achromatic

[In Fr. achromat i que; from Gr. ἀχρώματος ( αchromatos ) = colourle ἀ, priv.; χρῶμα (chroma) = colour.] Optics: Colourle . 60 1. Achromatic Telescope : The name given by Dr. Bevis to an improved form of the refracting telescope constructed by Dollond in 1761. When a single lens is used for the object-gla of a telescope, the image of the object is fringed with colour, and hence high magnifying powers cannot be used, unle the focal length of the lens is very considerable. Sir Isaac Newton, from experiments made on the refrangibility of light, had erroneously concluded that the size of the object-gla es of refracting telescopes could not be enlarged beyond three or four inches [APERTURE]: for this reason he turned his attention to reflected light, in which the image of the object is uncoloured. Reflecting telescopes of the Greg or i an form were from Newton's time generally used. In the middle of the last century, Dollond, a Spital fields weaver, undertook a course of experiments with the object of ascertaining the correctne of Newton's statements. His researches were rewarded by the valuable discovery that by using two different kinds of gla , and giving to the surfaces of each lens a different curvature-the focal lengths of the two lenses being in a certain ratio-an image of the object could be obtained free from colour; while, by a skilful arrangement of the radii of the surfaces of each gla , the errors arising from spherical aberration [ABERRATION] could be entirely removed. In the early telescopes made by Dollond and his son Peter, the object-gla was usually a double concave lens of flint enclosed between two con vex gla es of crown (Fig. 1); but modern object gla es have only a concave lens of flint combined with a convex of crown or plate (Fig. 2). A century ago flint-gla of a size suitable for large telescopes could not be obtained; but more recently the removal of the Fig. 1. Fig. 2. excise duty, and the succe attained by Guinand and others in gla manufacture, have enabled English and foreign opticians to construct achromatic telescopes of considerable magnitude, with object-gla es of twelve, fifteen, and even twenty-six inches diameter, the area of aperture having the property of increasing in a considerable ratio the power of the telescope to penetrate into space and render visible the minutest objects. Achromatic telescopes, from their convenient size and comparative cheapne , have been and still are generally used by astronomers in Great Britain, Europe, and America, and by their aid many modern iscoveries have been made. So perfect is the image formed by a well-corrected achromatic object-gla , that almost any magnifying power can be applied, and thus a telescope of this form three or four feet in length is superior in its definition and surpa es in magnifying power one of the old unwieldy telescopes 100 feet long. The eye-gla es of the telescope also require to be free from colour and aberration, and the correction of these defects is accomplished by an arrangement of the lenses forming the eye-piece. [See EYE-PIECE, OBJECT GLASS, APLANATIC.] 2. Achromatic Microscope : In a compound microscope an image of the object is first forined by the objective, and afterwards enlarged by the lenses constituting the eyepiece. Till about the year 1830 the object gla es of microscopes were mostly formed of single or combined lenses, the apertures of which, in order to obtain a distinct image of the object, were exceedingly small. The labours of modern opticians to adapt the achromatic principle to compound microscopes were rewarded by the construction of lenses in which the images of objects were rendered distinct in their minute details even when high magnifying powers were applied. In a modern microscopic objective, not only is the colour corrected and the image free from distortion, but by an increase in the angle of aperture [ANGLE OF APERTURE] the penetrating power of the objective is considerably increased, and le magnifying power is required from the eye-piece. With a good objective of one-eighth of an inch focus, magnifying powers ranging from 450 to 1,200 diameters can be obtained by using different eye-pieces. [OBJECTIVE.]