On the diatom prism, and the true form of diatom markings / by J.B. Reade.

  • Joseph Bancroft Reade
Date:
1869
Catalogue details

Licence: Public Domain Mark

Credit: On the diatom prism, and the true form of diatom markings / by J.B. Reade. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    great difference in the diameter of the hemispheres, and in their distance from each other. We are told, popularly, and in suffi- ciently vague language, so far as structure is concerned, that the “striae” range between about 30 and 100 in roWth of an inch, and, to adduce an example, that the striae of P. strigilis are much closer than those of P. formosum—a statement which gives no idea of the fact that the diameters of their hemispheres are the same. Doubtless it would be more accurate to give the number of the hemispheres and the measure of the space between them, or the ratios of the diameter and the interval. In my own microscope, with Boss’s Ath and a double D eye- piece, the diameter of the field of view at the distance of the stage from the eye is 12 inches, and this space represents the magnified image of xoVirth. of a inch, on a micrometer-slide ruled by Mr. Waterhouse. The magnifying power is therefore 12,000 linear. Using this arrangement, P. Quadratum has 40 hemispheres and 40 intervals in the diameter of the field, i. e. in 12 inches, which cover ToVoth of an inch on the micrometer-slide, and as each interval is equal to a radius of a hemisphere, the magnified diameter of each hemisphere covers f^ths, and the interval TVth of an inch. Therefore, the real diameter of the hemispheres is GOoooth, and of each interval i goV^th of an inch. The rows of hemispheres cross each other at an angle of 60°, as in P. angulatum, and are there- fore arranged in the order of the sides of an equilateral triangle. Hence, under the illusion of the common methods of illumination, which deal with shadows only, and under deep powers, the markings of these diatoms are described and figured as hexagons, with the sides and centre light and dark, or vice versa, and Photography stands by as an attesting witness. But this illusion arises from causing either the illuminated or the shaded portions of the hemi- spheres to run into each other, and so to form hexagons with either dark or light centres. In a valuable paper by Dr. Wallich, “ On the Development and Structure of the Diatom-valve,” communicated to the Microscopical Society in March, 1860, it is stated that “in P. formosum there exists good evidence to prove that the interlinear spaces are occu- pied by elevated rhomboidal papillae, which present facetted surfaces, whereas in P. balticum, instead of rhomboidal elevations, we have four-sided flattened pyramids, presenting, as in the former case, four sets of lines, of which those bounding the spaces, and not crossing them, are the predominant ones.” No one will be more pleased than Dr. Wallich with the very different, but more truthful representation of these valves when illuminated by the diatom- prism which I will presently describe. In both valves we have rows of siliceous hemispheres. Those in P. formosum are at right- angles to each other, and meet the longitudinal division of the valve
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