xxxiv INTRODUCTION. 



mirror somewhat, so as to render the Hght transmitted through the object unequally oblique, 

 the appearances will be reversed, a number of luminous dots being visible, bounded by dark 

 lines. In PL 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 photograph lent us by 

 Mr. F. H. Wenham (the lights and shades being, however, reversed) ; and PL 11. fig. 48 

 represents a portion of a valve of Gyrosigma 5^rf^osMm,magnified4700 diameters, as seen under 

 our own microscope. 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 Dia- 

 TOMACE^ must be consulted for further details in regard to the structure of these valves, 

 and the article Angular Aperture in regard to the changes produced in the appearances 

 of objects by variation of the angular aperture of the object-glass, and of the degree of obli- 

 quity of the transmitted light. But we may remark here, that these dots must not be com- 

 pared to cells, but to the depressions found upon the seeds of the white poppy, Parame- 

 cium, &c., in which forms resembling those resulting from the mutual pressm-e 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 structm*e, 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, wull denote the existence of ridges. Thus we have seen portions of air, 

 accidentally confined between the sm-face of a scale of Lepisma saccharina 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 transpa- 

 rent colourless 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 difficulty in deciding, the structure should be broken, if possible, and the margins ex- 

 amined. Sometimes the polariscope is of use ; the general substance may polarize light, but 

 the foramina will not do so of course. Charring the structure, or colouring it with re- 

 agents, if organic, will sometimes afford 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 agree with depressions ; 

 but they diff*er from these in their luminous appearance with high powers, and their not 

 being rendered more distinct by oblique light, but the reverse. 



p. When the structure 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 mistaken for 

 granules of pigment : immersion or maceration of the structure in oil of turpentine will, 

 however, fill them, and cause the dark appearance to vanish, whilst pigment would still be 

 visible. Examination by reflected light will also readily distinguish the one case from the 

 other. Also where this tubular structure is present, perpendicular sections will exhibit 

 furrows, which may be recognized as directed above. In distinguishing foramina, the higher 

 the power employed the less is the difficulty. 



q. It has sometimes to be decided, whether certain dark lines visible at the surface of objects. 



