The microscope : its history, construction, and application, being a familiar introduction to the use of the instrument, and the study of microscopical science / by Jabez Hogg.

Hogg, Jabez, 1817-1899.

Date:: 1898

Credit: The microscope : its history, construction, and application, being a familiar introduction to the use of the instrument, and the study of microscopical science / by Jabez Hogg. Source: Wellcome Collection.

Provider: This material has been provided by University of Bristol Library. The original may be consulted at University of Bristol Library.

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$perfection of the objective of the microscope so much depends. Chromatism arises from tlic uneqxial refrangibiUty and length of the different coloured rays of light that together go to make up white light; but which, when treated of in optics, is always associated with achromatism, so that a combination of prisms, or lenses, is said to be achromatic when the coloured rays arising from the dispersion of the pencil of light refracted through them are combined in due propor- tions as they are in perfectly white light. A lens, however, of uniform material will not form a single white image, but a series of images of all colours of the spectrum, arranged at difi'ereut distances, the violet being nearest, and the red the most remote, every other colour giving a blurred image; the superposition of these and the blending of the different elementary rays furnishing a complete explanation of the beautiful phenomenon of the rainbow. Sharpness of outline is rendered quite impossible in such a case, and this source of confusion is known as chromatic aberration. In order to ascertain whether it is possible to remedy this evil by combining lenses of two different materials, Newton made some trials with a compound prism composed of glass and water (the latter con- taining a little sugar of lead), and he found it impossible by any arrangement of these two, or by other substances, to produce devia- tion of the transmitted light without separation into its component colours. If this ratio were the same for all substances, as Newton supposed, achromatism would be impossible ; but, in fact, its value varies greatly, and is far greater for flint than for crown glass. If two prisms of these substances, of small refracting angles, be com- bined into one, with their edges turned in opposite directions, they will achromatise each other. The chromatism of lenses may, however, be somewhat further reduced by stopping out the marginal rays, but as the most perfect correction possible is required when lenses are combined for micro- scopic uses, other means of correction are resorted to, as will be seen hereafter. I shall first proceed to show the deviations which rays of white light undergo in traversing a lens. If parallel rays of light pass through a double-conve.K lens the violet rays, the most refrangible of them, will come to a focus at a point much nearer to the lens than the focus of the red rays, which are the least refrangible ; and the intermediate rays of tlie spcctrvmi$