MICROSCOPIC ANALYSIS. SURFACE. xxxvii 



glass cover applied, and the cover be moved backwards and forwards upon the slide with 

 the linger covered with a cloth, the drop of oil will be subdivided into globules of various 

 sizes, some of which will enclose globules of water ; thus we shall have globules of the 

 oil surrounded simply by water, globules of water enclosed in globules of oil, and some of 

 these globules will contain, within them globules of the other kind again, the globules of 

 oil being readily distinguished by their red colour. On examining the slide with a tole- 

 rably higli power, all the globules will appear bounded by a black circle, and present a 

 luminous point in the centre, when viewed separately and the focus suitably adjusted for 

 each. But when they are examined in comparison and together, they will be found to 

 exhibit characteristic appearances according to the variation of the focus. Thus, of the 

 simple globules, when their margin is most distinctly brought into focus, some will become 

 more luminous as the object-glass is depressed (PI. 49. fig. I a) these are globules of 

 water surrounded by oil ; others will become darker under the same circumstances (PL 49. 

 fig. 1 6), and very luminous as the object-glass is raised (PI. 49. fig. 1 c) these are globules 

 of oil ; and the nature of the components of the compound globules may easily be deter- 

 mined by the occurrence of the same phenomena. The globules of oil being more highly 

 refractive than the water, act like little convex lenses ; whilst the globules of water sur- 

 rounded by the oil, exerting a lower refractive power than the latter, act like concave 

 lenses, and their centre appears luminous because the rays which traversed them diverge 

 as they ascend, as if they emanated from a (virtual) focus situated beneath the globules, or 

 on the same side of them as the mirror. Hence these foci may be distinguished as the 

 " lenticular foci " of the objects. And when dots or markings upon objects are very 

 minute, frequently all that can be distinguished under the microscope are these lenticular 

 foci of the various parts. 



The same phenomena may be observed in air-bubbles immersed in water ; these corre- 

 spond with the globules of water surrounded by the oil. It need scarcely be remarked 

 that the object in colouring the oil is to allow of the control of the conclusions arrived at. 

 g. In the globules of sarcode or protoplasm and many cells, the vacuoles are easily shown, 

 by the same method, to be filled with a material of less refractive power than the general 

 substance of which they are composed : these vacuoles are frequently mistaken for nuclei 

 and nucleoli ; but they are readily distinguished from them by the dark appearance they 

 present when the object-glass is raised above the focus of their margins. 



h. The above principles are applicable to the determination of numerous cases where 

 the elevation or depression of a spot or marking upon a surface is called in question ; 

 for elevations on a surface will produce the general effect of convex lenses, whilst 

 depressions will produce that of concave lenses. In the above experiment, plano-convex 

 lenses of both oil and water are frequently seen, and readily distinguished by the above 

 means. 



Take also the instance of a Paramecium aurelia, either dried or immersed in water. 

 The surface is beautifully marked with pretty regular dots, which appear luminous as the 

 object-glass is depressed (PL 32. fig. 1 ), and dark as it is elevated (PL 32. fig. 1 6) ; hence 

 they consist of depressions upon the surface. Had they been elevations or little tubercles, 

 they would have become more luminous as the object-glass was raised, and vice versa. 



When an isolated granule of pigment or of any opaque substance is brought into focus, 

 on raising the object-glass a luminous spot appears to occupy its place ; hence it agrees 

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



