Air bubbles are a perplexing source of trouble. The better way of becoming accustomed to deceptive appearances of the kind is to compare the aspect of globules of oil in water with bubbles of air in water, or Canada balsam. The molecular movements of finely divided particles, seen in nearly all cases when certain objects are first suspended in water, or other fluids, are a frequent cause of embarrassment to beginners. If a minute portion of indigo or carmine be rubbed up with a little water, and a drop placed on a glass slide under the microscope, it will at once exhibit a peculiar _perpetual motion_ appearance. This movement was first observed in the granular particles seen among pollen grains of plants, known as _fovilla_, and which are set free when the pollen is crushed. Important vital endowments were formerly attributed to these particles, but Dr. Robert Brown showed that such granules were common enough both in organic and inorganic substances, and were in no way “indicative of life.”[42] Professor Jevons succeeded in throwing light on these curious movements. He showed that they were not due to evaporation, as some observers contended, as they continue when all possibility of evaporation is cut off, when the fluid is surrounded by a layer of oil, and enclosed in an air-tight case: but as Professor Jevons pointed out, these movements are greatly affected by the admixture of various substances with water, being increased by a small quantity of gum, and checked by a drop of sulphuric acid, or a few grains of some saline substance, which increases the conducting power of water for electricity. The Brownian movement, now termed _pedesis_, much depends upon the size of the particles, their specific gravity, and the nature of the liquid in which they are immersed. The correct conclusions to be drawn by the microscopist regarding the nature of an object will necessarily depend upon previous experience in microscopic observations, a knowledge of the class of bodies brought under observation, and the skill of the observer in the use of the instrument--that is, in securing the best focus possible with any objective brought into use. I am indebted to Messrs. Beck for the following series of illustrations, showing the effect of under and over correction of the objective. DIRECTIONS FOR FINDING THE BEST FOCUS. The method of finding and determining when the screw-collar adjustment of the high-power objective has arrived at a point of perfect definition and magnification is as follows:--