The microscope and its application to vegetable anatomy and physiology

Front matter

SECT. II.- NATURAL HISTORY.

THE MICROSCOPE.

AND ITS APPLICATION

Preface To The Second Edition

dition of the above four Chapters, I have been furnished by Dr. Schacht with a quantity of new matter in manu¬ script, being the result of his investigations since 1852, and this new matter has been incorporated in the text ; the present Edition is, therefore, considerably in advance of the original work. At the suggestion of Dr. Schacht, I have added the Chapters IX, X, and XI, which con¬ tain an interesting account of the embryogeny of the Coniferse, and are a translation of a portion of a work published by him in the course of last summer, en¬ titled “ Beiträge zur Anatomie und Physiologie der Gewächse/’ Chapters VII and VIII of the first Edi¬ tion, which related to drawing and to the preservation of objects, have been omitted; but the student will find what is necessary upon these points under the head of ‘‘Delineating Apparatus/' in Chapter III, and in Chap¬ ter IV, which treats of the preservation of specimens.

FREDERICK CURREY.

Blackheath,

December 19 th, 1851.

TRANSLATOR’S PREFACE

TO THE

FIRST EDITION.

The Work of Dr. Schacht, of which a Translation is now offered to the public, relates to a branch of Micro¬ scopical Science which has not hitherto formed the sub¬ ject of a separate Treatise ; and the high reputation of the Author, and the interesting nature of the subject, have induced a belief that the present Version is likely to meet with a favourable reception.

It has been thought advisable to omit the greater part of the description of foreign Microscopes and aux¬ iliary instruments contained in the Original Work. These details would, for obvious reasons, be uninterest¬ ing, if not useless, to the English reader. There is no doubt of the superiority of English instruments over those described by Dr. Schacht ; and the elaborate and able Treatise of Professor Quekett affords all the neces¬ sary information upon the subject of English Micro¬ scopes, &c.

The high price of good English Microscopes has hither¬ to been an impediment to the progress of Microscopy,

Vlll

Preface To The First Edition

and much attention has lately been directed to the production of cheaper instruments. A very useful and convenient form of Student's Microscope is represented in the Frontispiece, which has been designed by Mr. Samuel Highley, Jun., of Fleet Street, and may be had at a very moderate price.

The figures of the Original Work, and their descrip¬ tions, have been incorporated into the text of the Translation, by which means the inconvenience of con¬ stant reference to the plates and their explanation is avoided. The figures of the foreign instruments, and a few other figures, which were not essential for the elucidation of the subject, and which would have in¬ creased the expense of the Translation, have been omitted.

CHAPTER I. Preliminary Ideas upon Optics Theories of Light

Laws of Light Ray of Light Pencil of Light Focus ....

Principal Focus Angle of Incidence Opaque Bodies Transparent Bodies

Reflexion ....

Irregular Reflexion Law of Reflexion

Refraction ....

Law of Refraction

Chromatic Dispersion Solar Spectrum

Lenses .

Material of Lenses Forms of Lenses .

Convergent Lenses Divergent Lenses .

Formation of Images by Lenses

Rules applicable to two particular cases

Spherical Aberration

Longitudinal Spherical Aberration

Lateral Aberration

Lens of least Spherical Aberration

Chromatic Aberration .

Effect of Chromatic Aberration Compound Lens Dispersive Power .

Page o

md

6 6 6

K

i

. 11 b vni

CONTENTS.

CHAPTER II. Page

The Application of the Principles explained in Chapter I. to

Object of a Microscope . . . . . . . .12

Adaptive Power of the Eye . . . . . . .12

Effect of a Double Convex Lens . . . . . .12

Simple Microscope

Effect of, upon magnitude of object . . . . .12

Wollaston’s Doublet . . . . . . . .13

Effect of Wollaston’s Doublet upon Spherical and Chromatic

Holland’s Triplet ......... 14

Dissecting Microscope . . . . . . . . .15

Ross’s Dissecting Microscope . . . . t . .15

Quekett’s Dissecting Microscope ...... 15

Compound Microscope . . . . . . . . .16

Formation of the Image in a Compound Microscope . .17

Ross’s Compound Microscope . . . . . .17

Highley’s Compound Microscope ...... 17

Quekett’s Compound Microscope . . . . . .17

Mechanical Parts of the Compound Microscope . . . .17

The Stage . 18

The Adjustment . . . . . . . . .18

The Fine Adjustment ........ 20

Optical Parts of the Compound Microscope . . . . .20

The Object-glasses ......... 20

Formation of an Image by the Object-glass . . . .20

Achromatism of Object-glasses ...... 21

Angle of Aperture ........ 21

Adjustment for covered and uncovered Objects . . .22

Practical Directions with regard to Adjustment . . .23

Huyglienian Eye-piece . . . . . . . .24

CONTEXTS.

Application of Principles explained in Chap. T. — continued. Optical Parts of the Compound Microscope — continued.

Effect of Huyglienian Eye-piece in connection with the Over correction of the Object-glass .....

Ramsden’s Eye-piece .......

Negative Eye-piece .......

Positive Eye-piece .......

Chapter III

On the Accessory Apparatus and Chemical Re-agents necessary for the Conduct of Microscopical Investigations in Botany .

Bidl's-eye Condenser .

Spherical Aberration ........

Achromatic Condenser .........

Quekett’s Condenser ........

Polarizing Apparatus .........

Manner in which Light may be Polarized ....

Double Refraction ........

Absorption ..........

Different Properties of Common and Polarized Light Polariscope ..........

Polarizer ....... .

Nichol’s Prism . . . . . . . * .

Tourmaline ..........

Herapath’s Crystals ........

Coloured Phenomena of Polarized Light ....

Depolarizer ..........

Selenite Stage .........

Effects of Polarized Light upon different Kinds of Vegetable Cells .

Delineating Apparatus ........

Different Instruments employed ......

Camera Lucida ..........

Mode of Using .

Neutral Tint Glass .........

Relative Advantages of Neutral Tint Glass and Camera Lucida

h 2

X

CONTENTS.

Page

Accessory Apparatus and Chemical Re-agents — continued.

Steel Mirror . 34

Micrometer . 34

With Negative Eye-piece . . . . . . .34

"With Positive Eye-piece . . . . . . .34

Compressorium . . . . . . . . • .34

Fishing Tubes .......... 35

Lamps . 35

Cambridge Lamp ......... 35

Camphine Lamp ......... 35

Palmer’s Candle . . . . . . . . .35

Highley’s Achromatic Gas-lamp . 36

Dissecting Instruments ......... 36

Scalpels and Knives ........ 36

Valentine’s Knife ......... 33

Forceps . 39

Crossed Forceps ......... 39

Plate Glass Stage . 40

Use of corrosive Agents . . . . . . .40

Disks for Cleaning Thin Glass ....... 40

Method of using Disks ....... 40

Section Instrument ......... 40

Objects for which Section Instrument may be used . .41

• Sections of Wood ......... 41

Brushes and other Apparatus . . . . . . .41

Chemical Re-agents ......... 42

Alcohol . . - . 42

Solution of Iodine ........ 42

Concentrated Sulphuric Acid . . . . . .42

Diluted Sulphuric Acid ....... 43

Solution of Chloride of Zinc, Iodine, and Iodide of Potassium 43

Syrup . 43

Schultz’s Method of Maceration . . . . . .44

Oil of Lemons ......... 45

Method of applying Re-agents . . . . . .45

Pottles for lie-agents . 45

CONTENTS.

XI

CHAPTER IV. On the Preservation of scope

Opaque Objects Slides .

Plate-glass Slides Ebony Slides .

Mahogany Slides

Transparent Objects Cells

Page

Botanical Specim

NS FOR THE MlCllO-

Glass Cells

Diffei'ent Forms of Cells Thin-glass Cells

Cement Cells ....

Instrument for making Cement Cells Fixing the covers of Cells

Preservative Fluids

Spirit and Water .

Solution of Muriate of Lime Glycerine and Water Solution of Acetate of Alumina

Canada Balsam Mode of using

Deane's Gelatine Mode of using

Castor Oil

Difficulty in using

Distilled Water Labels for Slides .

47 47

48 48 48 48 48

Chapter V

General Rules for the Use of the Microscope, Preparation of Objects .

A N D

FOR the

Position of Apartment .

Management of Objects by Lamp-light „ „ Day-light

53 53

Xll

CONTENTS.

General Rules for Use of Microscope — continued. Use of different magnifying Powers Transmitted and Incident Light Examination of Objects under Water Use of covering-glasses .

Precautions to be observed in using Re-agents Cleansing of Object-glasses, &c.

Deceptions arising from foreign Substances .

„ ,, other causes .

Appearances of Motion .

Mouches Yolantes ....•• Preparation of different Objects Form of Razors — Method of cutting Removal of Air

Transferring Objects from Fluid Adjustment of Focus .

Use of Compressorium . ... .

Examination of the contents of Cells Importance of the use of Re agents

Page

. 54

. 54

. 55

. 55 . 55 . 56

. 64 65, 66 . 66 . 67 . 69

Chapter VI

On the different Appearances, Forms, and Arrangements of the Cells of Plants; and on the Methods of Procuring and

Examining them . ...

The Free Cell .

Colouring of the Cell-membrane by Iodine

and Sulphuric

Acid . .

The Primordial Utricle .....

-Effects of Iodine ....

„ Syrup .

Growth of the Cell ....

Increase of thickness of the Cell .

Spiral Bands .....

Lamination .....

i i

Pitted Tissue ......

Parenchyma and Prosenchyma

Spiral, annular, and scalariform Vessels .

Perforations in Cells

Different kinds of Tissue ....

Parenchymatal Tissue . . . . .

Formative Tissue ......

• • •

CONTENTS. Xlll

Page

On Appearances, Forms, &c., op Cells of Plants — continued.

Woody Tissue ......... 81

Epidermal Tissue . . . . . . . . .83

Vascular Bundles ......... 85

Cambium Cells ......... 86

Inter-cellular Passages . . . . . . . .88

CHAPTER VII. Concerning the Method op Investigation . 89

General Rules ......... 90

Importance of making Drawings of Microscopical Objects . 93

Examination of the Perfect Plant . 95

Cryptogams

„ Fungi . 95

„ Fucaceao . . . . 97

„ Florideae . 98

„ Mosses and Liver-worts . . . . .99

,, Rhizocarpeae . . . . . . .101

„ Lycopodiaceao, Equisetaceae, Pterideae . .101

,, Antheridia and Pistillidia .... 99-102

Phanerogams . . . 103

Examination of the Stem and Root . . . . . .103

Monocotyledonous and Ciyptogamous vascular bundles . 1()4 Monocotyledonous Roots . . . . . . .106

The Dicotyledonous Stem . . . . . . .106

Transverse Sections of the Pith, Wood, Cambium, and

Longitudinal Sections of ditto . . . . . .108

Radial Sections of ditto ....... 109-110

Medullary Rays . . . . . . . . .109

Section of Pitted Tissue . . . . . . . .111

Roots of Dicotyledons . . . . . . . .112

XIV

CONTENTS.

Page

Concerning the Method of Investigation — continued.

Examination of Leaves . . . . . . < . .113

Epidermis, Stomata, Cuticle, and Hairs . . . . .113

Contents of Cells ......... 114

Examination of the Flower and Fruit . . . . • .115

The Bract and the Calyx . . . . . • .116

The Petals and Stamens . . . . • • . H<

The Anthers and Pollen . . . . . • .118

The Style and Stigma ......•• H9

The Ovary and Placenta) ....... 120

The Ovule . 121, 122

The Ripe Fruit and Seed . . . . . . .123

Motion of the Juices of the Cell ....... 125

Investigation of the Developement of Plants ..... 126

The Germination of Cryptogams . 127

Formation and Germination of the Spores of Algte . . 127

Zoospores and Microgonidia ....... 127

Motionless Spores . . . . . . . . .128

Fungi and Lichens . . . . . . . . .129

Method of producing Germination of the Spores of Ferns and

Investigation of the Developement of the Stem, Leaves, and Foots . 131 The Cambium-ring and Vascular Bundles . . . 131,135

Formation of Leaves ........ 132

The Axillary Bud . . . . . . . . .133

Adventitious Buds . . . . . . . .133

Morphology and Anatomy of Buds . . . 134, 135

Cambium, Wood-cells, Vessels, and Liber-cells . . . 136

Difference between Cell-multiplication and Cell-extension . 136 Branching of the Vascular Bundles from the Stem into the

Developement of Roots . . . . . . . .138

Adventitious Roots . . . . . . . .138

Examination of the Developement of Flowers . . . . .138

Difficulties to be overcome . . . . . . .139

Examination of Transverse Sections, points to be observed . 141 ,, Longitudinal Sections . ,, „ . 142

Adhesion of different parts . . . . . . .143

CONTENTS. XV

Page

Concerning the Method of Investigation — continued.

Examination of the Developement of the Embryo . . . .143

Course of the Pollen-tube . . . . . . .144

Developement of the Ovule . . . . . . .145

Examination of the Ovule . . . . . . .145

Entrance of the Pollen-tube into the Micropyle . . .147

Origin of the Embryo in the extremity of the Pollen-tube . 149 Explanatory Examples . . . . . . . .150

Examination of the Developement of the Cell . 153

Examination of the Developement of Spores, Pollen, and

Use of Re-agents . . . . . . . . .155

Cell Formation ......... 156

Formation of the Membrane of the Cell .... 157

Zoospores of Chlamidococcus and other Alga) . . .157

Division of the Primordial Utricle ..... 157

Formation of the Covering of Cellulose . . . .157

Chapter VIII

Examples of the Developement of Flowers .... 15S Asclepias Syriaca . . . . . . . . .159

Stachys Coccinea . . . . . . . . .167

Cleome Arborea ......... 170

CHAPTER IX. On the Structure of the Pollen of the Coxifer.e .172

Formation of the Pollen-tube . . . . . .172

Cells in the Interior of the Pollen ..... 172 Theories of Geleznoff and Fritsche . . . . .173

Catkin-bud of Abies pectinata . . . . . .173

Cone-bud of Abies pectinata . . . . . .174

The Anthers and Pollen-grains . . . . . .174

Mother-cells of Abies pectinata and Picea vulgaris . .175

Ripe Pollen-grains of Abies pectinata and Picea vulgaris . 176

Pollen-grain of Larix Europsea . . . . . .177

Pollen-grain of Picea vulgaris . . . . . .173

XVI

CONTENTS.

Page

Structure of the Pollen of the Conifer.e — continued.

Origin of tlie Cellular Body in the Pollen-grains of the Coni¬ fer» . . . . . . . . . .179

Pollen-grain of Pinus sylvestris . . . . . .180

Pollen of Encephalartos Altensteini . . . . .180

Difference between the Reproductive Organs and the for¬ mation of the Embryo in the Conifer» and in other Phanerogams ......... 181

CHAPTER X. Further Observations upon the Structure of the Pollen of

the Conifers . 183

Formation of the Cellular Body in the Pollen-grain of the

Formation of the Cellular Body in the Pollen-grain of Cu-

pressus, Thuja, Juniperus, and Taxus .... 183

Pollen-sacs of the Conifer» . . . . . . .184

Anther of Podocarpus . . . . . . . .184

Contents of the Pollen-grains in the Conifer» . . .186

The two Modes of formation of Pollen-tubes in the Conifer» 186

Phytozoa in Pollen-tubes of the Conifer»

Free Cells in the Corpuscula of Conifer»

Mock Cells ......

Entry of the Pollen-tube into the Corpuscula

CHAPTER XI. Some further Remarks upon the Mode of Impregnation in Pinus

sylvestris and Taxus baccata . 188

Formation of the Cellular Body ...... 188

The Lower Rosette and Embryonal Tubes . . . .188

Course of the Pollen-tube in Taxus baccata .... 189

The Embryonal Tubes ,, ,,.... 190

The Rudiment of the Embryo „ „ . . . . 190

Position of the Corpuscula in Pinus . . . . .190

Formation of the First Cells of the Embryo in Phormium

CONTENTS.

XVI 1

Impregnation in Pinus sylvestris and Taxus baccata — continued.

Relation between the Conifers; and Cycadeee and the higher Cryptogamia ........

Relation between the Conifers) and Cycades) and other Phanerogams ........

Appendage to the Embryo of Tropseolum ....

Rudiment of the Embryo of the Coniferse ....

Office of the Pollen-tube of the Conifers) ....

Entrance of Phytozoa into the Germ-organ of Ferns

Page

191 19*2

192 192

Appendix

Unger’s Account of the Origin of the Embryo

AND ITS APPLICATION TO

VEGETABLE ANATOMY AND PHYSIOLOGY.

Chapter I. Preliminary Ideas Upon Optics

Before entering upon the description of the instruments and apparatus necessary for the conduct of microscopical investiga¬ tions, it may be interesting and useful to bring to the student’s notice a few of the leading preliminary facts connected with opti¬ cal science, in order that when he enters upon the use of the mi¬ croscope he may have some idea of the causes of the effects produced, and may not be in the position of merely seeing and admiring them.

Light is the agent by which the objects of the material world become visible to our senses. Upon the nature of the constitu¬ tion of light, different opinions have been entertained. Sir Isaac Newton and his followers supposed that light consisted of mi¬ nute, material particles, darted out in straight lines or rays from luminous bodies, and that these particles, by impinging upon the eye, produced the sensation of vision. According to the theory of modern philosophy, the sensation of light is pro¬ duced by the vibrations of the particles of a subtle elastic medium supposed to pervade all space, and to which the name Ether has been applied

It will not be necessary for our purposes to go into the ques¬ tion of these two theories, beyond remarking that the latter is the one which is generally admitted at the present day, although many optical phenomena are capable of explanation according to either the one or the other.

B

THE MICROSCOPE.

law* of Light. — The three principal laws of light are those of reflexion, refraction, and the chromatic dispersion of light. Upon each of these we shall have to say a few words, hot will first explain some terms of constant occurrence, and which it is necessary should be understood.

A ray is the smallest portion of light which we are capable of conceiving, and is represented by a line drawn in the direction in which the light is supposed to proceed. A small assemblage of rays constitutes a pencil of light, wliich may be either cojiical, as when

the rays diverge from a point ” 1 (fig. 1), or cylindrical, when

consisting of parallel rays

An assemblage of rays, if too large to come under the denomination of a pencil, is called a beam. When rays diverge from a point or con¬ verge to a point, the point from which they diverge or converge is called the focus.

The princi pal focus is the point to which parallel rays converge after reflection or refraction.

The normal to a surface at any point is a straight line per¬ pendicular to the surface at that point.

When a ray of light is incident upon a plane surface, the angle which the direction of the ray makes with the normal to the surface at the point of incidence, is called the angle of incidence ; and when the incident ray is reflected or refracted by the surface, the angles which the reflected and refracted rays respectively make with the normal, are called the angles of reflexion and refraction.

Bodies which do not admit of the passage of light through them are called opaque, whilst those through which light passes are called transparent.

There are no bodies in Nature which are either perfectly opaque, or perfectly transparent. Bodies composed of the densest material, such as gold, when sufficiently reduced in

THE MICROSCOPE.

thickness, admit of the passage of a small portion of light, and bodies apparently of the greatest transparency always in¬ tercept some portion of light, however small.

Reflexion. — When light is incident upon an opaque body, it is reflected, or thrown back, into the medium in which it is propagated, and this reflexion takes place in different ways, ac¬ cording to the nature of the surface upon which the light is incident. When the surface is rough and unpolished, the light is reflected in all directions from every point of the surface, each point becoming, as it were, the origin of a pencil of rays, which diverge in all directions. This mode of reflexion is called irregular reflexion, and by means of it the outlines and forms of bodies in general are rendered visible.

When light is reflected from a polished surface, the reflexion takes place according to a

certain law, which may be Fl&* 3*

thus expressed, viz. : —

The incident and reflected rays lie on opposite sides of the normal to the surface at the point of incidence , and the angle of incidence is equal to the angle of reflexion.

Suppose E B F in fig. 3 to be the reflecting surface,

A B a ray of light incident upon it at B, and let B D be per¬ pendicular to E B F ; then if the angle D B C bo taken equal to the angle A B D, B C will be the course of the ray A B after reflexion.

Refraction. - When a

ray of light passes obliquely from one transparent me¬ dium into another of diffe¬ rent density, it is bent oui of its course, or, as it is call¬ ed, refracted.

Fipr. 4.

B

Thus, let E D (fig. 4) be a ray of light incident upon A B,

b 2

THE MICROSCOPE.

which we may suppose to be the surface of water, then the ray E D will be turned out of its course, and will proceed in the direction D F. In this case, the density of water being greater than that of air, the refraction takes place towards the perpen¬ dicular (D L) to the surface. When the density of the medium into which the refracted ray passes is less than that out of which it passes, the refraction is from the perpendicular.

Refraction takes place according to a certain law, and in order to render that law intelligible, it is necessary in the first place to explain what is meant by the sine of an angle.

If A B C (fig. o) be any angle, and if from any point P in B A, a line P N be drawn perpendicular to B 0, then the ratio of the line

is called the sine of the angle A B C, or as it is expressed mathe- P N

matically, = sin. B.

The law of refraction is as follows : —

Tlue incident and refracted rays lie in the same plane with , but on opposite sides of the normal to the surface at the point of incidence , and the sine of the angle of incidence bears a certain ratio to the sine of the angle of refraction, which ratio depends upon the nature of the media between which the refraction takes place.

If the angle of incidence be represented by </>, and the angle of refraction by </>', this law may be shortly expressed thus where p is constant.

Chromatic JMspersion of light. — If a beam of solar light abc (fig. 6), be admitted through a round hole in the windowshutter of a darkened room, and received upon a screen, a circular spot of white light will be visible upon the screen at the point where the beam of light impinges upon it. If, now, a glass prism, A C B, be placed so as to intercept the course of the light, in the manner represented in the figure, the beam will

Fig. 5.

THE MICROSCOPE.

be decomposed ; the light will be refracted upwards, and an oblong image containing the colours, popularly known as the prismatic colours, will be visible upon the screen, instead of the spot of white light. These prismatic colours are seven in num-

Fig. 6.

ber, and succeed one another in the following order; viz., violet, indigo, blue, green, yellow, orange and red. The assemblage of them is called the solar spectrum, and the cause of the ap¬ pearance has been ascertained to be the different refrangibility of the different rays of light which enter into the comj>osition of a beam of ordinary sunlight. The violet rays l>eing more easily refracted than any others, are diverted from their course to a greater extent than the indigo, the indigo to a greater ex¬ tent than the blue, and so on, until we arrive at the red rays which undergo the least refraction of all.

Lense». — A lens is a portion of a transparent medium bounded by spherical surfaces. This definition would include a lens with one-side plane, such as a plano-convex lens, inasmuch as the plane side may be considered to be a spherical surface with an infinite radius.

For optical purposes glass is the material always employed in the manufacture of lenses. Although lenses might be made of

b 3

THE MICROSCOPE.

any transparent material, and with surfaces other than spherical, no material has yet been met with which combines so many ad¬ vantages as glass ; and this fact, together with the difficulty of grinding accurately any other than a spherical surface, has led to the universal adoption for microscopical purposes of glass lenses with spherical surfaces.

In fig. 7 are represented the principal forms of lenses, and the names attached to these different forms are as follows : —

Fig. 7.

1. Double convex; 2. Plano-convex; 3. Meniscus; 4. Con¬ cavo-convex ; 5. Double concave ; G. Plano-concave.

It will be observed, that in the meniscus, the radius of the convex surface is less than that of the concave surface ; whilst in the concavo-convex the contrary is the case, that is, the radius of the concave surface is less than that of the convex.

Nos. 1, 2, and 3 are called convergent lenses, because a pencil of parallel rays incident upon them is rendered convergent.

Nos. 4, 5, and 6 render a pencil of parallel rays incident upon them divergent, and are therefore called divergent lenses.

Whatever be the form of a convergent lens, a double convex lens with equal radii can always be found, which will produce the same optical effect; and, on the other hand, a double con¬ cave lens, with equal radii can always be found, which shall be optically equivalent to any form of divergent lens.

Formation of Images by Femes. — A full investigation of the manner in which images are formed by lenses would occupy more space than can be afforded in a work like the present, and would, moreover, involve a discussion of mathematical formula?, which would be out of place. The following rules, however, applicable to two particular cases of the formation of images, and

THE MICROSCOPE.

i

which will occur in the discussion of the construction of micro¬ scopes, may here be stated.

1. When an object is placed before a double convex lens, at a distance from the lens less than that of its principal focus, a vir¬ tual erect and magnified image of the object is formed on the same side of the lens as the object, and at a distance from the lens greater than that of the object.*

2. When an object is placed before a double convex lens, at a distance from the lens greater than that of the principal focus, but less than twice the focal length of the lens, a real, inverted, magnified image of the object is formed on the opposite side of the lens.

Fig. 3.

Fig. 8 will illustrate rule 1. Let a b be an object placed within the principal focus of the double convex lens e f and let eaf ebf be the pencils of light from the extreme points of the object; the rays a e, a/, (which are the extreme rays of the pencil e a f) will be refracted by the lens into the directions e m, fn , and after refraction will apparently diverge from the point c. In like manner the rays be, bf will be refracted so as to diverge apparently from the point cl, and thus a virtual, erect, and mag¬ nified image of the object a b is formed at c d.

* A virtual image, is one in which the rays of light do not actually pass through the points of the image, as is the case when the image is veal.

TITE MICROSCOPE.

Fig. 9 will illustrate rule 2. If a b be an object placed a little beyond the principal focus of the lens e f the pencil e a f diverging from the extremity a of the object, will, after refrac-*

Fier. 9.

tion, be brought to a focus at a point d on the other side of the lens, and in like manner the pencil e b /will be refracted to a focus at c, and thus a real inverted magnified image of the ob¬ ject will be formed at c d.

In the formation of these images we have supposed all the rays of each pencil to be brought to an exact focus by the lens. This, however, is never the case with an ordinary lens, on ac¬ count of the fact, that the Fig. 10.

rays which fall upon a lens near the margin are more strongly refracted than those which pass through nearer to the centre; hence arises an aberration known by the name of spherical aberration, which fig. 10 will serve to ex¬ plain. A pencil of light (here supposed to consist of parallel rays) falls upon the lens ; the rays A near the margin are refracted to the focus A, whilst the rays

THE MICROSCOPE.

B near the centre are refracted to B. It will at once be seen that the effect of this aberration will be to cause an indistinct¬ ness in the image. This indistinctness may be remedied by limiting the aperture of the lens, which may be done by cover¬ ing up either the margin or the centre of the lens ; in the former case, the image is formed by the central rays alone ; in the latter, by the peripheral rays alone. The distance, A B, is called the longitudinal spherical aberration ; and the line through B at right angles to B B, and which is the diameter of the circle of aberration over wliicli the rays are spread, is called the lateral aberration.