Treatise on mathematical instruments; their construction, adjustment, testing and use concisely explained - Softcover

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1866 Excerpt: ...similar changes of position: if the result be satisfactory, the metals may be considered as well placed, and well adapted for each other. To try whether the large speculum partake of the parabolic form, let the aperture be partially covered, first at the central part, and then round the circumference by tin, pasteboard, or stiff paper; and if on trial the same adjustment for distinct vision be good in both these cases, and also when the speculum is all exposed, the figure may be considered good. If these effect be not produced, the instrument will be incompetent to perform several of the nicer observations in astronomy. When a mistiness appears in the field, it is a proof that the aberrations are not corrected, and that the figure of at least one of the specula is not perfect. If a telescope is not good with its full aperture, its effect may be greatly improved, by putting a cover on the mouth, with a circular aperture, of about one-half the diameter that the tube has, in such a way that the diminished aperture may fall entirely at one side of the opening of the tube. THE SOLAR MICROSCOPE. fn this instrument the object itself is not viewed through a combination of lenses, as in the microscopes already described (pp. 76-81), but a magnified image of the object is formed by a combination of lenses, and received upon a screen The term solar is applied to the instrument, because the light of the sun, concentrated by a lens, is made use of to illuminate the object to be observed, and the construction is in all other respects identical with the common magic lantern, and the oxy-hydrogen microscope. In the case of the microscope, how ever, whether illuminated by the sun or the brilliant oxyhydrogen light, great regard must be had to the forms of the lenses and the...