Original paper
Diatom Research and the Scanning Electron Microscope
Wornardt, Walter Wm. jr.
Nova Hedwigia, Beihefte Beih. 31 (1970), p. 355 - 376
29 references
published: Apr 1, 1970
DOI: 10.1127/nova.beihefte/31/1970/355
ArtNo. ESP051003100012, Price: 29.00 €
Abstract
Ever since O. Müller described Vorticella pyraria (a species of Gomphonema) in 1773, the light microscope has been used as the basic tool in diatom research. In the early 19th century, Ehrenberg, the father of micropaleontology, described thousands of diatoms with microscope lenses that were rather poor. About the same time Kützing (1844) used morphologic features, that were difficult to resolve, as a basis for a diatom classification. This included the markings on the valves which he thought were thickenings of the wall substance. About 1851-1856, Smith used somewhat sophisticated optical lenses to describe the microstructures on the diatom valves. Somewhat later, Müller (1871) described the areolate structure of the valves of Triceratium is detail. About 60 years later, microscope objectives had very precise and high qualitiy optical lenses. Thus, Hustedt (1928-1959), the pre-eminent diatomist, used these excellent lenses to describe many minute features and to systematize much of the previous diatom taxonomy. Although optical lenses and objectives have improved drastically since the time of Anton van Leeuwenhoek (1632-1723), several inherent limitations have prevailed in the light microscope; they are, field depth, magnification, and resolution. Field depth or vertical resolution, is the most important factor that limits the examination of three dimensional objects. The field depth is controlled by the aperture of the objective. If the angle of the light rays that enter the lens decreases, the field depth will increase. As this angle is reduced, there is an appreciable loss in the amount of light gathered by the lens and therefore a reduction in the numerical aperature. Inasmuch as the N. A. is nearly directly proportional to the resolution of a lens, the reduced N. A. of the objective results in a corresponding loss in resolution. Magnification is also a direct function of the N. A. and it should not exceed 1000 times the numerical aperature of the objective. Inasmuch as the better condensers and objectives have a N. A. of about 1. 3, most photomicrographs should not exceed 1300 diameters. The finest lenses and objectives available have a large numerical aperature, high resolution and magnification, but a shallow depth of field. This is due to an optical law that states, the vertical magnification in an image is the square of the lateral magnification. Therefore, the high magnification is obtained only at the expense of less field depth.
Keywords
diatom • SEM • Scanning electron microscope