  129 

colonies  of  such  Monads,  as  they  are  called,  are  to  be  found  on 
weeds,  ceaselessly  lashing  the  water  with  their  flagelia,  causing 
a  current  which  brings  particles  of  food  within  their  reach. 


Fig.     120.— 

Monad. 


Fig.   121.— Collarerl 
Monad, 


Fig.   122.— Collared 
Monad  in  Shells. 


A  further  elaboration  of  this  cell  wall  is  found  in  the  Collared 
Monads,  which  are  possessed  of  a  transparent  cup  made  from  an 
extension  of  the  hardened  margin  of  their  body.  In  the  centre  of 
this  the  flagellum  vibrates,  bringing  a  steady  flow  of  water  into 
this  cup  or  collar.  This  is  the  simplest  form,  but  in  a  more 
complicated  one  these  Collared  Monads  have  provided  themselves 
with  transparent  shells  of  most  elegant  forms,  to  the  bottom  of 
which  they  anchor  themselves.  They  retreat  right  into  them 
for  protection  from  danger,  but  are  found  extended  when  engaged 
in  finding  their  food. 

Thus  a  series  of  creatures  are  met  with  which  possess  a  shell 
of  the  same  simple  type,  consisting  of  nothing  but  a  piece  of  jelly 
with  a  nucleus  and  a  bubble,  but  showing  great  diversity  of^form 

as    regards    the    struc- 
ture of  the  wall  of  the 

cell  in  which  the  jelly 

is  contained. 

The  development  of 

a  single  Flagellum  has 

been   traced,    but  now 

we  come  to  the  Ciliata, 

which  have  rows  of  hairs. 

If  we  imagine  the  soft, 

jelly-like  exudations  of 

the     Heliozoa     to     be 

hardened   and  given  a 
vibratile  motion,  we  have  the  simplest  form  of  Ciliate,  just  a  tiny 
ball  with  rapidly  vibrating  hairs  all  over  it,  these  giving  it  a  con- 
tinuously rolling  movement.   Myriads  of  such  creatures  in  different 
forms  exist,  some  briUiantly  coloured,  some  perfectly  transparent. 


Fig.  123.— Ciliata. 


Fig.   124.— 
Vorticellae. 


130 


THE  MICKOSCOPE 


c 

Fig.  125.— 
Stentor. 


The  Vorticellse  are  a  particularly  lovely  family  resembling 
groups  of  dainty  lilies.  They  have  a  circle  of  vibrating  hairs 
around  the  mouth  of  a  bell-shaped  body,  and  are  anchored  down 
by  a  long  stalk.  If  there  is  a  sudden  shock  and  they  are  alarmed, 
the  stems  shut  down  like  corkscrews,  and  down  they  go  in  a 

flash,  taking  refuge  till  the  danger  is  over,  and 
coming  out  slowly  and  carefully  a  few  moments 
later. 

A  somewhat  similar  species,  the  Stentor,  has 
a  horn-shaped  body,  with  a  powerful  ring  of 
hairs  around  its  upper  surface.  It  is  a  most 
voracious  animal  and  eats  almost  anything  that 
is  brought  to  it  by  the  strong  current  of  water 
which  its  vibrating  hairs  set  up.  One  species 
somewhat  like  the  Stentor  has  a  brown  shell  in 
which  it  lives.  This  is  fitted  with  a  trap-door 
attached  to  the  body  of  the  creature  in  such  a 
manner  that  when  it  retreats  into  its  cell  or  case 
the  shell  closes  like  the  nest  of  a  trap-door  spider. 
The  Protozoa,  therefore,  display  in  their  living  representa- 
tives what  looks  like  a  fairly  complete  history  of  their  original 
development  as  regards  external  structure,  indicating  the  creation 
of  a  shell  or  covering,  and  the  creation  of  cilia  and  of  swim- 
naing  apparatus.  It  is  now  interesting  to  examine  the  question 
of  their  feeding  from  the  same  point  of  view. 

The  Amoeba  simply  pours  itself  round  and  engulfs  any  object 
it  meets  and  wishes  to  feed  upon,  the  object  being  then  gradually 
dissolved.  Some  of  its  constituents  mix  with  the  jelly  and 
are  absorbed,  and  the  creature  gradually  increases  in  bulk  until, 
being  too  large  for  comfort  or  convenience — if  such  terms  can  be 
applied  to  such  a  primitive  creature — it  splits  itself  into  two  parts, 
each  of  which  is  a  perfect  animal. 

Those  portions  of  the  food  which  are  insoluble 
are  allowed  to  escape  from  the  jelly,  but  there  are 
other  portions  which,  although  dissolved,  are  not 
suitable  or  required  for  nourishment  and  growth. 
Water  is  also  taken  in  with  the  food  particles.  It 
would  not  do  for  the  Amoeba  to  be  constantly 
filling  itself  up  with  useless  material,  neither  would 

it  be  satisfactory  for  it  to  be  continuously  diluting    -p^^    ^^q 

itself.  ^  Trap  -  door 

Some  means  must  be  found  to  get  rid  of  these  Animalcule, 
waste  products,  and  the  means  employed  are  ex- 
tremely simple.  The  water  and  the  unnecessary  products  of 
the  dissolving  process  form  into  a  bubble,  and  as  soon  as  such 
a  bubble  approaches  conveniently  near  to  the  surface  of  the 
animal,  it  bursts,  discharging  its  contents  into  the  surrounding 
water.      This  is  certainly  the  simplest  form  of    digestion  that 


THE  MICROSCOPE  AS  A  RECREATION  131 

can  be  imagined,  but  it  fulfils  all  the  necessary  functions,  and, 
moreover,  the  constant  introduction  of  water  into  different  parts 
of  the  jelly  tends  to  supply  the  necessary  oxygen  to  keep  the 
animal  healthy. 

This  Contractile  Vacuole,  as  the  bubble  is  named,  is  the  earliest 
germ  of  both  a  digestive  and  a  respiratory  system,  and  we  shall 
now  see  how  from  this  simple  commencement  a  gradual  growth 
in  complexity  can  be  traced. 

The  bubble  of  the  Amoeba  forms  at  any  convenient  position 
within  its  substance,  and  in  some  of  the  Protozoa  several  of  such 
bubbles  form ;  but  the  second  development  is  to  be  found  in  those 
allied  creatures  in  which  the  bubble  is  always  in  the  same  place 
in  the  same  species,  and  the  water  drains  from  the  rest  of  the  body 
to  that  spot. 

In  the  next  stage  the  bubble  is  no  longer  a  perfect  sphere, 
but  has  one  or  more  extensions,  until  in  the  final  stage  there  are 
minute  channels  all  over  the  creature  which  communicate  with 
the  main  bubble,  thus  creating  a  complex  drainage  system,  and 
this  is  as  far  as  digestion  is  developed  in  the  Protozoa. 

The  development  of  a  mouth  presents  features  of  equal 
interest. 

The  Amoeba  has  no  mouth — it  does  not  eat,  it  engulfs. 

The  Difflugia  is  similar,  but  its  area  of  action  is  reduced  by 
the  fact  that  a  large  portion  of  its  body  is  enclosed  in  a  shell. 

The  Heliozoa  also  have  no  special  feeding  organ.  When  an 
object  becomes  entangled  in  their  fine  filaments  of  jelly,  it  may, 
if  very  small,  find  its  way  for  digestion  into  the  interior  of  the 
shell,  but  as  likely  as  not  the  fine  rays  may  join  up  around  the 
object  outside  the  main  body,  and  digestion  will  proceed 
there  just  as  weU  as  in  the  interior. 

There  is  another  series  of  animals  called  the  Acineta,  which 
somewhat  resemble  the  Heliozoa  in  that  they  are  provided  with 
long,  ray-like  projections  which  are  hard 
or  leathery  except  at  the  tips.  Here  they 
swell  out  into  small  knobs  of  soft  material, 
through  which  small  portions  of  food  can 
be  taken  in.  Such  organisms  may  be  con- 
sidered as  having  hundreds  of  mouths. 

Another    series    shows    a    much    more 
direct  development.     On  these  the  skin  is 
hard  except  in  patches,  where  alone  food 
can  enter;  and  around  these  soft  patches     Fig.  127.— Acineta. 
there   is   usually  a  ring  of  rapidly  vibra- 
ting   hairs,    which     create    a    current,    bringing    the    floating 
particles  into  contact  with  the  absorbent  portion  or  portions  of 
the  body.      The  Vorticellse   (Fig.  124)  have  a  disc-shaped   ab- 
sorbent surface  surrounded  by  the  strong  ring  of  vibrating  hairs 

The  Stentor  (Fig.  125),  which  is  larger  and  a  most  voracioua 


132 


THE  MICROSCOPE 


creature,  has  also  a  large  disc-like  surface  where  there  is  prac- 
tically no  skin  to  its  jelly.  The  cilia  which  vibrate  around  this 
disc  cause  a  powerful  current  to  flow,  and  it  is  amazing  to  watch 
the  smaller  kinds  of  Protozoa  being  hustled  into  the  creature's 
body,  where  they  swim  about  for  a  second  or  two  and  are  then 
still. 

As  a  final  stage  we  find  creatures  with  only  one  small  open- 
ing where  food  can  be  absorbed,  and  the  complete  development 
of  the  mouth  is  here  concluded. 

One  of  the  earlier  figures  shows  the  dainty  little  Collared 
Monad,  which  consists  of  a  single  cell  with  a  vibrating  whip,  or 
flagellum,  and  a  very  perfect  little  cup  or  collar  of  transparent 
material.  They  are  often  found  in  large  colonies  on  the  surface 
of  weeds.  To  them  we  owe  our  sponges.  A  microscopic  examina- 
tion of  one  of  the  holes  of  a  growing  sponge  reveals  a  colony  of 
these  little  organisms,  closely  arranged  aU  round  the  interior 
of  its  surface.     These  have  the  power  of  creating  instead  of 

shells  a  fibrous  material,  which  forms 
the  matrix  in  which  they  are  embedded. 
In  place  of  shells  they  deposit  hard,  flinty 
spines  in  the  substance  of  this  matrix, 
and  these  are  called  spicules.  Thus  a 
sponge  consists  of  myriads  of  colonies  of 
Collared  Monads,  their  vibrating  flagellae 
causing  a  current  of  water  to  rush  through 
every  cavity  of  the  entire  sponge,  in  order 
to  provide  the  food  and  oxygen  necessary 
for  the  support  of  the  community. 

The  Protozoa  show  in  a  series  of  inter- 
esting stages  the  gradual  development  of  creatures  of  one  cell. 
Each  cell  is  complete  in  itself,  though,  as  in  the  case  of  the 
sponge,  an  approach  to  a  more  elaborate  form  is  seen.  Never- 
theless, here  each  organ  eats  for  itself,  breathes  for  itself,  re- 
produces itself  by  splitting  in  half,  and  is  an  individual. 

Later  stages  of  development  show  creatures  of  more  than 
one  cell,  in  which  some  cells  perform  one  function  and  some 
another,  and  none  are  complete  by  themselves ;  and  the  develop- 
ment of  the  simplest  form  of  life  into  a  more  complex  animalcule 
as  indicated  by  a  study  of  the  Protozoa  is  but  an  indication  of 
the  interest  that  can  be  obtained  by  the  use  of  the  microscope. 

The  more  elaborate  and  highly  organised  creatures  met  with 
in  water  have  equal  charm  and  variety.  The  manner  in  which 
they  feed  upon  each  other,  the  manner  in  which  some  become 
parasites,  and  the  methods  of  reproduction,  are  all  subjects  which 
well  repay  investigation.  The  development  of  many  of  the  animal- 
cula  from  the  egg  to  the  finished  and  perfect  creature  has  a  special 
fascination,  because  naturalists  have  discovered  that  in  this 
change  from   stage   to   stage  which   certain  forms  go  through 


Fig.   128.— Sponge. 


Fig.   129. — Holopediura. 


THE  MICROSCOPE  AS  A  RECREATION  133 

there  is  a  history  in  an  abbreviated  form  of  the  stages  througli 
which  the  species  originally  developed.  The  so-called  water- 
fleas,  for  instance,  are  little  crustaceans  Hke  small  shrimps. 
They  are  hatched  out 
from  eggs  as  small  oval 
bodies  with  short  legs,  and 
very  little  else  except  one 
eye.  After  a  time  the 
young  creature  casts  off 
its  skin  and  becomes  rather 
more  elaborate  in  form. 
This  goes  on  stage  by  stage 
till  it  develops  into  a 
creature  with  the  most 
complete  series  of  legs, 
antennae,  tail,  and  other 
appendages.  It  has  as- 
sumed the  appearance  of  a  small  shrimp.  In  some  species  it 
goes  further,  and  after  having  for  a  short  time  lived  a  free  and 
energetic  life  it  develops  into  nothing  but  a  bag  and  suckers, 
which  attach  themselves  to  fishes  and  suck  their  nutriment  from 
the  fish's  body. 

This  points  to  a  degeneration  in  the  development  which  has 
taken  place  in  the  history  of  a  race  who  found  it  less  fatiguing, 
if  less  honourable,  to  live  on  other  people  rather  than  to  fight 
their  own  battle  in  life. 

These  small  Crustacea,    generally  known  as  water-fleas,  are 

one  of  the  chief  foods  of  fish,  both  salt 
and  fresh  water.  They  exist  in  such 
enormous  numbers  in  some  parts  that 
they  even  satisfy  the  appetite  of  the 
whale.  The  sea  is  sometimes  of  a  blood- 
red  colour  due  to  the  myriads  of  a 
coloured  form  of  these  creatures.  There 
is  no  pond  that  has  not  many  varieties, 
and  they  can  be  best  captured  with  a 
collecting  net.  Certain  forms  are  phos- 
phorescent, but  all  are  more  or  less  trans- 
parent, and  can  be  thoroughly  investi- 
gated under  the  microscope.  Fig.  129 
shows  one  form  found  in  the  lakes  of 
Cumberland,  which  is  supposed  to  be  a 
delicacy  beloved  by  the  salmon  trout  and 
the  char.  This  curious  species  is  em- 
bedded in  an  envelope  of  jelly  much 
larger  than  itself.  It  is  quite  transparent.  The  rolling  of  its 
single  eye,  the  beating  of  its  heart,  and  the  digestion  of  its  food, 
can  all  be  watched  under  quite  a  low-power  object  glass. 


I 


Fig. 130 

trepes. 


Bytho- 


134 


THE  MICKOSCOPE 


Another  form  with  a  tail  like  a  long  spine  and  an  eye  that 
fills  most  of  its  head  is  shown  in  Fig.  130. 

The  study  of  the  development  of  the  cell  structure  in  vegetable 
life  is  equally  fascinating — how  cells  which  in  their  simplest 
forms,  having  all  similar  functions,  group  themselves  together 
into  colonies.  Some  of  the  constituents  take  on  certain  functions 
only,  leaving  others  to  accomplish  difierent  work,  until  a  complex 
vegetable  growth  is  built  up  of  cells,  all  of  which  have  their  own 
characteristics. 

The  circulation  of  the  sap  in  plants  can  be  readily  observed. 
The  breathing  apparatus  of  plants  where  they  absorb  carbonic 
acid  and  liberate  oxygen  can  be^  found  on  the 
under-surface  of  most  leaves.  The  hairs  of  plants 
form  a  study  in  themselves.  Fig.  131  shows  the 
hair  of  the  stinging  nettle :  on  the  left  it  is  in  its 
undamaged  condition.  It  has  a  knob  on  the  end, 
and  a  closed  canal  can  be  seen  running  up  the 
centre.  A  light  touch  knocks  off  the  knob,  leaving 
a  sharp  pointed  end  which  will  pierce  the  sldn; 
and  the  canal  being  opened  by  the  removal  of  the 
knob,  the  poison  that  it  contains  can  enter  the 
prick  made  by  its  sharp  point. 

The  seeds  and   pollen   of  plants  are   wonderful 
in  the  elegance   of   their   design   and   the   variety 
of  their  structure. 

The  spore  cases  of  ferns,  with  their  apparatus  like  tiny  spiral 
springs  for  hurling  the  spores  to  a  distance  when  ripe,  can  be 
found  as  brown  patches  on  the  under-surfaces  of  the  fronds. 

Perhaps  nothing  will  create  more  amusement  and  interest 
than  the  examination  of  the  contents  of  an  open  umbrella  after 
it  has  been  held  under  the  bushes,  on  a  hot  summer  day,  while 
the  bushes  are  lightly  beaten  with  a  stick.  No  one  could  have 
imagined  what  a  variety  of  tiny  microscopic  insects  exist  of  which 
most  people  are  entirely  unaware.  The  eyes,  legs,  wings,  pro- 
boscis, and  other  parts  of  the  insects  should  be  examined,  and 
the  habits  of  the  voracious  little  creatures  will  surprise  even  the 
naturalist  who  is  used  to  the  curious  manners  and  customs  of 
the  larger  animals. 

These  few  notes  on  the  employment  of  the  microscope  for 
the  less  serious  subjects  than  those  from  which  it  is  a  necessary 
as  a  scientific  tool,  do  not  do  more  than  indicate  a  few  directions 
in  which  enjoyment  can  be  obtained  from  its  use. 


FiQ.   131.— 
Nettle  Hair. 


17     0     0 


PRICE    OF    INSTRUMENTS   AND   APPARATUS 
DESCRIBED   IN   PREVIOUS  PAGES   (1922) 

MICROSCOPE  STANDS 

No.        Page  £     3.    d. 

3210  97     Standard  London  Microscope,  with  plain  sub- 

stage,  with  iris  diaphragm;   stand  only,   in 

case  .  .  .  ,  .  .  .      10  10     0 

3211  93     Standard  London  Microscope,  with  spiral  screw 

focussing  substage ;   stand  only,  in  case  .      11   15     0 

3212  100     Standard  London  Microscope,  with  rack  and 

pinion  substage  ;    stand  only,  in  case  .  .      13  10     0 

3213  99     Standard     London     Microscope,     with      rack 

and  pinion  centring  focussing  substage ;  stand 

only,  in  case      .  .  .  .  .  .      14  10     0 

3214  104     Standard  London  Microscope,  with  standard 

body,  circular  rotating  stage,  screw  focussing 

substage  ;    stand  only,  in  case     .  .  .14     5     0 

3215  104     Standard  London  Microscope,  with  standard 

body  circular  rotating  stage,  rack  and  pinion 

focussing  and  centring  substage  ;   stand  only, 

in  case      ....... 

3216  103     Standard    London    Microscope,    with    circular 

rotating  stage,  large  2-inch  body,  rack  and 
pinion  focussing  and  centring  substage ; 
stand  only,  in  case     .  .  .  .  .      19     0     0 

3217  103     Standard    London   Microscope,   with   circular 

rotating  stage,  with  double  extension  rack 
and  pinion  drawtubes,  rack  and  pinion 
focussing  and  centring  substage ;  stand  only, 
in  case       .  .  .  .  .  •  .      23     0     0 

—       116     Binocular   body  in  place  in  ordinary  body  on 

Stands 18     0     0 

3221  101     Standard  London  Portable  Microscope,  with 

rack    and    pinion    focussing    and    centring 
swing-out  substage  and  mechanical  stage,  in 
case  ....... 

3227     117     Standard    London   Metalliu-gical   Microscope; 

stand  only 15   10     0 

3225  118     Standard  Microscope,  with  Rowley  metallur- 

gical attachment ;   stand  only,  in  case  .      23     2     0 

3226  120     Metallurgical  Bench  Microscope  Stand,  without 

case,  but  with  vertical  prism  illuminator 
lamp,  with  cover  tubes  and  fittings  complete 
for  plugging  into  220- volt  or  100- volt  circuit.      13   10     0 

3222  123     Standard  Petrological  Microscope,  with  nose- 

piece  analyser,  eyepiece  with  cross-wires, 
polarising  and  analysing  prisms,  converging 
system  of  lenses,  Becke  lens,  mica  J-wave 
plate,  and  Klein's  quartz  plate  ;  stand  only, 
in  case     ...•••• 

135 


28     0     0 


29     0     0 


136 


THE  MICKOSCOPE 


No.        Page 
3223     123 


—       122 


3352     122 


3351 

122 

3350 

122 

3218 

116 

116 

3219 

103 

3219a 


/103 
\  52 


1103 
"    \104 

3200a  104 


3201a  104 


—  116 

—  103 


Standard  Petrological  Microscope,  with  eye- 
piece analyser,  with  cross-wires,  polarising 
prism  ;   stand  only,  in  case .... 

Independent  swing -out  and  focussing  arm, 
which  enables  the  condenser  to  be  swung  out 
of  the  axis         ....  extra 

Becke  lens  and  converging  system  of  lenses, 
in  fitting  in  stage-plate  with  small  apertures 
for  Becke  shadow  test  to  fit  under  polariser, 
and  plate  with  apertures  of  various  sizes 
for  limiting  the  field  to  fit  into  eyepiece  slot    . 

Plain  quartz  wedge,  ungraduated   . 

Quartz  wedge,  cemented  on  gypsum  plate  (red 
1st  order),  graduated  in  Retardations  . 

High-power  Binocular  Micioseope,  with  square 
stage  and  rack  and  pinion  focussing  and 
centring  substage  ;   stand  only,  in  case 

Stand  as  No.  3218,  but  with  detachable 
mechanical  stage        ..... 

High  Binocular  Microscope,  with  plain  circular 
lotating  stage  and  centring  adjustments ; 
stand  only,  in  case      ..... 

Microscope  as  No.  3219,  with  addition  of 
circular  rotating  mechanical  stage  ;  stand 
onlj',  in  case      ...... 

Interchangeable  extra  monocular  large  body, 
with  rack  and  pinion  drawtubes  extending 
to  260  mm.        ...... 

Massive  Model  Microscope,  with  plain,  square 
stage  and  attachable  mechanical  stage,  as 
on  page  99,  Abbe  condenser,  dark-ground 
illuminator  in  interchangeable  mounts,  and 
one  extra  substage  slide  ;   in  case 

Massive  Model  Microscope,  with  mechanical 
stage  as  illustiated  on  page  105,  with  dry  and 
immersion  achromatic  condenser  and  focuss- 
ing dark  ground  illuminator  and  one  extra 
substage  slide  ;   in  case        .... 

Intel  changeable    high-power    binocular    body 

extra 

Double  extension  drawtube,  with  rack  and 
pinion  adjustment      .  .  .  extra 


£    s.    d. 

24     7     0 

1     5     0 


3  15     6 
2  15     0 

4  10     0 


32   10  0 

38  10  0 

35     0  0 

47     0  0 

7     0  0 


44  10     0 


3300 
3301 
3280 

3281 
3282 


3230 
3231 
3232 
3234 
3236 


NOSEPIECE   AND  OBJECT  GLASS   CHANGERS 

20  Dust- tight  double  nosepiece  .... 
20  Dust- tight  triple  nosepiece  .... 
20     Sloan  object  glass  changer,  adapter,  spanner, 

and  two  fittings  ..... 

20     Extra  fittings  for  above  .  .  .  each 

20     Case  to  hold  three  fittings,  with  object  glasses 

attached  in  dust-tight  spring  holders    . 

OBJECT  GLASSES   AND   EYEPIECES 

77     li-in.  Achromatic  object  glass  (32  mm.) . 
77     2/3-in.  Achromatic  object  glass  (16  mm.) 
77     1/3-in.  Achromatic  object  glass  (8  mm.)  ♦ 
77     1/6-in.  Achromatic  object  glass  (4  mm.)  . 
77     1/8-in.  Achromatic  object  glass  (3  mm.),  Oil- 
immersion  ...... 


70 

2 

6 

20 

0 

0 

5 

0 

0 

S 

1 

1 

7 
10 

6 
0 

1 
0 

7 
5 

6 

0 

0  11     6 


2  5  0 
1  10  0 
4     5  0 

3  15  0 

6  17  6 


PRICE   OF  INSTRUMENTS  AND  APPARATUS     137 


No. 

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88 

3270 

88 

3275 

68 

3263 

73 

3264 

73 

3273 

73 

1/12-in.  Achromatic  object  glass  (2  ram.),  Oil- 
immersion  ..... 
li-in.  Apochromatic  object  glass  (40  mm.) 
2/3-in.  Apochromatic  object  glass  (16  mm.) 
2/3-in.  Apochromatic  object  glass  (14  mm.) 
1/3-in.  Apochromatic  object  glass  (8  mm.) 
1/6-in.  Apochromatic  object  glass  (4  mm.'i 
1/6-in.  Apochromatic  object  glass  (4  mm.),  with 

correction  collar  .... 

1/12-in.  Apochromatic  object  glass  (2  mm.),  Oil 

imm.ersion  ..... 

42-mm.   x  6  Huyghenian  eyepiece  . 
25-mm.   x  10  Huyghenian  eyepiece 
17-mm.   X  15  Huyghenian  eyepiece 
45-mm.   X  6  Compensating  eyepiece 
30-mm.  X  8  Compensating  eyejjiece 
22-mm.   X  11  Compensating  eyepiece 
15-mm.  X  17  Compensating  eyepiece 
10-mm.   X  25  Compensating  eyepiece 
Beck  micrometer  eyepiece 
Eyepiece  with  indicator 
Eyepiece  with  cross- wires 
Erecting  eyepiece  .... 


£    s. 


8  10 

0 

4  10 

0 

7  15 

0 

.   7  15 

0 

9  10 

0 

.   11  0 

0 

.   12  0 

0 

.   18  0 

0 

0  12 

0 

0  12 

0 

0  12 

0 

2  2 

0 

2  2 

0 

2  10 

0 

2  10 

0 

2  10 

0 

2  2 

0 

0  18 

0 

0  17 

0 

1  10 

0 

LARGE  SIZE  EYEPIECES  1-41  INCH  DIAMETER  OF  TUBE 


3257 

85 

3253 

88 

3254 

88 

3255 

88 

Eyeshade  ..... 
42-mm.  X  6  Huyghenian  eyepiece  . 
25-mm.  X  10  Huyghenian  eyepiece 
17-mm.  X  15  Huyghenian  eyepiece 


0 

2 

0 

2 

5 

0 

2 

5 

0 

2 

5 

0 

APPARATUS  FOR  ILLUMINATING  OBJECTS 


3285  26 

3285p     33 

3286  26 


3286p     33 
3287       27 


3288       27 


3288p  33 
3291   28 


3284   33 
3295   34 


3296   34 


Small  Abbe  condenser,  fitting  with  iris  dia- 
phragm on  Microscopes  No.  3210  and  3211  . 

Set  of  patch-stops  for  above    .... 

Large  Abbe  condenser  in  fitting  with  iris  dia- 
phragm and  swing-out  tray  for  colour  screens 
with  coloured  and  ground  glass  . 

Set  of  three  patch-stops  for  above  . 

Beck  dry  achromatic  condenser,  1  N.A.,  lenses 
only  in  mount,  with  standard  object  glass, 
screw  thread      ....•• 

Beck  dry  achromatic  condenser,  1  N.A.,  com- 
plete in  fitting  with  iris  diaphragm  and  swing- 
out  tray,  grey  and  green  glass     . 

Set  of  three  patch-stops  for  No.  3288  or  3290  . 

Beck  dry  and  immersion  achromatic  and  apla- 
natic  condenser,  1-3  N.A.,  in  mount,  with  iris 
diaphragm  and  tray  for  patch-stops  and  \yith 
ground  and  green  glass  double-wedge  light 
moderator  ...••• 

Traviss  expanding  iris  patch-stop  .  .  . 

High-power  dark-gromid  illuminator,  optical 
portion  only  in  mount,  with  standard  object 
glass  thread       .  .  •  •  •  • 

Illuminator  as  above  in  plain  substage  fitting   . 


1 

0 

5 

7 

0 
6 

2 
0 

10 

7 

0 
6 

4     5     0 


5   15     0 
0     7     6 


9  15 

0 

0  12 

6 

2  0 

0 

2  10 

0 

No. 

Page 

3297 

34 

3298 

34 

3293 

35 

3294 

35 

3215 

38 

3216 

38 

3360 

40 

3361 

40 

3362 

40 

3363 

41 

3364 

41 

3328 

32 

3366 

43 

3335 

44 

3330 

46 

£ 

B. 

d. 

3 

5 

0 

0 

2 

6 

5 

7 

6 

6 

2 

6 

2 

10 

0 

1 

5 

0 

2 

2 

0 

2 

17 

6 

1 

7 

6 

1 

7 

6 

1 

17 

6 

3 

17 

6 

2 

10 

0 

2 

15 

0 

138  THE  MICROSCOPE 


Uliuninator  as  above  in  centring  substage  fitting 
Stop  to  fit  1/12-in.  oil-immersion  object  glass  to 

reduce  aperture  for  dark-ground  illumination 
Beck  patent  focussing  dark-ground  immersion 

illuminator  in  plain  substage  fitting 
Ditto,  in  centring  substage  fitting  . 
Bull's-eye  condenser,  2|^-in.  diameter,  on  heavy 

stand,  with  full  adjustments 
Bull's-eye    condenser,     H-in.    diameter,    on 

smaller  stand  with  full  adjustments 
Parabolic  reflector  ..... 

Parabolic  reflector  with  Sorby  reflector   . 
Thin  glass  reflector         ..... 

Thin  glass  vertical  illuminator 

Prism  illuminator ...... 

Double-wedge  light  moderator 

Colour  trough        ...... 

Paraffin  lamp        ...... 

Beck  electric  lamp,  complete   with  bull's-eye 

condenser,  ground  glass,  signal-green  glass, 

and  metal  filament  60  candle-power  lamp      .        8   10     0 

3331  46     Beck  electric  lamp  as  No.  3330,  but  with  100 

candle-power  half- watt  lamp        .  .  .900 

3332  46     Beck  electric    ]amp   as    No.    3330,   but    with 

"  Pointolite  "  electric  lamp,  with  resistance  to 

work  off  direct  current  from  100  to  250  volts     14  15     0 

3333  42     Set    of     9     Wratten  &   Wainwright's  colour 

screens,  for  use  with  above  lamps,  com- 
prising :  light  blue,  to  give  daylight  colour. 
No.  78  ;  dark  blue,  dominant  wave-length, 
5,000  ;  blue-green,  dom.  wave-length,  4,700  ; 
light  green,  dom.  wave-length,  5,500  ;  green, 
dom.  wave-length,  5,350;  yellow,  dom.  wave- 
length, 6,000 ;  orange,  dom.  wave-leng-th, 
6,300  ;  red,  dom.  wave-length,  6,500  ;  neutral 
tint,  passing  1/10     . 

—  42     Single  filters,  as  above  .  .  .   each 

—  —     Stand  to  hold  two  filters,  as  above 

—  Circular  screens  to  fit  substage  rings  can  be 

supplied  .....  each 

3337  48     Incandescent  gas  lamp  .... 

—  Extra  mantles       ...... 

3338  48     Incandescent  spirit  lamp        .... 

—  Extra  mantles       .....  each 
3336       45     Electric  lamp  on  stand  to  take  ordinary  bulb  . 

APPARATUS  FOR  HOLDING  SPECIMENS 

3307       51     Sliding  ledge 0  14     0 

3305  99     Detachable  mechanical  stage,  as  illustrated  on 

Microscope  No.  3213  (page  99)    .      .  .600 

3306  52     Concentric  rotating  mechanical  stage,  as  illus- 

trated   12     0     0 

3400  52     Glass  slips,  3x1,  best  quality,  ground  edges, 

approximate  thickness  1  mm.  per  doz. 

per  gross 

3401  62     Ditto,  second  quality,  ground  edges       per  doz. 

per  gross 


3 

12 

6 

0 

6 

6 

1 

10 

6 

0 

5 

6 

2 

5 

0 

0 

1 

0 

2 

17 

6 

0 

1 

0 

1 

10 

0 

0 

0 

9 

0 

8 

6 

0 

0 

7 

0 

6 

6 

PRICE  OF  INSTRUMENTS  AND  APPARATUS     139 


No. 
3405 


Page 
65 


3390   63 


3391 
3392 

3393 
3394 

3395 
3388 

3406 
3409 
3410 
3412 
3413 
3414 
3415 
3416 
3420 
3321 
3421 
3422 
3425 


53 
63 

53 
53 

53 
53 

64 
55 
55 
56 
56 
56 
56 
56 
57 
57 
57 
57 
68 


3386  58 

3325a  59 

3325b  59 

3325c  59 

3325s  61 


3222 

61 

3223 

69 

3224 

59 

3274 

63 

3384 

62 

64 

1290 

64 

1292       64 


Glass  slips,  3x1,  ground  edges  and  excavated 
hollow      .....       per  doz. 

Cover  glasses,  No.    1,  average  thickness  -006, 
circular     .....         per  oz. 

Ditto,  square         ....  per  oz. 

Cover  glasses.  No.  2,  average  thickness  -008, 
circular     .  .  .      '    .  .         per  oz. 

Ditto,  square         ...... 

Cover  glasses.  No.  3,  average  thickness  -01, 
circular     ....... 

Ditto,  square         ...... 

Micrometer  screw  gauge  for  measuring  thickness 
of  cover  glass  slips,  etc.       .... 

Glass  slips  with  ledge   ..... 

Cells,  metal,  circular       .  .  .  per  100 

Cells,   glass.  ^         .  .  .      per  doz. 

Troughs  on  3  X  1  slips,  small 

Troughs  on  3  X   1  slips,  larger 

Troughs  on  3  X   1^  slips 

Tirough,  1|  X   U  X  ^  in. 

Beck's  glass  trough 

Live  box 

Rousselet's  live  box 

Beck  compressor  . 

Stage  forceps 

Case  of  apparatus  for  holding  objects,  including 
3  slips,  2  slips  with  hollow,  slip  with  ledge, 
trough  on  3  X  1  slip,  Beck's  glass  trough. 
Beck's  compressor,  stage  forceps,  live  box 
and  thin  glass    ...... 

Xylol 2  oz. 

Benzol  .  .  .  .  .  .  2  oz. 

Canada  balsam,  pure,  in  benzol  or  xylol       25  gr. 

Hollis  glue per  bottle 

Turntable  .  .  •  •        _  • 

Thoma-Hawksley  Hsemacytometer,  with  two 
pipettes  and  covers  in  case .... 

Ditto,  but  with  Zappert,  Turck,  Fiichs  and 
Rosenthal,  or  Breuer  ruling 

Ditto,  Biirker  model      ..... 

Hsemacytometer  chess-board  pattern  squared 
glass  plate  to  drop  into  eyepiece,  25  squares  of 

1  mm.,  9  squares  of  2  mm.,  or  squares  over 
entire   field  either  I  mm.,  J  mm.,  1  mm.,  or 

2  mm each 

Cotmting  chamber  for  use  with  chess-board 

Pipette  for  red  corpuscles 

Pipette  for  white  corpuscles    . 

Microspectroscope  eyepiece     . 

Simple  warm  stage 

S.I.R.A.  wax,  per  stick  6  X  |  in 

Beck    grinding    and    polishing    machme     for 

preparation  of  metallurgical  specimens; 
standard  machine  without  motor,  with  one 
polishing  disc,  cover,  catcher,  disc-lifter,  1 2 
feet  connecting  wire  plug  adapter,  and  tin 
of  grease  ...•••• 
Extra  poli  shing  discs  .••.•• 
Motors  fitted  at  makers'  current  prices. 


£      B. 


0     1     6 


0 

7 

6 

0 

6 

0 

0 

6 

0 

0 

4 

6 

0 

4 

6 

0 

3 

6 

3 

3 

0 

0 

0 

6 

0 

6 

0 

0 

5 

0 

0 

1 

0 

0 

2 

0 

0 

2 

6 

0 

4 

6 

0 

9 

0 

0 

8 

6 

0 

17 

6 

1 

1 

0 

0 

14 

0 

3  15     0 


0 

1 

6 

0 

1 

6 

0 

2 

6 

0 

1 

0 

1 

I 

0 

o 

10 

0 

2 

12 

6 

3 

10 

0 

0  12  6 

0     2  6 

0     7  6 

0     7  6 


0     7 
0     1 


1    15     0 


6 
6 


21     0     0 
1    15     0 


140 


THE  MICROSCOPE 


No. 

Page 

3429 

65 

3430 

65 

Pipettes 

Pipettes,  glass,  with  teat 


each 


£  s.  d. 
0  0  3 
0  0  6 


3431 
3432 
3433 

3434 
3435 
3436 
3437 
3438 
3439 
3446 
3441 
3442 
3443 
3444 
3445 
3446 
3447 
3448 
3449 

3450 
3451 


0 
0 
0 


3 

0 
6 
6 
6 
6 
6 


DISSECTING    INSTRUMENTS 

Blow-pipes  with  stiletto  .  .  .  .016 

Razor,  hollow  ground  one  side,  flat  the  other   .        0     2     9 

Scalpels,  with  ebony  handles,  in  three  sizes  of 
blade         ......    each 

Needles  in  wooden  handles  with  ferrule    . 

Needles  in  bayonet  shape  in  wood  handles 

Platinum  needles  in  glass  handles   . 

Scissors,  curved    ..... 

Scissors,  elbow      ..... 

Scissors,  straight,  4- inch,  with  fine  points 

Scissors,  straight,  4^  inches   . 

Scissors,  straight,  6  inches 

Seekers  in  ebony  handles 

Chain  hooks  ..... 

Forceps,  without  guide  pin,  4|  inches 

Forceps,  without  guide  pin,  6  inches 

Forceps,  with  guide  pin,  2h  inches  and  4  inches 

Forceps,  Comett's  cover  glass 

Section  lifter,  copper     .... 

Section   lifter   and    seeker    combined,    nickel 

plated       .  .  .  .  .  .  .010 

Metallic  hone,  for  sharpening  razors,  scalpels,  etc.       0     2     6 

Case  of  dissecting  instruments,  consisting  of 
three  pairs  of  scissors,  two  scalpels,  razor, 
two  pairs  of  forceps,  combined  seeker  and 
section  lifter,  blow-pipe,  two  needles,  pipette 
with  teat,  magnifier,  in  walnut  case       .  .        2     5     0 


0  2  9 
0  0  6 
0  0 
0  2 
0  6 
0     5 

4 

3 

5 

0     0     9 
0     3     0 


0 

2 

6 

0 

5 

0 

0 

5 

9 

0 

2 

6 

0 

0 

6 

COLLECTING    APPARATUS 

3452  66     Tow-net   for   collecting   marine   specimens,  11 

inches  diameter,  with  fine  muslin  bag  and 
bottle  attached,  with  24  yards  stout  cord  on 
wood  frame       ...... 

3453  66     Ditto,  made  of  bolting  silk     .... 

3454  — .     Dredge  for  bottom  sea  collecting,  canvas  and 

net  bag,  10  inches 

3455  —     Ditto,  14  inches    . 

3456  —     Ditto,  16  inches    . 

3457  —     Ditto,  18  inches    . 

3458  —     Ditto,  24  inches    . 

3459  66     Collecting   stick   with   inner  lengthening  rod, 

total  extension  5  feet  6  inches,  polished  cane 
with  crook  handle      ..... 

3460  66     Net    ring    for    attaching    to    above,  5    inches 

diameter,  with  bolting  silk  net  and  bottle 

3461  66     Ditto,  but  with  6-inch  diameter  net . 

3462  66  Flanged  tube  for  collecting  nets      . 

3463  —  Cutting  hook  for  cutting  weeds 

3464  —  Collecting  bottles,  6  inches  X  1  inch  per  doz. 

3465  —         4  inches  x  1  inch       .          .          .  per  doz. 


0  15 

0 

1  12 

6 

0  18 

6 

1  2 

6 

1  10 

0 

1  17 

6 

2  5 

0 

0  13     6 


0 

5 

6 

0 

7 

6 

0 

0 

6 

0 

3 

6 

0 

3 

6 

0 

3 

0 

PRICE   OF  INSTRUMENTS  AND   APPARATUS     141 

No.        Page 

3466  —  Collecting  bottles,  3  inches  x  1  inch   , 

3467  —         3  inches  X  J  inch 

3468  —         2  inches  x  f  inch 

3469  —         If  inch  x  i  inch 


£ 

8.   d. 

per  doz. 

0 

2  9 

per  doz. 

0 

2  0 

per  doz. 

0 

1  0 

per  doz. 

0 

1  0 

SUNDRY   APPARATUS 

3279       67     Glass  plate  ruled  in  squares   . 

3277  67     Stage  micrometer  with  engraved  scale.  1/10  and 

1/100  of  a  millimetre 

3278  67     Stage  micrometer  with  engraved  scale,   1/100 

and   1/1000  of  an  inch 
3276       68     Eyepiece  micrometer,  glass  plate  to   fit  into 

eyepiece  ....... 

3480       67     Glass  scale  with  100  divisions  etched  on  in  milli 

metres  for  use  in  making  drawings 
Cross- wire  for  eyepiece .... 
Beck  micrometer  eyepiece 
Beck  horizontal  camera  lucida 
Beck  vertical  camera  lucida. 
Abbe  camera  lucida       .... 
Simple  type  of  Abbe  camera  lucida 
Drawing  table       ..... 
Iris  diaphragm  to  fit  between  object  glass  and 

nosepiece.  ..... 

Polariscope  for  use  on  microscope  . 
Photomicrographic  camera,  vertical  type,  with 

one  dark  slide,  J-plate  size 

—  —     Extra  double  plate-holders     . 
3340       75     Photomicrographic   camera,    horizontal   type 

J-plate  size,  with  one  dark  slide  . 

—  —     Extra  double  plate-holders    . 
3343       75     Focussing  glass     ..... 
3483  '    63     Handcentrifugewithtwotest-tubesinaluminium 

covers       ....... 


3265 

73 

3275 

68 

3368 

69 

3369 

70 

3370 

71 

3371 

71 

3375 

71 

3358 

3345 

72 

3342 

74 

0  10  6 

0  12  6 

0  12  6 

0  10  6 

0  10  G 

0  5  0 
2  2  0 

1  17  6 

2  10  0 
4  5  n 

3  3  0 
0  15  0 


1   15 
3  10 


0 
0 


3     3  0 

0  13  6 

8  15  0 

0  13  6 

0  15  0 

3     3  0 


INDEX 


Abbe  camera  lucida,  7 1 

,,     condenser,  26 
Achromatic  condenser,  27 

,,  object  glasses,  76 

Acineta,  131 

Adjustable  patch -stop,  33 
Adjustment,  focussing,  18,  93 

,,  of  vertical  illuminator,  41 

Amoeba,  126 

Angle  of  aperture,  14,  16,  17 
Aperture,  14,  15,  16,  17,  77 
Apochromatic  object  glasses,  83 
Apparatus  for  holding  objects,  60 

,,  sundry,  67 

Base  of  microscope,  92 
Beck  binocular,  107 
,,  compressor,  57 
„  glass  trough,  56 
Bench  metallurgical  microscope,  119 
Binocular,  Abbe,  108 

„  Beck,  107 

„  illumination  with,  111 

,,  optical  path.  111 

„  Powell  and  Lealand,  108 

„  resolution  with,  110 

„  short  tube,  112 

„  Wenham,  107 

Biology,  best  object  glasses  for,  90 
Blood  counts,  68 

„     films,  64 
Body  of  microscope,  8,  93,  103 
Botany,  best  object  glasses  for,  91 
Box  for  Sloan  object  glass  changer,  21 

,,    live,  57 
Bull's-eye  condenser,  24,  38 
Bythotrepes,  133 

Camera  lucida,  69 

„       photomicrographic,  74 
Case  of  apparatus  for  holding  objects, 

68 
Cedar-wood  oil,  17 
Cells,  65 
Cements,  58 
Centrifuge,  63 

Centring  of  dark -ground  illuminator,  34 
ofhght,  22 

,,         of  object  glass,  21 

,,  of  rotating  stage,  52 

,,         of  substage  condenser,  29 
Ciliata,  129 
Circular  stage,  52,  120 
Cleaning  cover  glasses,  54 


Cleaning  object  glasses  and  eyepieces, 

76 
Coarse  adjustment,  18,  93 
Collecting  net,  66 
Colour  screens,  42,  81 
Concave  mirror,  23 
Condenser,  Abbe,  26 

,,  achromatic,  27 

bull's-eye,  24,  38 

,,  for    dark -ground    illumina- 

tion, 27,  33 

,,  immersion,  28 

,,  substage,  24,  26 

„  use  of  substage,  28 

Cone  of  illuminating  light,  27 
Conjugate  images,  78 
Construction  of  eyepieces,  14 
Convergent  light,  23 
Compressor,  57 
Cork  holder  for  objects,  67 
Correction  collar,  81 

,,  for  cover  glass,  79,  80 

„  of  object  glasses,  78 

Counting  blood  corpusc'es,  68 
Cover  glass  corrections,  79,  80 

,,  „     thickness,  16,  63,  64 

Cross-lines  in  eyepiece,  73 
Cultiire  plates,  62 

Dark -ground  illuminator,  34 

Depth  of  focus,  18 

Diameter  of  eyepieces,  88 

Diatom  periodic  structure,  26 

Difflugia,  127 

Dimensions  of  microscopes,  96 

Direction  of  light,  22 

Dirt  on  lenses,  76 

Dissecting  instruments,  73 

Divergent  hght,  23 

Double  wedge  light  moderator,  32 

Drawing  table,  7 1 

,,        with  the  microscope,  69 
Drawtube,  14,  93,  103 
Dry  mounted  objects,  36,  58 

Electric  lamp,  46 
Entomostraca,  133 
Erecting  eyepiece,  73 
Eyepiece,  11,  13,  15,  88 

compensating,  89 

erecting,  73 

for  photography,  89 

Huyghenian,  89 

micrometer,  68 


142 


INDEX 


143 


Eyepiece,  projecting,  89 

},         with  cross-lines,  73 

»  ,,     indicator,  73 

Eyepoint,  13 
Eyeshade,  73 

Field,  flatness  of,  18,  81 

„     ofview,  14,  82 
Films,  blood,  58 
Finder  divisions,  51,72 
Fine  adjustment,  19,  93 
Flagellata,  128 
Flat  mirror,  23 
Flatness  of  field,  18,  81 
Flattening  objects,  54,  57 
Focal  length,  12 
Focussing  adjustment,  18,  93 

,,         best  method  of,  19 

„         dark -ground  illuminator,  35 

„         glass,  75 

,,         substage  condenser,  28 
Foraminifera,  128 
Forceps,  stage,  57 
Freehand  drawing,  67 

Gas  lamp,  48 
Glass  cover,  53 

„     focussing,  75 

„     plate  ruled  in  squares,  6 1 

,,     slip  with  ledge,  54 

,,     shps,  62 
Grinding  machine,  64 

Haemacytometer,  58 

Hair  of  nettle,  134 

HeHozoa,  127 

High-power  illuminator,  dark -ground, 

34 
Histology,  best  object  glasses  for,  90 
Holding  specimens,  50 
Holopedium,  133 

Illumination,  21,  23,  28 

„  intensity  of,  37,  43,  44 

„  opaque,  38,  39 

,,  with  condenser,  26,  38 

,,  ,,    dark -ground,  34 

,,  ,,    mirror,  23 

Illuminator,  parabolic,  40 

,,  vertical,  40 

Image  formed  by  eyepiece,  9,  13 
,,  ,,  ,,  object  glass,  9,  11 

„      incorrect,  31 
Immersion  condenser,  28 

,,         fluids,  17 

„         object  glasses,  17,  34 
Inclined  position  of  microscope,  50 
Indicator  eyepiece,  73 
Initial  magnifying  power,  68,  77 
Instrument,  dissecting,  73 
Intensity  of  light,  44 
Iris  diaphragm,  23,  26 

Joint  of  microscope  stand,  94 

Lamp,  electric,  45,  46 
„'      gas,  48 


Lamp,  paraffin,  44 

,,        spirit,  48 
Ledge,  sliding,  51 
Length  of  body,  79,  93 
Light,  centring,  15 

,,       cone  of  illuminating,  26 

,,       convergent,  23 

,,       correct  direction,  22 

„       divergent,  23 

,,       intensity,  44 

,,       moderator,  32 

,,       scattered,  24 
Limb  of  microscope  stand,  94 
Linear  magnifying  power,  13 
Live  box,  57 
Living  specimens,  55 
Lucida  camera,  69 

Magnifying  power,  13,  77,  88 

Measuring  cover  glass,  53 
,,  specimens,  67 

Mechanical  stage,  52 

Metallurgical  microscopes,  115 
,,  specimens,  63 

Metallurgy,  object  glasses  for,  91 

Micrometer,  67 

Microscope  as  a  recreation,  125 
„  binocular,  107 

,,         massive  model,  104 
,,         metallurgical,   115 
,,         petrological,  120 
„         portable,  100 
„         Standard  London,  95 
,,         stands,  9,  92 

Mirror,  23,  28,  95 

Moderator,  light,  32 

Monad,  129 

Mounting  cells,  55,  58 
,,         specimens,  58 

Net  collecting,  66 

Nosepiece,  20 

Numerical  aperture,  15,  17,  70 

Object  glass,  11,  12,  17,  76 
„      centring,  21 
,,  ,,      changer,  21 

,,         ,,      characteristics  of   differ- 
ent, 82 
,,         ,,      examination  of  back  lens. 

29 
,,  ,,      selection  of,  87 

Object,  scattering  of  light  by,  24 
Objects,  flattening,  54,  57 
Oil-immersion,  78 

Optical  quahty  of  substage  condenser, 
26 

Parabolic  illuminator,  40 
Patch-stop  adjustment,  33 
Pathology,  best  object  glasses  for,  87 
Penetration,  18 
Periodic  structure,  25,  31 
Petrological  microscope,  120 
Photomicrography,  camera  for,  74 
,,  eyepiece  for,  89 

Pillar  of  microscope  stand,  62 
Pinhole  aperture,  12 


lU 


INDEX 


Pipettes,  59,  66 

Plates,  cvilture,  62 

Podura  scale,  31 

Pointolite  lamp,  46 

Polarising  apparatus,  72 

Polishing  machine,  64 

Portable  microscope,  100 

Power,  magnifying,  13,  16 

Preparing  metallurgical  specimens,  63 

Prism  illuminator,  41 

Protozoa,  126 

Quality  of  substage  condenser,  26 

Ramsden  circle,  13 
Recreation,  microscope  as  a,  125 
Reflection  from  objects  on  dark  ground, 

37 
Reflector,  Sorby's,  40 

,,  thin  glass,  40 

Requirements  of  microscope  stand,  92 
Resolution,  15,  31,  37,  86 

,,  binocular,  110 

Rotating  stage,  52 

Rowley  metallurgical  attachment,  118 
Rulings  for  blood  counts,  60 

Scatteredlight,  24,  31 

Screens,  colour,  42,  81 

Screw,  object  glass,  82 

Seeds  and  pollen,  134 

Selection   of  object  glasses  and  ^eye- 
pieces, 90 

Shape  of  limb,  94 

Size  of  mirror,  95 

Sliding  ledge,  51 

Slip,  glass,  52 
,,     with  ledge,  54 

Sloan  object  glass  changer,  20 

Specimens,  how  to  hold,  50 
living,  55 
,,  mounting,  58 

Spirit  lamp,  48 

Sponge,  132 

Spores  of  fern,  134 

Squares  ruled  on  glass  plate,  67 

Stage  clips,  50 
,,      forceps,  57 


Stage,  mechanical,  51 

,,       rotating,  52 
,,       warm,  62 
Stand  of  microscope,  9,  92 
Standard  London  microscope,  95 

„         object  glasses,  77 

„         of  magnification,  13 

,,         screw,  82 

„         thicknessof  cover  glass,  53,  79 

„  tube  length,  15 

Stentor,  130 
Stereoscopic  vision,  113 
Structure  revealed  by  dark  ground,  37 
Substage,  8 

„  condenser,  24,  25,  26 

Table,  drawing,  66 

,,        of  drawtube  corrections,  80 

,,        ,,   field  of  view,  77 

,,       ,,   object  glasses,  70 
Thickness  of  cover  glass,  53 

„  „  slip,  52 

Traviss  patch-stop,  33 
Troughs,  56 
Trypanosome,  128 
Tube  length,  16,79,93 
Tubercle  bacillus,  31 
Tiu-ntable,  58 

Use  of  dark-ground  illuminator,  34 
„    „  substage  condenser,  28 

Varying  magnifying  power,  15 
Vernier,  72 

Vertical  camera  lucida,  69 
,,        illuminator,  40 
,,        photomicrographic      camera, 

74 
,,        position  of  microscope,  50 
View,  field  of,  14 
Villous  amoeba,  126 
Virtual  image,  13 
Vision  of  natural  objects,  22 
Vorticella,  129 

Warm  stage,  62 

Wedge,  light  moderator,  32 

Working  distance,  12 


Printed  by  Hazell,  Watson  &   Viney,  Ld.,  London  and  Aylesbury