Half-hours with the microscope : a popular guide to the use of the microscope as a means of amusement and instruction

Preface To Third Edition

The success of this little work has greatly exceeded the Author’s most sanguine expectation upwards of 10,000 copies having been already sold. A new edition being now required, he gladly avails himself of this opportunity to make several important additions, consisting of fresh matter interspersed with woodcuts, descriptive of the Compound Microscope and its various appendages, and an additional chapter on the Structure and Uses of the Polariscope, by Mr. F. Kitton.

The Publishers hope that it may, in its present shape, meet with the same favourable reception, and prove a still more useful aud reliable guide to the Amateur Microscopist.

b

WELLCOME INSTITUTE

LIBRARY

Coll.

we’MOmec

Call

No.

CONTESTS

CHAPTER I. PaGB

ji HALF-HOUR ON THE STRUCTURE OF THE MICROSCOPE 1

CHAPTER IL

A HALF-HOUR WITH THE MICROSCOPE IN THE GARDEN 30

CHAPTER HI.

A HALF-HOUR WITH THE MICROSCOPE IN THE COUNTRY 47

CHAPTER IV.

f HALF-HOUR WITH THE MICROSCOPE AT THE POND-8IDE *6

CHAPTER V.

4 HALF-HOUR WITH THE MISCROSCOPE AT THE SEA-SIDE fit

CHAPTER VI.

A riAL*-HOUR WITH THE MICROSCOPE IN- DOORS TO

CHAPTER VII.

A HALF-HOUR WITH POLARIZED LIGHT

Appendix

TUE PREPARATION AND MOUNTING OF OBJECT* t

DESCRIPTION OE PLATES.

In the examination of these Plates the observer is requested to remember that they are not all drawn to the same scale. Some objects, adapted for low powers, are only magnified a few times, whilst smaller objects are magnified many hundred times. All objects, oj course, vary in apparent size, according to the powers with which they are examined. Descriptions of the objects will be found in the pages indicated.

PLATE I. to face page 1.

FIO. PAGE

1. Vegetable cells with nucleus from apple 31

2. Cellular tissue from pith of elder 31

3. Stellate cell-tissue from rush 32

4. Flat tabular cell from surface of tongue 91

5. Ciliated cell from windpipe of calf 91

6. Human blood corpuscles 91

7. Blood corpuscles from fowl 92

8. Blood corpuscle from frog 92

9. Blood corpuscle from sole 92

10. Blood corpuscle from beetle 92

11. Filament of a species of Zygnema, a plant 60 a. Portion of a filament of the same, the cellcontents becoming changed into zoospores. Zoospore more highly magnified.

XII

THE MICROSCOPE.

PLATE III. to face page 40.

FIG. PAGE

54. “Glandular” woody tissue 34

65 Transverse section of glandular woody tissue .... 34

56. Transverse section of oak 34

57. Long section of oak 34

58. Oblique section of oak 34

59. Section of cork 35

60. Transverse section of coal 36

61. Longitudinal section of coal 86

62. Wheat starch 37

63. Oat starch 37

64. Potato starch 37

65. Tous-les-mois starch 37

66. Indian com starch 38

67. Sago starch 37

68. Tapioca starch 37

69. Acicular raphides from garden hyacinth 38

70. Bundle of ditto from leaf of aloe contained in a cell 38

71. Compound raphides from stalk of garden rhubarb. . 39

72. Tabular prismatic raphides from outer coat of onion 39

73. Circular crystalline mass from a cactus 39

74. Simple vegetable hair from leaf of a common grass 40

75. Rudimentary hair from flower of pan3y 40

76. Simple club-shaped hair 40

77. Club-shaped hair from leaf of dock 41

78. Hair from throat of pansy 40

79. a. Hair formed of two cells from flower of white

dead-nettle 41

79, L Al any-jointed tapering hair with nuclei from

common groundsel 41

DESCRIPTION OF PLATES. xii”

PIG P tGE

81. Glandular hair from leaf of common tobacco 41

82. Hair from leaf of garden chrysanthemum 41

83. Rosette-shaped glandular hair from flower of verbena 41

84. Stellate hairs from the hollyhock [Althaa rosea).. 41

85. a. Stellate hair from leaf of lavender 41

85, b. Hair from leaf of garden verbena, with warty

surface 41

86. Hair from leaf of white poplar ( Populus alba) .... 41

57. Base of a hair on a mass of cellular tissue 41

88, a. A sting from common nettle 42

88, b. Portion of a leaf of Valisneria 42

PLATE 1"V. to face page 48.

90. Yeast plant 48

91. Portions of vinegar plant 48

92. So-called cholera fungus obtained from the air. ... 48

93. Red rust of wheat 49

94. Puccinia graminis — mildew 49

95. Penicillium glaucum — common mould 49

96. Botrytis from mouldy grape 49

97. Fungus from mouldy bread ( Mucor Mucedo) .... 49

98. Fungus from human ear 49

69. Fungus from leaf of bramble (Phragmidium bub

bosum) 49

100. Vine blight (Oidium Tuckeri) 50

101. Potato blight {Botrytis in/estam) 50

102. a. Pea blight ( Erysiphe Pisi ) 50

6. Asci and sporidia of pea blight 50

103. Fungus from a decayed Spanish nut 50 xir

THE MICROSCOPE.

FIG. PAGE

104. Curious fungus from oil casks 50

105. Fungus of common ringworm (A chorion SchVnlenii) 50

106. Fungus on stem of duckweed 50

a. Another within the cells.

107. a. Branched cells from stem of mushroom 51

b. Branched cells from rootlets of mushroom .... 51

C. Reproductive bodies borne in fours on the gills

of mushrooms 51

108. Section through a brilliant orange-coloured peziza 52

109. Section through the common yellow lichen of trees

and walls 52

110. Leaf of Sphagnum, — bog moss 52

б. Spore.

c. Portion of Bispore ; and

d. „ Tetraspore.

e. Antheridia.

112. Reproductive organs of a moss, a species of Tortnla 52

а. The calyptra.

б. The operculum.

c. The peristome.

d. The teeth.

e. The spores.

11 3- Fructification on back of frond of male fern 53

114. Fructification on back of frond of common brake? 53

115. Capsules of Scolopendrium — hartstongue. The

sporules seen escaping 53

a. One of the latter more magnified 53

116. Fructification of Equisetum — horsetail 55

a. Shield-like disk of ditto, separated, surrounded

by thee® 55

DESCRIPTION OF PLATES.

XV

116. 6. Spore, much magnified, with elastic filaments coiled closely round 55

c. Spore expanded 55

117, <*. Fructification of Lycopodium — club moss 54

b. Sporules 54

c. Sporules more highly magnified

PLATE V. to face page 56

118. Delicate spiral cells from anthers of furze 44

119. -Large well-developed spiral cells from anthers of

hyacinth, with minute raphides in intercellular spaces 44

120. Irregular deposit in cells of anthers of white dead-

nettle 44

121. Annular ducts from anthers of narcissus 44

122. Stellate cells from anthers of crown imperial .... 44

123. Ovate pollen cells 44

124. Triangular pollen cells from hazel 44

125. Pollen cells of heath 44

126. Pollen cells of dandelion 44

127. Pollen cells of passion flower 45

128. Pollen cells of mallow 45

129. Red poppy seed 46

130 Black mustard seed 46

131. Seed with deep and curved furrows 46

132. Great snapdragon seed 46

133. Chickweed seed 46

134. Umbelliferous seed or fruit 46

135. Zygnema, conjugating 6q

136. Vlosterium, conjugating 57

XVI

THE MICROSCOPE.

*'lt* PAGE

137. Cosmarium, conjugating 57

138. Epithemia gibba, conjugating 43

139. Melosira nummuloides, conjugating 59

140. Transverse section of common sponge 63

141. Transverse section of common British sponge .... 68

a. Spicules of the same more magnified.

Calcareous spicules of Grantia ciliala. 68

142. Pin-like spiculum from Oliona, a boring sponge . . 69

143. Spiculum from Spongilla, a fresh-water sponge . . 69

144. Spiculum from unknown sponge 69

145. Spiculum from Tethea *59

146. Common Hydra 79

a. Stinging organ from common Hydra

147. A species of Sertularia, a zoophyte 71

148. Campanularia integra, a zoophyte 71

149. “Cup” of Campanularia volubilis, a zoophyte. . .. ,71

150. Spicula of Gorgonia verrucosa 71

151. Transverse section from base of spine of Echinus

neglectus 72

152. Calcareous rosette from sucker of Echinus 72

153. Pedicellaria from Echinus 72

154. Pedicellaria from star-fish 72

PLATE VI. to face page 72.

155. Lcpralia, a polyzoon 72

156. Bowerbanhia densa, a polyzoon 73

157- Tobacco-pipe, or bird’s-head processes of Notamia 73

a. Bird’s-head process.

158. Bugula avicularia 73

159 Bird’s-head process of Bugula Murrayana . ...... 73

DESCRIPTION OF PRATES. XVli

FIG. r*AGK

160. Scrujndaria scruposa, with bird’s-head processes

(avicularia) and sweeping bristles ( vibracula ). . 73

163. Plumatclla repens, a fresh-water polyzoon 74

164. Egg of Cristatella Mucedo, a fresh-water polyzoon 74

165. Transverse section of shell of Pinna, showing

prismatic shell structure 74

166. Longitudinal section of shell of Pinna 75

167. Transverse section from oyster-shell 75

168. Section of shell of Anomia, with tubular borings. . 75

169. Section of mother of pearl 75

170. Prawn-shell viewed as a transparent object 75

171. Teeth of whelk 75

172. Teeth of limpet 7 g

173. Teeth of periwinkle 75

174. Teeth of Limneus yg

175. Scale of sturgeon — ganoid yg

176. Prickle from back of skate — -placoid yg

177. Borings by a minute parasite in a fossil fish-scale. . yy

178. Scale of sole — ctenoid yy

179. Scale of whiting — cycloid yy

a. Calcareous particles, magnified.

180. Scale of sprat — cycloid 7i

181. Section of egg-shell 90

182. From soft egg.

183. Section of egg-shell of emu 90

PLATE VI I to face page 80.

184. Human hair . . >f 7§

a. Transverse section of human hair.

XV1I1

FIG* PAGE

185. a. Small mouse-hair 76

b. Larger mouse-haii.

c. Plain mouse-bait.

d. Minute hair from ear of mouse.

1 86. Hair of long-eared bat 80

1 87. Transverse section of hair of peccary 80

188. Pith-like hair of musk-deer 80

189. Hair from tiger caterpillar 80

190. a. Branched hairs from leg of garden spider

( Epeira diadema) 81

b. Spine, with spiral flutings, from the same.

C. Small brush-like hairs from an Australian spider.

191 Hair from flabellum of crab 81

192. Portion of four of the barbs of a goose-quill 81

193. Portion of the same more magnified 81

194. Swan’s-down 81

195. Head and mouth of a flea 82

196. Head and mouth of a bug 82

197. Mandible of humble bee 84

198. Head and mouth of louse 83

199. Head and mouth of gnat 83

200. Extremities of barbs of the sting of common bee. . 84

201. Head of honey bee 38

a. Piece of the tongue more magnified.

202. Mouth of blow-fly 84

298. Head and mouth of butterfly 8s

204. One of the fangs of a spider, showing the poison-

bag and duct 86

206. Eoot of Empis, a species of fly 87

206. Foot of bee 87

207. Foot of spider 37 description of plates.

XIX

PAG*

FIG. _ '

208. Head of common spider, showing eight simple eyes.

a. Cornea of one of these more magnified

209. Skin of garden spider

21 C Portion of compound eye of fly

211. Portions of the two wings of bee in flight

a. Nervule of wing.

212. Spiracle of fly *

213. Spiracle of

214. Threads of garden spider (Epeira diadema )

86 83 88

St

Simple thread of the same.

Thread of a concentric circle with viscous dots.

PLATE VIII. to face page 88.

215. Fore leg of Gyrinus natator, whirligig beetle .... 87

216. Middle leg of the same 87

217. Hind leg of same 87

218. Fore leg of male Dytiscus 87

219. Middle leg of the same.

220. a. Gizzard of cockroach 89

5. Ditto, cut open.

221. a. Gizzard of cricket 89

b. Ditto, cut open.

222. Trachea from caterpillar 86

223. Proleg of caterpillar of common garden white

butterfly, with the membranes in which the nooks are seated, expanded as in action 88

224. Part of leg of cockroach

225. Battledore scale from blue argus butterfly 88

226. Scale of ordinary shape from same 88

227. Scale from meadow-oro-vn butterfly 88

228. Scale of gnat 88

XX THE MICROSCOPE.

FIG.

229. Scale reduced to a hair from clothes-moth &d

230. Hair-like scale from clothes-moth, with three

prongs 89

231. Cartilage from mouse’s ear 91

232. Transverse section of human bone 90

233. Striped muscular fibre from meat 92

234. a. Liber fibre of flax, natural state 79

b. Ditto, broken across at short intervals

235. Wool from flannel 79

237. Cotton hair. 79

238. Crystal of honey 39

239. Thick crystal of ordinary sugar — same angles .. .. 39

240. Crystals of sugar from adulterated honey . 4u

241. Cuticle from berry of holly 33

242. Transverse section of whalebone 90

243. Transverse section of plum-stone 31

244. Transverse section of testa of seed of Guelder rose 13

245. Fruit of groundsel — opaque 42

246. One hair of pappus of dandelion 42

247. Cottony hair of burdock 42

248. Portion of pappus of goats-beard 42

249. Wood of youDg shoots of vine, the cells containing

starch S3

250. Spiral fibres from testa of wild sage seed 35

PLATE IX. Frontispiece to face title-page.

1. Iodo-sulphate of Quinine

2. Salicine

3. Aspartic Acid

4. Sulphate of Copper in Gelatine

5. Grey Hair (human)

6. Scales of Hyppophae rhamnoirtes

HALF-HOURS WITH THE MICROSCOPE.

OHAPTER I.

A HALF-HOUR ON THE STRUCTURE OF THE MICROSCOPE.

The Microscope is often regarded merely as a toy, capable of affording only a certain amount of amusement. However much this might have been the case when its manufacture was less perfectly understood, it is now an instrument of so much importance that scarcely any other can vie with it in the interest we attach to the discoveries made by its aid. By its means man increases the power of his vision, so that he thus gains a greater knowledge of the nature of all objects by which he is surrounded. What eyes would be to the man who is born blind, the Microscope is to the man who sees only with his naked eye. It opens a new world to him, and thousands of objects whose form and shape, and even existence, he could only imagine, can now be observed with accuracy.

Nor is this increase of knowledge without great advantages. Take for instance the study of plants and animals. Both are endowed with what we call life : they grow and .perform certain living functions ; but as to the mode of their growth, and the way in which their functions were performed, little or nothing was known till the Microscope revealed their minute structure, and showed how thei»' various parts were related to each other. The

Microscope has thus become a necessary instrument in the hands of the botanist, the physiologist, the zoologist, the anatomist, and the geologist.

Let us. then, endeavour to understand how it is this little instrument has been of such great service in helping on the advancement of science. Its use depends entirely on its assisting the human eye to see — to see more with its aid than it could possibly do without it. This it does by enabling the eye to be brought more closely in contact with an object than it otherwise could be.

Just in proportion as we bring our eyes close to objects, do we see more of them. Thus, if we look at a printed bill from the opposite side of a street, we can see the larger letters only ; but if we go nearer we see the smaller letters, till at last we get to a point when we can see no more by getting closer. Now suppose there were letters printed on the bill so small that we could not see them with the naked eye, yet, by the aid of a lens — a piece of convex glass — we could bring our eyes nearer to the letters, and see them distinctly. It would depend entirely on the form of the lens, as to how close we could bring our eyes to the print, and see; but this great fact will be observed, that the nearer we can get our eyes to the print, the more we shall see. The most important part of a Microscope, then, consists of a lens, by means of which the eye can be brought nearer to any object, and is thus enabled to see more of it. Magnifying-glasses and Simple Microscopes consist mainly of this one element. ] n order, however, to enable the eye to get as close as possible to an object, it becomes convenient to use more than one lens in a glass through which we look. These lenses, for the sake of convenience, are fixed in a brass frame, and attached to the Simple Microscope; when there are two lenses they

THE MICROSCOPE.

aie called doublets, and when three they are termed triplets. The magnifying-glasses which are made to be held in the hand, frequently have two or three lenses, by which their power may be increased or decreased. Such instruments as these were the first which were employed by microscopic observers : and it is a proof of the essential nature of this part of the Microscope, that many of the greatest discoveries have been made with the Simple Microscope.

The nearer the glass or lens is brought to an object, so as to enable the eye to see, the more of its details will be observed. So that when we use a glass which enables us to see within one inch of an object, we see much more than if we could bring it within only an inch and a half or two inches. So on, till we come to distances so small as the eighth, sixteenth, or even fiftieth of an inch.

Although a great deal may be seen by a common hand-glass, such as may be purchased at an optician’s for a few shillings, yet the hand is unsteady ; and if these glasses were made with a very short focus, it would be almost impossible to use them. Besides, it is very desirable, in examining objects, to have both hands free. On these accounts the glasses, which in such an arrangement are called object-glasses (see fig. 3), are attached to island, and placed in an arm, which moves up and down with rackwork. In this way, the distance of the object from the glass can be regulated with great nicety. Underneath the glass, and attached to the same stand, is a little plate or framework, to hold objects, which are placed on a slide of glass. This is called the stage. (Fig. 1, G.) Sometimes rack-work is added to this stage, by which the objects can be moved upon it backwards and forwards, without being moved by the hand. Such an arrangement as this is

4:

called a Simple Microscope. Of course many othei things may be added to it, to make it more convenient for observation ; but these are its essential parts.

But, although the Simple Microscope embraces the essential conditions of all Microscopes, and has, In the hands of competent observers, done so much for science, it is, nevertheless, going out of fashion, and giving way to the Compound Microscope. (Fig. 1, p. 5.) This instrument, as might be inferred from its name, is much more complicated than the Simple Microscope, but it is now constructed with so much accuracy, that it can be used with as great certainty and ease as the Simple Microscope itself. In order to understand the mechanism of the Compound Microscope we must first of all study the principles on which it is constructed. If we take a common convex lens and place any small object on one side of it, so as to be in its focus, and then place on the other side a sheet of white paper, we shall find at a certain point that an enlarged picture of the object will be produced on the paper ; and this is the way in which pictures are formed by the camera of which the photographic artist avails himself for his portraits and sun-pictures. Now L we look at. this picture with another lens of the' same character but of somewhat less magnifying power, we shall obtain a second picture larger than the first, and this is the principle involved in the Compound Microscope. The superiority of this instrument over the Simple Microscope consists in an increase of magnifying power. There is, however, a limit to the utility of this magnifying power; for when objects are greatly magnified they become indistinct. This is seen in the Oxyhydrogen and Solar Microscopes, where the images are thrown, by means of highly magnifying lenses, on a white sheet : and, although made enormously large their details

THE MICROSCOPE.

are much less clear than when looked at by a lens magnifying much less. Another advantage of the

Compound Microscope.

Compound Microscope is the distance at which the eye is placed from the object, and the facility with

* In this little work we have purposely abstained from mentioning either the names or the Microscopes of our principal makers, lest we should thereby seem to give »

which the hands may be used for all purposes of manipulation.

A brief description, aided by the accompanying illustration, will, it is hoped, suffice to make the beginner acquainted with the various parts of this important instrument.

We have already mentioned that when powerful lenses are used in the examination of small objects the hand is not sufficiently steady to give a firm support to the lens employed, and this is equally true of the hand that holds the object. It is also essentially requisite to have both hands free, for the purpose of manipulation. Hence it becomes necessary to devise some mechanical means for the support of both the lens and the object. How these wants have been supplied by the enterprising skill and ingenuity of our opticians will be best seen as we describe the various parts of which the Compound Microscope consists.

The most important part of the instrument is undoubtedly that which carries the various lenses or magnifying powers. These are contained in the interior of the tube or body, A, which is usually constructed of brass, and from 8 to 10 inches in length. At the upper end of the tube is the eye-piece, B, so named from its proximity to the eye of the observer. It consists of two plano-convex lenses, set in a short piece of tubing, with their flat surfaces turned towards the eye, and at a distance from each other of half their united focal lengths. The first of these lenses is the eye-glass, while that nearest the objective is termed the field lens. The use of the latter is to alter the course

preference to any. The general excellence of these instruments is so well known and the names of their makers areno universal that the student will find no difficulty in providing himself with an efficient instrument at a moderate cost.

THE MICROSCOPE.

of the light’s rays in their passage to the eye, in such manner as to bring the image formed by the object-glass into a condition to be seen by the eye-glass. A stop also is placed between the two lenses in such a position that all the outer rays, which produce the greatest amount of distortion, arising from spherical and chromatic aberration, are cut off. The short tube carrying the lenses (fig. 2) slides freely, but without looseness, into the upper end of the compound body, A, an arrangement which affords a ready and convenient method for changing the eye-pieoe.

Compound Microscopes are generally fitted up with two eye-pieces, the one deep and the other shallow. The last has its lenses close together, and magnifies the most, whilst the other has them farther apart, and magnifies less. In the use of these eye-pieces, it should never be forgotten that the one which magnifies least is generally the most trustworthy.

At the opposite end of the tube A is the object-glass C. The use of this lens is to collect and bring to a point the rays of light that proceed from any object placed in its focus. At this point an enlarged image of the object will be formed in the focus of the eye-glass. We have only to look through the latter at the picture thus formed in order to obtain a second image larger than the first. And this is the way in which minute objects are made to appear so much larger than when seen by the unassisted eye. It will at once be seen how

Fig. 2. Eye-piece.

much of the utility of a Microscope depends on good object-glasses. Where they are faulty, the image they form is also faulty ; and when these faults in the first image are multiplied by the power of the eye-piece, they become — like the faults of our friends when viewed through a similar medium — of great magnitude.

A good object-glass may be known by its giving a clear and well-defined view of any object we may wish to examine ; while a bad lens may be equally well known by the absence of these qualities. In short, a badly constructed objective is more apt to mislead than to guide the student, by the fictitious appearances it creates — appearances that may be erroneously taken for realities, which have no existence in the object itself. The object-glasses of our best opticians consist of several lenses arranged in pairs, set in a small brass tube. A screw at one end serves to attach them to the lower extremity of the compound body, A. (Fig. 3.) The body of the Microscope is supported by a stout metal arm, D, into the free end of which it screws. The opposite end of the arm is secured to the stem, E, by a screw, on which it moves as on a pivot. By this means the tube of the Microscope can. be turned away from the stage — an arrangement that gives this form of Microscope an advantage over those that are not so constructed. To the stem, E, which woi’ks up and down a hollow pillar by rack-work and pinion, is attached the stage, G. This, in its simplest form, consists of a thin flat plate of brass, for holding objects undergoing ex-

Ftff. 3. Object-Glass.

THE MICROSCOPE.

y animation. In the centre is a circular opening, for the passage of the light reflected upward by the mirror, H. There is also a sliding ledge, I ; against this the glass slide, on urhich the object is mounted, rests, when the Microscope is inclined from the perpendicular.

In a stage of this kind the various parts of an object can only be brought under the eye by shifting the slide with the fingers. But in more expensive instruments the stage is usually constructed of one or two sliding plates, to which motion is given by rackwork and pinion ; the whole being brought under the hand of the operator by two milled heads, a mechanical arrangement which enables him to move with ease and certainty the object he may wish to investigate.

Underneath the stage is the diaphragm, K, a contrivance for limiting the amount of light supplied by the mirror,

H. It consists of a thin, circular, flat plate of metal, turning on a pivot, and perforated with three or four circular holes of varying diameter (fig. 4), the largest only being equal to the aperture in the stage. By turning the plate round, a succession of smaller openings is brought into the centre of the stage, and in one position of the diaphragm the light is totally excluded. By this small but useful contrivance the Microscopist can adjust the illumination of the mirror to suit the character of the object he may be investigating. In some Microscopes the diaphragm is a fix-

THE STRUCTURE OP ture, but in the better class of instruments it issimply attached to the under part of the stage by a bayonet catch, or by a sliding plate of metal (fig. 5), and can be readily removed therefrom when it is desirable to employ other methods of illumination.

In working with the Microscope it is necessary to adopt some artificial means for ensuring a larger supply of light than can be obtained from the natural diffused light of day, or from a lamp or candle. For this purpose the Microscope is furnished with a double mirror, H, having two reflecting surfaces, the one plane and the other convex. The latter is the one usually employed in the illumination of transparent objects ; the rays of light which are reflected from its concave surface are made to converge, and thus pass through the object in a condensed form to the eye. The plane mirror is used generally in conjunction with an achromatic condenser, when parallel rays only are required. The whole apparatus is attached to that portion of the hollow pillar continued beneath the stage, in such a manner that it can be moved freely up and down the stem that supports it. This motion enables the Microscopist to regulate the intensity of his light by increasing or decreasing the distance between the mirror and the stage ; while the peculiar way in which the mirror itself is suspended on two points of a crescent-shaped arm, turning on a pivot, gives an almost universal motion to the reflecting surfaces. The observer by this means can secure any degree of oblique illumination he

Fig. 5. Diaphragm.

THE MICROSCOPE.

may require for the elucidation of the structure undergoing examination.

We next come to the stand, which, though the most mechanical, is at the same time a very important part of the Compound Microscope. On the solidity and steadiness of this portion of the instrument depends in a great measure its utility. The form generally adhered to is that represented in our diagram (fig. 1, p. 5.) It consists of a tripod base, P, from which rise two flat upright pillars, U. Between these, on the two hinge-joints shown at L, is suspended the whole of the apparatus already described : namely, the body carrying the lenses, the arm to which it is attached, the stage, and the mirror underneath it. By this contrivance the Microscope can be inclined at any angle between a vertical and horizontal position — an advantage which can be duly appreciated by those who work with the instrument for two or three hours at a time. Close to the points of suspension are the milled heads, M ; these are connected with a pinion working in a rack cut in the stem, E. By turning the milled heads the tube is made to approach or recede from the stage until the proper focus of the object-glass is found. This is termed the coarse adjustment, and is generally used for low powers, where delicate focussing is not required. But when high magnifying powers are used, that require a far greater degree of precision, we have recourse to the fine adjustment, A, which consists of a screw acting on the end of a lever. The head of the screw by which motion ic communicated to the object-glass is divided into ten equal parts, and when caused to rotate through any of its divisions slightly raises or depresses the tube, carrying the objective with it. As the screw itself contains just 150 threads to an inch one revolution of its head will cause an alteration of the 150tb of an inch in the distance of the lens from the object. When moved through only one of its divisions we obtain a result equal to the 1500th of an inch, and by causing it to rotate through half a division we secure a movement not exceeding the 3000th part of an inch in extent. Such nicety in the adjustment of the optical part of the Microscope may seem to the beginner unnecessary, but when he comes to work with high powers he will find that he needs the most delicate mechanical contrivances to enable him to secure the proper focus of a sensitive object-glass.