xxxii INTRODUCTION. 



water in which the objects are contained j of course this is only of use if the objects be 

 living ; the particles will then be set in motion, and their motion may be distinguished from 

 molecular motion by the definite direction in which the particles move. 



/. The nature of many markings, spots, &c. is best determined by comparing the effects of 

 the refraction of the transmitted light at different foci produced by the markings themselves, 

 and the substances in which they are situated ; and these phenomena may be conveniently 

 illustrated by their occurrence in known objects. If a drop of oil of turpentine, which has 

 been digested with alkanet root so as to become coloured, be placed upon a slide, a drop of 

 water added to it, a thin glass cover applied, and the cover be moved backwards and for- 

 wards upon the slide with the finger covered with a cloth, the drop of oil will be subdivided 

 into globules of various sizes, some of which will enclose globules of water j 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 tolerably high power, all the globules will appear bounded by a black circle, and pre- 

 sent a luminous point in the centre, when viewed separately, and the focus suitably ad- 

 justed 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. 40. fig. 1 a), these are globules 

 of water surrounded by oil ; others will become darker under the same circumstances (PI. 40. 

 fig. 1 6), and very luminous as the object-glass is raised (PL 40. 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 traverse 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. 



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 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 ele- 

 vations 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 (PI. 25. fig. 1 a), and dark as it is elevated (PI. 25. fig. 1 b) ; hence they con- 

 sist 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 opake substance is brought into focus, on 

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



