employed, as a microscope, a drop of Canada balsam or turpentine varnish upon a thin plate of glass, of which the surfaces were exactly parallel. This is a very ready way of forming a plano-convex lens, and if kept free from dust will last some time.

The compound or achromatic microscope consists of four lenses and a diaphragm, placed in the following order: the object-lens; the diaphragm; the amplifying lens, so called because it amplifies or enlarges the field of view; the field-lens; and the eye-lens. The relations between the focal lengths and intervals of the lenses, and the distance of the diaphragm from the object-lens, are determined, so that the combination may be achromatic, aplanatic, and free from spherical confusion. The field-lens and eye-lens form what is called the eye-piece; and the object-lens and amplifying lens form, or tend to form, an enlarged image of the object, in the focus of the eye-piece, which image is viewed through the eyepiece. When the focus of the eye-piece is beyond the fieldlens, so that the image is formed between the amplifying lens and the field-lens, the eye-piece is called a positive eye-piece; but when the focus of the eye-piece is between the two lenses of which it is composed, in which case its effect corresponds with that of a concave lens, it is called a negative eye-piece. With a negative eye-piece the pencils proceeding from the amplifying lens are intercepted by the field-lens before forming an image, and the image is formed between the field-lens and the eye-lens, in the focus of the latter.

The best microscopes are constructed with compound objectlenses, which are both achromatic and aplanatic; and by this means the aperture, and consequently the quantity of light, is much increased. Good compound lenses possessing the required properties have been formed of a concave lens of flint glass, placed between two convex lenses, one of crown glass, and the other of Dutch plate.

The magnifying power of any refracting microscope or tele scope may be practically found, by pointing the object-end of the instruments towards the light, and receiving the image of the object-glass formed by the other lenses upon a screen placed at the eye-end of the instrument, and at a proper distance from it, which may be determined by trial. Then the ratio of the diameter of the object-glass, or of the diaphragm, in the case of the compound microscope, to the diameter of its image upon the screen, gives the magnifying power of the telescope or microscope. In all microscopes it is necessary

to illuminate the object strongly, in consequence both of the diffusion of the small portion of light, received from the object, over the magnified image, and of the absorption of the light by the several lenses.

The Reflecting Microscope.-In this instrument a concave speculum of short focal length is substituted for the objectlens. The object is placed on one side of the axis of the instrument, so that its perpendicular distance from the axis, together with the distance from the speculum of the point where this perpendicular meets the axis, may be a little greater than the focal length of the speculum. A small plane reflector is placed upon the axis of the instrument at the point where the perpendicular from the object meets it. This reflector is set at an angle of 45° to the axis, and having its plane perpendicular to the plane through the object and the axis. The object being strongly illuminated, the pencils of rays proceeding from it, after reflection at the plane reflector and concave speculum, tend to form a magnified image, but are intercepted by the field-glass of the negative achromatic eye-piece, called the Huyghenian eye-piece (p. 82); and the image formed after the transmission of the rays through the field-glass is viewed through the eye-glass.

In the examination of small objects with a high power, it is necessary that the microscope should be perfectly free from all tremor, the slightest motion being so magnified as to prevent a good view from being obtained. Regard must be had, therefore, to solidity and accuracy in the fitting of all the joints and screws: in the choice of an instrument, and for a first-rate instrument, recourse should be had only to a maker of well-known talent, as many so-called opticians are mere sellers of articles of the qualities of which they are totally ignorant. The adjustment of the eye-piece should be obtained through the medium of a clamp and slow motion screw of the best kind, in which the screw acts upon a spiral spring, and by means of which the adjustment for a good focus may be obtained with the greatest possible accuracy, and without the slightest tremor. If the workmanship and fittings of the instrument appear to be satisfactory, a few test objects should be examined with it, to try the quality of the combination of lenses. Two of the best test objects are the Podura plombea, or Skiptail, a small wingless insect, the size of a flea, found in damp cellars, and the Navicula Sigma, a small

*The best clamp, referred to in the text, is Dollond's, or a modification of Dollond's clamp.

shell found in fresh water pools. The surface of the scales of the Podura plombea should appear covered with a great number of delicate marks, like notes of admiration. The Navicula Sigma should appear completely chequered with a number of longitudinal and transverse lines. Should the instrument show these test objects well, it may at once be deemed a good one.

TELESCOPES.

The Refracting Telescope consists of a convex object-glass, which forms an image of a distant object, and an eye-piece of one or more lenses, which per

forms the office of a microscope for viewing this image. The most simple form of the tele

scope is that called the astronomical telescope, and consists of two convex lenses, the object-glass o, of as great focal length, and, consequently, low magnifying power, as the size of the telescope will permit, and the eye-glass e, of small focal length, and, consequently, high magnifying power. When arranged for distinct vision of a distant object, the distance between the two lenses is equal to the sum of their focal lengths: an inverted image, i, of the object is, consequently, formed in the common focus of the two lenses, and the pencils proceeding from the image consist, after refraction at the eyeglass, of parallel rays, which are the most favourable for distinct vision.

The magnifying power of this instrument is represented by the ratio of the focal length of the object-glass to that of the eye-glass, and may therefore be increased either by increasing the focal length of the object-glass, or by diminishing that of the eye-glass. The latter means, however, cannot be resorted to without increasing both the chromatic dispersion and the spherical aberration. Hence, before the means were discovered of forming achromatic and aplanatic object-glasses, the only unobjectionable way of increasing the power of the telescope was by increasing the focal length of the object-glass, and astronomers used to attach the object-glass to the end of a long pole. This contrivance was called an aërial telescope. Huyghens used one of 123 feet in length, and Cassini one of 150 feet.

That the field of view should be as bright as possible, the image of the object-glass formed by the eye-glass at the place of the eye should not be larger than the pupil of the eye; and

the brightness will then vary directly as the square of the diameter of the object-glass, and inversely as the square of the magnifying power. The brightness is also diminished by passing through the refracting media; and hence it is always an object to employ as few lenses as possible, consistently with the attainment of the other requisites of a good telescope.

Refracting telescopes for astronomical observations are now constructed with achromatic object-glasses, and eye-pieces of two lenses, called celestial eye-pieces, which are of one or the other of the two following constructions :

1. The Huyghenian Eye-piece consists of two convexo-plane lenses, with their plane sides, consequently, turned towards the eye, their focal lengths and the interval between them being as 3, 1, and 2. The lens of greatest focal length, f, is

next the object-glass, and is called the field-lens, because it enlarges the field of view. When the telescope is arranged for distinct vision of a distant object, the field-lens is placed between the object-glass and its focus, at a distance from the latter equal to half its own focal length. The pencils of rays from the object-glass, tending to form an image at a distance from the field-lens equal to three-fourths of the interval between the two lenses of the eye-piece, are intercepted by the field-lens and brought sooner to a focus so as to form the image i, half way between the two lenses, and consequently in the focus of the eye-lens e. In this eye-piece the refractions of the axes of the pencils are equally divided between the two lenses, by which the spherical confusion is much diminished; the forms of the lenses are also such as to diminish the spherical aberration, and the relation between the focal lengths of the lenses and the interval between them is such as to satisfy the conditions of achromatism. This eyepiece, called a negative eye-piece (p. 79), is always to be preferred, when we are only seeking to obtain the best defined and most distinct view of an object, and is the best eye-piece for all reflecting telescopes; but when it is necessary to place cross-wires or spider-lines at the place of the image in the field of view, for the purpose of accurately measuring the position of an object, at the time of observation, or to apply an apparatus, called a micrometer, for measuring the dimensions

of an image, the Huyghenian eye-piece can no longer be employed.

We have then recourse to Ramsden's Eye-piece, called a Positive Eye-piece (p. 79). This consists of two lenses of equal

focal lengths, one plano-convey, and the other convexo-plane, so that the convex sides are turned towards one another, the interval between them being equal to two-thirds of the focal length of either. When the telescope is arranged for distinct vision of a distant object, the field-lens f, is placed at a distance from the object-glass o, greater than the focal length of this glass by one-fourth of its own focal length. The focus of the object-glass is then also the focus of the entire eye-piece, and the rays proceeding from the image at i, emerge from the eye-lens e, parallel, or in the condition best adapted for distinct vision. This eye-piece is not achromatic, but the spherical aberration is less with it than with the Huyghenian eye-piece. Whether the eye-piece be positive or negative, a diaphragm is placed at the place of the image so as to intercept all the extraneous light.

With the eye-pieces of which we have been speaking, the object appears inverted, which is no inconvenience when this object is one of the heavenly bodies. These eye-pieces are consequently called celestial eye-pieces. For the convenient observation of stars near the zenith, a plane reflector or prism is placed in the eye-piece, by which the directions of the pencils are turned, so that the axis of the eye-lens is at right angles to the axis of the instrument. Such an eye-piece is called a diagonal eye-piece.

When terrestrial objects are to be viewed, it is generally necessary that they should appear erect, for which purpose the inverted image formed by the object-glass must be again inverted by the eye-piece. The terrestrial, or erect eye-piece, used for this purpose, is coincident with the compound microscope already described (p. 79), consisting of an object-lens, a diaphragm, amplifying lens, field-lens, and eye-lens, the two latter forming either a negative or positive eye-piece. In con sequence of the loss of light consequent upon this construc tion, portable telescopes with celestial eye-pieces are used by navigators for descrying objects at night, and these telescopes are, consequently, called night-glasses.