MAGNIFYING INSTRUMENT.

355. Simple microscope.-Microscopes are instruments which, giving very magnified images, enable us to observe objects which are too small to be seen with the naked eye. Two kinds are distinguished, the simple and the compound microscope.

The first of these is nothing more than a small highly convergent lens, which is used as a magnifying glass, as seen in fig. 275. The object observed is then placed between the lens and its principal focus, and the magnifying power is greater the more condensing is the lens. When it is rather large it is mounted in horn or in ivory, and is then known as a lens. It is frequently used to assist the

sight of the aged, or to facilitate certain kinds of work, which, as in watchmaking and engraving, require great accuracy. But no great magnification is thus attainable, and, in order to observe very small objects, the compound microscope is used, which is so called, for it is made up of several lenses.

356. Compound microscope.-Fig. 276 represents the mode of using a compound microscope, and fig. 277 the path of the luminous rays in the interior of the apparatus. The object observed, which is always very small, is placed at a, between two glass plates on a support called the stage. OAo is a brass tube in which are two condensing lenses, the object-glass, o, at the bottom, and the eyepiece, O, at the top. The object, a, being placed very little beyond the

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Compound Microscope.

355

principal focus of the eyepiece, we know that a real, erect, and greatly magnified image will be formed at bc (328). But as the eyepiece, O, is at such a distance that the image, bc, is between this glass and its principal focus, F, it follows that for an eye looking through it the

eyepiece acts as a lens (329), and gives at BC a virtual and amplified image of the first image, bc. It may thus be said, that the compound microscope is nothing more than a simple microscope

applied not to the object, but to its image already magnified by the first lens.

The magnification depends more especially on the object-glass. In order to increase its power it consists of two or three small lenses, superposed, as seen in H, on the right of the drawing (fig. 277). To the eyepiece a second glass is used, the object of which is less to obtain increased magnification than to render the images more defined by diminishing, as in telescopes, chromatic and spherical aberration. All the glasses are, moreover, achromatic. The magnifying power in compound microscopes has been carried to 1,800 times, and even more, but then what is gained in magnification is lost in definiteness. A good magnification does not exceed 600 in length and breadth, which amounts to a superficial enlargement of 360,000 times.

a

M

Fig. 277.

B

From the great magnification of the image the object must be powerfully illuminated. For this purpose, when it is sufficiently transparent, it is illuminated from below by a concave mirror, M, which concentrates upon it a large quantity of light, as shown in fig. 277. If the object is opaque it is illuminated above by a condensing lens, L (fig. 276); the focus of which is formed upon the object itself. 357. Origin and use of the miThe invention of the croscope. microscope does not extend further back than the commencement of the

seventeenth century, which is surprising, for it had long been known that a drop of water placed in a small hole in a thin opaque plate magnified objects seen through it. From the commencement of the first century, A. D., the philosopher Seneca announced that writing appeared larger under a glass globe containing water. Finally, in the thirteenth century, spectacles were used, that is, magnifying glasses, to assist the sight of the aged. The inventor

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Optical Recreations.

357

of the microscope is not known; it has, probably, only acquired its present form after numerous successive improvements.

The microscope has been the origin of discoveries in the vegetable and animal kingdom, as curious as they are varied. Botanists owe to it their most beautiful discoveries on the structure of the cellular tissue in plants, the circulation of the sap, the function of leaves in the respiration of vegetables. In entomology it has enabled us to discover a crowd of small animals which would otherwise have remained unknown from their extreme minuteness. Thus there have been observed, in vinegar and in sour paste, thousands of small grigs called vibrions; in stagnant waters myriads of animalcules, as remarkable for their fantastical forms as for their beautiful colours, their instincts, their warlike or sociable manners. Mould presents the appearance of small mushrooms with the most brilliant colours. In short, any object seen through the microscope becomes an object of astonishment and admiration: thus, for instance, a hair, a piece of silk thread, the eye or wing of a fly, a bee's sting, a spider's claw, a cat's or mouse's hair, the down of fruit, the scales of a butterfly's wing or of fish, starch grains, spiderweb, etc., etc., everywhere we recognise the infinite perfection of nature's works.

The microscope may also be advantageously used to recognise the fraudulent mixtures in cloths of various kinds, by giving a means of ascertaining whether they contain wool or silk, linen or

cotton.

CHAPTER VIII.

OPTICAL RECREATIONS.

358. Magic lantern. In the instruments that still remain to be described, the object is to project on a screen reduced or enlarged images of an object, so as to exhibit them to a number of spectators, or to utilise them for drawing.

The oldest and most simple of these apparatus is the Magic lantern, which, as everyone knows, is one of the first physical instruments placed in children's hands. It was invented by Father Kircher, a German Jesuit, about 200 years ago, and is used to project a magnified image of small objects painted on glass on a white screen in a dark room (fig. 278). It consists of a tin plate box

in which there is a lamp placed in the focus of a concave mirror, M (fig. 279). The reflected rays fall upon a condensing lens, L, which concentrates them on the figure painted on a glass plate, ab. There is a system of two lenses, m, acting as a single one of great magnifying power, at a distance from ab of rather more than its focal distance. At this distance the system of two lenses acts as in the experiment (fig. 278); that is, a real and very much magnified image of the figure on the glass is produced on the screen. The

image is made erect by placing in the lantern the glass painted in such a manner that the design is reversed. The image, AB, is formed at so much the greater distance, and is so much the more amplified, the nearer the glass, ab, is to the principal focus of the system of lenses, m, and the greater the magnification of this system.

359. Phantasmagoria. This is only a modification of the magic lantern, and dates from the end of the eighteenth century: its name is derived from two Greek words, which signify assemblage of phan