xxxviii INTROD UCTION. 



aud may usually 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 Faramecium), 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. 32. fig. 1 6). When the substance of the object is some- 

 what 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. 



In relation to this matter, the meaning of the " optical section " of minute objects viewed 

 under the microscope is important to be considered. Taking a globule of oil, or a pollen- 

 grain, and bringing the object-glass to focus upon the upper surface, at first the portion of 

 the surface which is in focus is visible; on lowering the object-glass, the surface becomes 

 invisible or indistinct, while a portion of the margin comes into focus, forming a ring. 

 This enlarges until the most convex portion is reached, when it diminishes until the lower 

 portion of the globule or grain is visible. In this way we obtain the same views of the 

 object, as if so many transverse sections or planes were examined. The better the correc- 

 tions of the object-glass, and the larger the angular aperture, the more distinctly will these 

 phenomena be developed. 



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

 general surface, and the dots or depressions present an angular outline, these dots cannot 

 possibly 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 ex- 

 erted 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 larger Diatoinaceae, the examination 

 of the margins of crushed and perfectly flat portions is important and sometimes conclusive ; 

 for it may be found, as in Isthmia (PI. 17. fig. 26) 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 

 Piatomacese (Pleurosic/ma e.), in which this observation is difficult to be made, the point 

 is important that the line of fracture of the broken valve passes through the rows of dark 

 dots or the lines corresponding to them, showing 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 Pleurosiyma, has long formed a disputed point. In 

 distinguishing in general minute points, as the little siliceous spines of the cuticle of Equi- 



