APPEARANCES OF GLOBULES OF AIR, ETC. 185 



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 

 convergence above the globule (i. e. between the globule and the 

 objective), its brightest image is given, when the object-glass is 

 removed somewhat further from it than the exact focal distance 

 of the object. On the other hand, the globule of wat^r 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 greatest lumi- 

 nosity will be found, by causing the object-glass to approach 

 within the proper focus. Now in the "protoplasm" of the cells 

 of the lower Plants, and in the " sarcode" of the lower animals, 

 oil-particles and vacuoles (or void spaces) are often interspersed ; 

 and present, at first sight, so very striking a resemblance, that 

 the inexperienced observer may well be pardoned for mistaking 

 the " vacuoles" for larger globules of a material more refractive 

 than the gelatinous substance around them. But the difference 

 in the effects of alterations of focus on the two sets of appear- 

 ances, at once serves to make evident the difference of their 

 causes; and this, moreover, is made obvious by the effect of 

 oblique light, which will cause the strongest shadow to exhibit 

 itself on opposite sides, in the two cases respectively. It will be 

 obvious that minute elevations and depressions of the surface of 

 the object will exert an influence upon the course of the rays 

 which it transmits, very similar to that which proceeds from the 

 presence of globular spaces, filled with transparent substances of 

 greater or less refracting power, in its interior ; and that the dis- 

 crimination between the two may be made by the same' means. 

 For if the dots appear more luminous as the object-glass is raised, 

 and darker as it is depressed, they may be interpreted as being 

 due to convexities upon the surface ; but if the contrary is the 

 case, they may be referred to concavities. 



100. Among the sources of fallacy by which the young Micro- 

 scopist is liable to be misled, one of the most curious is the Mole- 

 cular Movement which is exhibited by the particles of nearly all 

 bodies that are sufficiently finely divided, 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 pollen-grains of plants (sometimes termed the 

 fovilla\ and which are set free by crushing these grains ; and it 

 was imagined that they indicated the possession of some special 

 vital endowment of these particles, analogous to that of the sper- 

 matozoa of animals. In the year 1827, however, it was an- 

 nounced by Dr. Bobert Brown, that numerous other substances, 

 organic and inorganic, when reduced to a state of equally minute 



