a small shell found in fresh water pools. The scales of the Podura plumbea should appear covered with a great number of delicate marks, like notes of admiration. The Navicula pleuroigma 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, ƒ, 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-convex, 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. By substituting for the convex eye-lens of the astronomical telescope a concave eye-lens of the same focal length, a simple telescope is formed with only two lenses, which shows objects erect. This is called the Galilean telescope, and is the construction used for opera glasses. When arranged for distinct vision of a distant object, the object-glass and eye-lens are separated by a distance equal to their focal lengths. The pencils of light proceeding from the object, after refraction at the object-glass o, tend to form an image of the object in the common focus of the two lenses; but, being intercepted by the concave eye-lens e, their rays are rendered parallel, and, consequently, adapted to produce distinct vision to an eye placed behind this lens. The magnifying power, as in the astronomical telescope, is represented by the ratio of the focal length of the object-glass to that of the eye-lens. Reflecting Telescopes.-Since the discovery of the methods of forming achromatic and aplanatic object-glasses, the magnitude and available magnifying powers of refracting telescopes are theoretically unlimited; but the difficulty of procuring flint glass of even texture and free from flaws, in pieces of any considerable magnitude, has hitherto practically placed a limit upon the magnitude and available power of refracting telescopes. By the substitution, however, of reflectors, which are always achromatic, for the object-glasses, telescopes of colossal magnitude have been most successfully constructed. Of reflecting telescopes there are four kinds-the Newtonian, the Gregorian, the Cassegrainian, and the Herschelian. The Newtonian telescope consists of a concave object-speculum, s, a plane reflector m, making an angle of 45° with the axis of the S m telescope, placed between the object-speculum and its focus, and an eye-piece. The pencils of light proceeding from a distant object tend to form an image after reflection at the object-speculum, but are bent by the plane reflector, so that the image is formed at i, on the axis of the eye-piece, and in the focus of the eye-lens. The Gregorian telescope consists of a concave object-speculum, s, a small concave speculum, r, whose focal length is short compared object - specu- an small speculum is placed so that its focus is near the focus of the object-speculum, but a little further from this speculum. The pencils of light proceeding from a distant object, after reflection at the object-speculum, form an inverted image, h, of the object at the focus of this speculum, and after reflection again at the small speculum form a second image, i, inverted with respect to the former, and, consequently, erect with respect to the object. This telescope has, for terrestrial purposes, the advantage over the Newtonian telescope, of showing objects erect, but yields to it both in the brightness and perfection of the image, because the second mirror increases the spherical aberration produced by the first, and it is extremely difficult to give the mirrors the proper curvature to remedy this evil. The Cassegrainian telescope consists of two specula and an eye-piece, like the Gregorian, but the second speculum is convex instead of concave, and is placed between the objectspeculum and its principal focus, at a distance from this focus somewhat less than its own focal length. The pencils of light proceeding from a distant object, after reflection at the objectspeculum, tend to form an inverted image of the object, but are intercepted, before doing so, by the convex speculum, and made to form the image still inverted, in the focus of the eyelens. Objects, therefore, are still inverted; but the spherical aberration of the convex speculum being opposite to that of the concave object-speculum, the whole spherical aberration is diminished. This telescope is also shorter than the Gregorian. It is, however, inferior to the Newtonian telescope for celestial observations, and not well adapted for terrestrial purposes on account of the inversion of the object. When light is reflected at a mirror or speculum, there will always be a waste and dispersion; and in consequence of the two reflections, and also of the light intercepted by the plane mirror, or second speculum, the loss of light in all the reflecting telescopes hitherto described is considerable. Sir W. Herschel, by a very simple contrivance, obtained what is called the front view; but this construction is only applicable to instruments of very large dimensions. In the Herschelian telescope the axis of the object-speculum, s, is slightly inclined to |