: be submitted to various tests. Air bubbles, striæ, sandholes, scratches, and so forth, of course, prima facie, are bad, but as they are not wholly incompatible with satisfactory performances, overmuch attention need not be paid to them. The Rev. T. W. Webb writes as follows:-"The image should be neat and well-defined, with the highest power, and should come in and out of focus sharply: that is, become indistinct by a very slight motion on either side of it. A proper test-object must be chosen the Moon is too easy; Venus too severe, except for first-rate glasses; large stars have too much glare; Jupiter and Saturn are far better; a close double star is best of all for an experienced eye; but for general purposes a moderate-sized star will suffice. Its image in focus, with the highest power, should be a very small disc, almost a point, accurately round; without "wings," or rays, or mistiness, or false images, or appendages, except 1 or 2 narrow rings of light, regularly circular and concentric with the image; and in an uniformly dark field a slight displacement of the focus either way should enlarge the disc into a luminous circle. If this circle be irregular in outline, or much better defined on one side of the focus than the other, the telescope may be serviceable but is not of much excellence. The chances are many, however, against any given night being fine enough for such a purpose, and a fair judgment may be made by day from the figures on a watch-face, or a minute white circle on a black ground, placed as far off as is convenient. An achromatic, notwithstanding the derivation of its name, will shew colour under high powers where there is a great contrast of light and darkness. This outstanding' or uncorrected colour results from the want of a perfect balance between the optical properties of the 2 kinds of glass of which the object-glass is constructed it cannot be remedied, but it ought not to be obtrusive. In the best instruments it forms a fringe of violet, purple, or blue, round luminous objects in focus under high powers, especially Venus in a dark sky. A red or yellow border would be bad; but before condemning an instrument from such a cause several eyepieces should be tried, as the fault may lie there, and be easily and cheaply remedied." : c Celest. Obj, p. 3. The achromatism may be judged of by directing the telescope to the Moon, or to such a planet as Jupiter, and alternately pushing in and pulling out the eye-piece backwards and forwards through the place of correct focus: when so pushed in, a ring of purple will be formed round the margin of the planet; on the other hand, when the magnifier is drawn out, the contour will be of a light green tinge, that colour being the central one of the prismatic spectrum, the whole performance shewing that the extreme colours red and violet are corrected. The "wings" spoken of in the above extract arise from the glass not being in every place of uniform refractive power-a defect sometimes partially remediable, but never altogether curable. For obvious reasons smaller telescopes are less liable to have their efficiency impaired by this cause than larger ones; and when, in using these latter, great precision of definition is particularly desired for any purpose, the defective portion may be covered over by a cardboard screen, and increased sharpness of outline secured at the expense merely of light-an alternative not always beneath notice. Supposing the reader to have become possessed of a refracting telescope, he is strongly recommended never to attempt to take to pieces the object-glass. If the lenses require any adjustment the maker is in every case the best person to take them in hand : unskilful treatment of them may cause much annoyance. It is different with eye-pieces, in consequence of their being less liable to derangement. The Galilean refracting telescope, so called from its inventor, the Fig. 189. THE GALILEAN TELESCOPE. illustrious Florentine, consists of a double convex object-glass, the eye-glass being a double concave lens placed in front of the image formed by the object-glass. The common opera-glass is a which the innermost is called the field-glass, and the outermost the eye-glass. The focal lengths are equal to each other; and the field-glass should be so far within the focus of the eye-glass, that particles of dust upon the former cannot be seen when looking through the latter. Fig. 191. THE POSITIVE EYE-PIECE. To find the single lens equivalent to an eye-piece of this description, Divide the product of the focal lengths of the component lenses by their sum, minus the distance between them. Thus, if the focal length of each lens be 1.5 inch, and the distance between them I inch, the length of the equivalent single lens will be The positive eye-piece having its focus beyond the field-glass, is suited for use with micrometers, and other instruments having wires in the focus of the object-glass-a case to which the negative eye-piece, in consequence of its having, as we shall see, the focus between the glasses, is not suited. A negative eye-piece consists also of 2 plano-convex lenses, the convex sides of both being in this case turned towards the object d A simple method of finding the focal lengths of small convex lenses is given by the Rev. T. W. Webb, in Month. Not. R.A.S., vol. xvii. p. 269. glass. The ratio of the focal lengths of the lenses is usually 3 to 1, the latter representing the eye-glass. In order that the combination may be achromatic, it is indispensable that the distance between the lenses be equal to half the sum of their focal lengths. a being the field-glass, a stop, cc, to limit the field of view, Fig. 192. THE NEGATIVE EYE-PIECE. a is placed in the focus of the eye-glass, and the eye-hole d is of such magnitude and distance from the eye-glass that the emergent pencils just find a passage through it. The passage of rays proceeding from an achromatic object a focus at e, at which The rays again diverge, glass is shewn in the figure, where it will be seen that after refraction by the field-glass they come to place the image of the object is formed. and by passing through the eye-glass towards the point d, where they enter the eye, and form an inverted image on the retina. are in turn converged To find the single lens equivalent to an eye-piece of this description, Divide twice the product of the focal lengths of the component lenses by their sum. Thus, if the focal length of the field-lens be 3, and of the eyelens be 1, the length of the equivalent lens will be— Negative eye-pieces are almost universally used for all astronomical purposes, except in those cases where they are inadmissible, and which have already been pointed out when speaking of the positive eye-piece. The following should be borne in mind— 1. That in an astronomical refracting telescope the distance between the object-glass and the eye-piece is the sum of their focal lengths, and the magnifying power is the ratio of their focal lengths. Thus, let the focal length of the object-glass of a telescope be 10 feet and of its eye-glass inch. Find the magnifying power. 2. That in a Galilean telescope, the distance between the glasses is the difference of their focal lengths, and the magnifying power is the ratio of their focal lengths. Thus, let the focal length of the object-glass of a telescope be 6 inches, and of its eye-glass inch. Find the magnifying power. As 6: 1 x. : 8. An eye-piece, called, from the name of its inventor, Kellner's, is much approved of by some observers, as offering a larger field than a Huyghenian of equivalent power, and with equally good definition. It consists of 2 lenses, the innermost, or fieldglass, being a double convex crossed one-that is, having surfaces of different radii, the most convex towards the object-glass; and the outermost, or eye-lens, a meniscus of great convexity and small concavity. In practice it is not easy to determine with the requisite nicety the focal lengths of small lenses, and a better method of ascertaining the magnifying power of a telescope may be given. : Accurately focus the telescope on a distant object, turn the instrument towards the sky, and a small luminous circle will be seen near the eye-piece this circle is nothing more than the image of the object-glass. Measure by a fine scale of equal parts the diameter of the object-glass and the diameter of the luminous circle the ratio will be the magnifying power required. The accuracy of the resulting evaluation will depend on the precision with which the spot in the eye-piece has been measured, and a micrometrical contrivance, called, from its inventor, Ramsden's Dynameter, is in general use for this purpose. : Mr. Dawes has contrived an eye-piece for diminishing the apertures of telescopes without disturbing them, which is equivalent to the application of a diaphragm of the required dimensions |