from the tube of the telescope and replaced with great facility without loss of adjustment. A tight-fitting brass cap closes the inner cell, and protects the silvered surface when not in use. A reference back to figs. 179-81 will shew the small mirror in each case mounted on a single stout arm, and this has been (I believe) always the method adopted; but Browning employs 3 thin strips of chronometer-spring presented edgeways to the axis of the telescope, as in figs. 187-8. This plan of mounting the smaller mirror has the important advantage that the 3 slender springs offer much less obstruction to the passage of the optic rays than does the I thick arm commonly used. I shall not enter at any greater length into the consideration of reflecting telescopes, for they have fallen somewhat into disuse, since the more durable character and greater manageability of refractors, together with the lessening in their cost, brought about of late years, have made them much more sought after than was formerly the case. A refracting telescope in its simplest form consists merely of a double convex lens which forms an image of the object to be viewed (and hence termed the object-glass), and a second and smaller double convex lens (called the eye-piece), used as a simple microscope to examine the image formed by the first. For astronomical purposes the object-glass is double (sometimes triple) for the purpose of neutralising certain optical inconveniences, called spherical and chromatic aberration, and the eye-glass is generally composed of 2 lenses suitably combined, which together form the eye-piece. It will be readily understood that it is no part of my present object to furnish an elaborate account of the theory of astronomical optics; the remarks which follow must therefore be understood to have reference merely to a few general propositions, with which every amateur astronomer ought to be acquainted. Spherical aberration arises from the circumstance that there are no curvatures practically attainable for a lens such that all the rays of light coming from a distant point can be exactly united in a common focus chromatic aberration depends on the unequal refrangibility of the different coloured rays which together make white light-so that when an observer views an object through a lens he will not see the image perfectly defined and colourless, but more or less. fringed with colour. In order to lessen as far as possible this latter annoyance, the telescope-makers of by-gone days constructed lenses of great focal length (some of them had foci as long as 300 feet); by this means the chromatic dispersion was diminished in comparison with the size of the image formed. Dollond, from an examination of the different kinds of glass then in use, found that some specimens dispersed colours under a given mean refraction much more than others; and starting from this as a basis, the above-named optician elaborated the compound and achromatic object-glass now in use. This invention was made in 1758, and may well be looked upon by Englishmen as a great national triumph; for it is not too much to say that by it optical science has been revolutionised. This object-glass is formed of a convex lens of crown-glass of a greenish hue and a concave lens of flintglass of a yellowish-white hue, placed in contact, the former outermost. By a proper arrangement of their focal lengths (a subject of considerable theoretical and mechanical difficulty) any 2 selected parts of the spectra formed by these lenses can be united, and the chromatic dispersion is thus to a great extent got rid of, without destroying the refractive power of the object-glass. When a ray of light falls upon such a combination it is acted upon by each component: the crown-glass renders it convergent and disperses it; the concave lens neutralises this dispersion, and an uncoloured image is formed at the focus. A mounted object-glass, which it is proposed to purchase, may |