MICROSCOPIC ANALYSIS. SURFACE. xxxix 



set urn, the vevy short spines on the wings of many insects {Tipididce Sec), ov the minute 

 spheroids in Schultze's siliceous films, it may aid somewhat to remember that prominences 

 are usually most distinct under open central illumination, while depressions are most evi- 

 dent under the central-stop illumination. If we take a flat fragment of an Irfhmia, and 

 examine it by the aid of the condenser with a central stop and an object-glass of lower power, 

 care being taken that the condenser and stop are perfectly central, it will exhibit a series 

 of angular dark or black dots bounded by luminous lines separating them (PI. 15. fig. 47), 

 and this when all parts of the object are best in focus ; for when the object-glass is elevated 

 or depressed, the whole becomes indistinct. The black dots in this instance clearly coin- 

 cide with the depressed portions of the surface of the valve. The same phenomena may 

 be observed in many other Diatoinaceae, as Triceratium (PI. 17. fig. 29), Coscinodiscus 

 (PL 51. fig. 1), &c. But when we examine those which have very fine markings, as the 

 valves of Pleurosigma (PL 15), the appearances vary greatly, according to the method of 

 illumination. The dark lines (PL 15. figs. 10, 12, and 25), which are brought to view by 

 oblique illumination, correspond to the rows of the black dots of Isthmia (the thinner and 

 weaker portions of the valves), and with stop-illumination are resolved into the separate dots 

 (PL 15. fig. 40), the rest of the valve appearing white and uniform. But at a different 

 focus, especially under immersion-lenses, the white portions of the valve present the 

 appearance of rows of bright pearls (PL 15. fig. 40), the other portions of the valve appear- 

 ing dark. The same brilliant pearls are seen in the finer valves of Coscinodiscus with the 

 open central illumination. The article Diatomaceje 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 degrees of obliquity of the transmitted fight. 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 saccharina and the thin 

 glass covering it, assume an elongated form, being limited laterally by the ridges upon the 

 scale (PL 34. fig. 3). 



o. 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 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 examined. Sometimes the polariscope is of use : the general substance may 



