MICROSCOPIC ANALYSIS. SURFACE. xxxiii 



with a highly refractive granule. The appearance, however, arises from diffraction, and 

 may be distinguished from that produced by refraction, by the luminous spot equalling or 

 exceeding the granule in size, whilst in the latter it is smaller and more brilliant. 



i. In all these experiments, the less oblique the light the more certain will be the results. 

 But this method is inapplicable to decide whether the less refractive portions are simply 

 depressions or cells. This may often be determined by examining the margin of the object, 

 where possible (as in Paramecium), and observing whether there are depressions upon it 

 corresponding to the parts at which the dots are situated, and whether these depressions are 

 continuous with the dots (PI. 25. fig. 1 6). When the substance of the object is somewhat 

 firm, drying it, if moist, will cause the dots to become filled with air ; they will then, if cells, 

 appear infinitely blacker than if simply depressions, and visible as readily by direct as by 

 oblique light; and after the object has been moistened with water or oil of turpentine, if it 

 be immediately examined, the blackness of the dots will appear still greater, and they will 

 be distinctly visible by direct light ; whilst depressions are much more easily filled with 

 liquid, and then, if minute, will only be visible by oblique light. 



k. If it can be shown that the parts corresponding to the dots are depressed below the gene- 

 ral surface, and the dots or depressions present an angular outline, these dots cannot possi- 

 bly represent cells ; because, if the angularity of the outlines of cell structures arose from 

 the pressure of surrounding or adjacent cells, this pressure would necessarily be exerted also 

 upon the free or external portion of each cell, so as to render it convex, or at any rate, not 

 concave. The firmness of the substance of the object must be attended to ; because where 

 it is absent, as the cells part with the liquid portion of their contents, the outer portion of 

 the cell-wall may become approximated to the inner, and thus no space be left for the air 

 to enter ; as in the exuviae of a Triton for instance. 



/. In brittle objects, as the siliceous valves of the Diatomacese, the examination of the mar- 

 gins of the fractured portions is important and sometimes conclusive, for it may be found, 

 as in Isthmia, &c., that the depression of the object-glass requisite to bring into focus the 

 margins of the thin depressed portion, is much greater than that required for the intermediate 

 thicker parts. In the valves of the more delicate Diatomacese (Gyrosigma, &c.), in which 

 this observation is difficult to be made, the point is important that the line of fracture of the 

 broken valves passes through the rows of dots or the dark lines corresponding to them, show- 

 ing that they are thinner and weaker than the rest of the substance ; had these dots 

 represented elevations, the valves would have been stronger at these parts. The nature 

 of the markings upon the siliceous valves of the Diatomacese, especially the species of 

 Gyrosigma, has long formed a much-disputed point. The continental microscopists take 

 little or no notice of them ; in fact, they are not generally acquainted with them, because 

 they do not comprehend the importance of the use of either the condenser with its stops, the 

 adjustment for the thickness of the glass cover, or the methods of compensating for the defi- 

 ciency of angular aperture which their glasses possess; and without attention to these circum- 

 stances they cannot be seen. If we take a flat fragment of an Isthmia, and examine it by 

 the aid of the condenser with a central stop and an object-glass of low 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. 11. 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 coincide with the depressed portions of 

 the surface of the valve. On examining a fragment of the valves of a Gyrosigma strigosum 

 or angulatum, &c., exactly the same phaenomena'are witnessed when the parts of the object in 

 view appear at their most distinct focus, the black dots being bounded by angular short con- 

 tinuous lines, giving them the appearance of being distinctly hexagonal. On inclining the 



