MANAGEMENT OF THE MICROSCOPE. 153- 



ing a thin-glass cover upon them, and then moving the cover several 

 times backwards and forwards upon the slide. Now when such a mix- 

 ture is examined with a sufficiently high magnifying power, all the glo- 

 bules present nearly the same appearance, namely, dark margins with 

 bright centres; but when the test of alteration of the focus is applied to 

 them, the difference is at once -revealed; for whilst the globules of Oil 

 surrounded by water become darker as the object-glass is depressed, and 

 lighter as it is raised, those of Water surrounded by oil become more 

 luminous as the object-glass is depressed, and darker as it is raised. The 

 reason of this lies in the fact that the high refracting power of the Oil 

 causes each of its globules to act like a double-convex lens of very short 

 focus; and as this will bring the rays which pass through it into conver- 

 gence above the globule (L e., between the globule and the Objective), its 

 brightest image is given when the object-glass is removed somewhat 

 farther from it than the exact focal distance of the object. On the other 

 hand, the globule of Water in oil, or the minute bubble of air in water 

 or balsam, acts, in virtue of its inferior refractive power, like a double- 

 concave lens; and as the rays of this diverge from a virtual focus below- 

 the globule (i. e., between the globule and the mirror), the spot of great- 

 est luminosity will be found by causing the object-glass to approach within 

 the proper focus. A thorough mastery of these appearances is very im- 

 portant in the study of the ' protoplasm ' of Plants the ' sarcode ' of 

 Animals, which includes oil-particles, together with spaces occupied by 

 a watery fluid, which (having been at one time supposed to be void) are 

 known as 'vacuoles.' 



155. Among the sources of fallacy by which the young Microscopist is 

 liable to be misled, one of the most curious is the movement exhibited by 

 very minute particles of nearly all bodies that are sufficiently finely di- 

 vided, when suspended in water or other fluids. This movement was first 

 observed in the fine granular particles which exist in great abundance 

 in the contents of the Pollengrains of plants (sometimes termed the 

 fovilla), and which are set free by crushing them; and it was imagined 

 that they indicated the possession of some special vital endowment by 

 these particles, analogous to that of the Spermatozoa of animals. In the 

 year 1827, however, it was announced by Dr. Kobert Brown that numer- 

 ous other substances. Organic and Inorganic, when reduced to a state of 

 equally minute division, exhibit a like movement, so that it cannot be 

 regarded as indicative of any endowment peculiar to the fovilla granules;, 

 and subsequent researches have shown that there is no known exception 

 to the rule that such motion takes place in the particles of all substances, 

 though some require to be more finely divided than others before they 

 will exhibit it. The closer the conformity between the specific gravity 

 of the solid particles and that of the liquid, the less minute need be that 

 reduction in their size which is a necessary condition of their movement: 

 and thus Carmine, Indigo, or Gamboge rubbed up with water, show it 

 extremely well; whilst the particles of Metals, which are from seven to 

 twenty times as heavy as water, require to be reduced to a minuteness 

 many times greater, before they will exhibit it. The movement is chiefly 

 of an oscillatory kind; but the particles also rotate backwards and for- 

 wards upon their axes, and gradually change their places in the field of 

 view. The movement of the smallest particles is the mosb energetic, and 

 tho largest (exceeding l-5000th of an inch) are quite motionless, whilst 

 those of intermediate size move with comparative inertness. A drop of 

 common ink which has been exposed to the air for some weeks, or a drop 



