The micrographic dictionary : a guide to the examination and investigation of the structure and nature of microscopic objects / by J.W. Griffith and Arthur Henfrey.

Date:
1875
Catalogue details

Licence: Public Domain Mark

Credit: The micrographic dictionary : a guide to the examination and investigation of the structure and nature of microscopic objects / by J.W. Griffith and Arthur Henfrey. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    The mirror should have one plane or fiat face, and another concave. It should not be too small, and its centre should coincide with the axis of the body of the microscope. A double arm enables the mirror to be brought more considerably to either side, so as to throw more oblique light upon an obj ect. So long as the above conditions are fulfilled, the general form and arrangement of the stand and its parts are of little consequence. It must also be remembered that the complication and accuracy of the apparatus required will vary according to the hind of investigations pursued ] thus the structure of the various tissues of animals, and that of most plants, can be satisfac- torily studied with apparatus which is totally insufficient to display the structure of certain of the more minute and difficult objects. But, on the other hand, it follows that if a peculiar structure can be shown to exist in any kind of objects by a complicated apparatus, which cannot be demonstrated by a more simple or less perfect apparatus, the study of the struc- ture of any object not previously examined must always be attended with uncertainty so long as it has not been tested by the more perfect kind of apparatus,—provided the micro- scopist has not acquired the art of replacing the imperfection of his apparatus by superior tact and management, which can be done to a great extent. Object-glasses.—The goodness of the object-glasses depends mainly upon their freedom from chromatic and spherical aberration, and upon the magnitude of their angular aperture. The freedom from the former renders them good in defining power, i. e. in exhibiting clearly the margins of objects, whilst large angular aperture renders them capable of penetration, or of rendering markings upon the surface of objects visible or distinct. At least this is the ordinary statement made in regard to the relations of defining and penetrating power; but it is only partially true, and there are two kinds of penetrating power, as we shall show in the article Test-Objects, where we have entered more fully upon this subject. As in the case of the stand &c. of microscopes, so in regard to the object-glasses; the best are made in this country, and can be obtained of first-rate quality of the three makers above-mentioned. But the palm in regard to the highest powers must be given to Powell and Lealand, who alone construct a of an inch object-glass, and a -^-^ with an angle of aperture of 175°. At the same time, the modern German immersion-lenses, as they are called (Object-glasses), resolve perfectly most of the difficult valves of the Biatoma- cepe ; and they are cheaper than the English glasses. Some of the American oljject-glasses also, which are but little known in this country, must stand in the first rank in regard to excellence in defiuing, and especially penetrating power. When a glass of unknown value, however, presents itself, it should be tried upon the test-objects. The defining power may be tested by the examination of the objects figured in Plate 1. figs. 1 to 4. The outlines or margins of these objects must appear black, well defined, and perfectly free from colour, not misty and red or green ; they should retain this- appearance when the higher eyepieces are used, of cotu-se some allowance being made in regard to this sharpness of outline, which will appear slightly broader and less defined, but nowise interfering with the distinctness of the image of the object. The various parts of an object lying in the same plane, as a transverse section of whalebone, should also be visible at the same focus; the lines upon a micrometer used as a slide will also serve to test this point. It is not, however, of very great importance, especially with high powers; but it is a character of a superior object-glass. If the definition of the glass be good, the field flat, and the power adequately high, it will also exhibit the structure of the objects in Plate 1. figs. 5, 6, 10, 12, and 13 clearly and distinctly ; it is then of sufficiently good quality for nearly all the pui'poses required in the investigation of animal and vegetable structures. Some German and French glasses