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a certain limit, according to the direction with respect to a certain fixed line in the crystal, called its optical axis. Suppose, then, to take the simplest case, that the eye-lens of a telescope, instead of glass, were formed of such a crystal (say of quartz, which may be worked as well or better than glass), and of a spherical form, so as to offer no difference when turned about on its centre, other than the inclination of its optical axis to the visual ray. Then when that axis coincides with the line of collimation of the object-glass, one image only will be seen, but when made to revolve on an axis jierpendicular to that line, two will arise, opening gradually out from each other, and thus originating the desired duplication. In this contrivance, the angular amount of the rotation of the sphere affords the necessary datum for determining the separation of the images.

(203.) Of all methods which have been proposed, however, the simplest and most unobjectionable would appear to be the following. It is well known to every optical student, that two prisms of glass, a flint and a crown, may be opposed to each other, so as to produce a colourless deflection of parallel rays. An object seen through such a compound or achromatic prism, will be seen simply deviated in direction, but in no way otherwise altered or distorted. Let such a prism be constructed with its surfaces so nearly parallel that the total deviation produced in traversing them shall not exceed a small amount (say 5'). Let this be cut in half, and from each half let a circular disc be formed, and cemented on a circular plate of parallel glass, or otherwise sustained, close to and concentric with the other by a framework of metal so light as to intercept but a small portion of the light which passes on the outside (as in the annexed figure), where the clotted lines represent the radii sustaining one, and the un


dotted those carrying the other disc. The whole must be so mounted as to allow one disc to revolve in its own plane behind the other, fixed, and to allow the amount of rotation to be read off. It is evident, then, that when the deviations produced by the two discs conspire, a total deviation of 107 will be effected on all the light which has passed through them; that when they oppose each other, the rays will emerge undeviated, and that in intermediate positions a deviation varying from 0 to 10', and calculable from the angular rotation of the one disc on the other, will arise. Now, let this combination be applied at such a point of the cone of rays, between the object-glass and its focus, that the discs shall occupy exactly half the area of its section. Then will half the light of the object lens pass undeviated — the other half deviated, as above described; and thus a duplication of image, variable and measureable (as required for micrometric measurement) will occur. If the object-glass be not very large, the most convenient point of its application will be externally before it, in which case the diameter of the discs will be to that of the object-glass as 707 : 1000; or (allowing for the spokes) about as 7 to 10.

(204.) The Position Micrometer is simply a straight thread or wire, which is carried round by a smooth revolving motion, in the common focus of the object and eye-glasses, in a plane perpendicular to the axis of the telescope. It serves to determine the situation with respect to some fixed line in the field of view, of the line joining any two objects or points of an object seen in that field — as two stars, for instance, near enough to be seen at once. For this purpose the moveable tli read is placed so as to cover both of them, or stand, as may best be judged, parallel to their line of junction. And its angle, with the fixed one, is then read off upon a small divided circle exterior to the instrument. When such a micrometer is applied (as it most commonly is) to an cquatorially mounted telescope, the zero of its position corresponds to a direction of the wire, such as, prolonged, will represent a circle of declination in the heavens — and the "angles of position" so read off are reckoned invariably from one point, and in one direction, viz., north, following, south, preceding; so that 0° position corresponds to the situation of an object exactly north of that assumed as a centre of reference, — 90° to a situation exactly eastward or following; 180° exactly south; and 270° exactly west, or preceding in the order of diurnal movement.









(205.) Geography is not only the most important of the practical branches of knowledge to which astronomy is applied, but it is also, theoretically speaking, an essential part of the latter science. The earth being the general station from which we view the heavens, a knowledge of the local situation of particular stations on its surface is of great consequence, when we come to inquire the distances of the nearer heavenly bodies from us, as concluded from observations of their parallax as well as on all other occasions, where a difference of locality can be supposed to influence astronomical results. "We propose, therefore, in this chapter, to explain the principles, by which astronomical observation is applied to geographical determinations, and to give at the same time an outline of geography so far as it is to be considered a part of astronomy.

(206.) Geography, as the word imports, is a delineation or description of the earth. In its widest sense, this comprehends not only the delineation of the form of its continents and seas, its rivers and mountains, but their physical condition, climates, and products, and their appropriation by communities of men. With physical and political geography, however, we have no concern here. Astronomical geography has for its objects the exact knowledge of the form and dimensions of the earth, the parts of its surface occupied by sea and laml, and the configuration of the surface of the latter, regarded as protuberant above the ocean, and broken into the various forms of mountain, table land1, and valley; neither should the form of the bed of the ocean, regarded as a continuation of the surface of the land beneath the water, be left out of consideration: we know, it is true, very little of it; but this is an ignorance rather to be lamented, and, if possible, remedied, than acquiesced in, inasmuch as there are many very important branches of inquiry which would be greatly advanced by a better acquaintance with it.

(207.) With regard to the figure of the earth as a whole, we have already shown that, speaking loosely, it may be regarded as spherical; but the reader who has duly appreciated the remarks in art. 22. will not be at a loss to perceive that this result, concluded from observations not susceptible of much exactness, and embracing very small portions of the surface at once, can only be regarded as a first approximation, and may require to be materially modified by entering into minutiae before neglected, or by increasing the delicacy of our observations, or by including in their extent larger areas of its surface. For instance, if it should turn out (as it will), on minuter inquiry, that the true figure is somewhat elliptical, or flattened, in the manner of an orange, having the diameter which coincides with the axis about jJuth part shorter than the diameter of its equatorial circle; — this is so trifling a deviation from the spherical form that, if a model of such proportions were turned in wood, and laid before us on a table, the nicest eye or hand would not detect the flattening, since the difference of diameters, in a globe of fifteen inches, would amount only to ^th of an inch. In all common parlance, and for all ordinary purposes, then, it would still be called a globe; while, nevertheless, by careful measurement, the difference would not fail to be noticed; and, speaking strictly, it would be termed, not a globe, but an oblate ellipsoid, or spheroid, which is the name appropriated by geometers to the form above described.

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