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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    cide with the depressed portions of the surface of the valve. On examining a fragment of the valves of a Gyrosigma strigosimi or angulatum mider a high power, for they are not visible under a low one, exactly the same phenomena are witnessed when the parts of the object in view are perfectly fiat and appear at their most distinct focus, the blaclj dots being bounded by angular short continuous lines, giving them the appearance of being distinctly hexagonal. On inclining the mirror somewhat, so as to render the light trans- mitted through the object irregular or unequally oblique, the appearances will be reversed, a number of luminous dots resembling pearls (PI. 11. fig. 46) being visible, bounded by dark spaces. These are the lenticular foci of the little knots formed by the union of the raised bars existing between the]^depressed portions of the valve. In PI. 11. fig. 41 is a diagram of a portion of a valve of Gyrosigma angulatum, magnified to the enormous extent of 15,000 diameters, taken from a photogTaph lent us by Mr. F. H. Wenham; and the same appearances may be seen imder a lower power in a very large number of the Diato- macese. The black hexagonal dots in the latter figure correspond to the black dots seen in Isthmia, and represent the depressed portions of the valves. The article Diatomacb^ must be consulted for fm-ther details in regard to the structure of these valves, and the article Angulab Aperture in regard to the changes produced in the appearances of ob- jects by variation of the angular aperture of the object-glass, and of the degree of obliquity of the transmitted light. But we may remark here, that these dots must not be compared to cells, but to the depressions found upon the seeds of the white poppy, Paramecium, &c., in which forms resembling those resulting from the mutual pressure of adjacent cells are present, but do not arise, so far as we know, from this cause. m. No special remarks are required in regard to furrows, as these are only elongated depressions. n. When ridges are present, these are frequently left projecting at the margin of a frag- ment; sometimes they project naturally; and it may readily be known that they are thicker portions of structure, by their blacker margins and their exhibiting the characters of elon- gated convex or plano-convex lenses. In some cases, the position assumed by confined portions of air, when the object is im- mersed in liquid, will denote the existence of ridges. Thus we have seen portions of air, accidentally confined between the surface of a scale of Lepisma saccliarina and the thin glass covering it, assume an elongated form, being limited laterally by the ridges upon the scale (PL 27. fig. 3). 0. Foramina or holes are in general readily distinguished by their dark and defined margins, and the absence of colour when they exist in coloured structures; when existing in transparent colom'less objects, the latter mostly exhibit minute irregularities, by which the presence of some kind of matter is indicated, whilst these are absent in the foramina. Where there is difiiculty in deciding, the structure should be broken, if possible, and the margins examined. Sometimes the polariscope is of use; the general substance may polarize light, but of course the foramina will not do so. Charring the structure, or colouring it with reagents, if organic, will sometimes afibrd decisive proof. Foramina cannot be mistaken for elevations on the surface, because they do not become more luminous as the object-glass is raised, after their margin has been brought most distinctly into focus; in fact the reverse occurs : hence they so far agi'ee with depressions; but they difier from these in their luminous appearance with high powers, and their not being rendered more distinct by oblique light, but the reverse. 2). When the structm-e in which they are situated is somewhat thick, and they form rather tubes than foramina, as the axes of these can hardly coincide with the direction of the transmitted light, their orifices will appear dark or black; hence they might be mis- taken for granules of pigment: immersion or maceration of the structure in oil of turpeu-