When we put a drop of liquor on the table of the double microscope, although horizontally placed, and in the most advantageous situation, we still see one common motion in the liquor, which forces all what it contains to one side. We must wait till the fluid is in an equilibrium and at rest, before we make our observations; for it often occurs, that this motion of the fluid hurries away many globules, and forms a kind of whirling motion, which returns one of these globules in a very different direction to the others. The eye is then fixed on the globules, and seeing one take a different course from the rest, supposes it an animal, or at least a body, which moves of itself, whereas its motion is only owing to that of the fluid; and as the liquor is apt to dry and thicken in the circumference of the drop, endeavours must be made to fix the lens on the centre of it. The drop should also be as large as possible, and contain as much liquor as will permit a sufficient transparency, to see perfectly what it contains. Before we begin to make observations, we should have a perfect knowledge of our microscope. There is no glass whatsoever but in which there are some spots, bubbles, threads, and other defects, which should be nicely inspected, in order that such appearances should not be represented as real and unknown objects: we must also endeavour to learn what effect the imperceptible dust has which adheres to the glasses of the microscope; a perfect knowledge of which may be acquired by observing the microscope several times. To make proper observations, the sight, or focus, of the microscope must not precisely fall on the surface of the liquor, but a little above it; as not so much reliance should be placed on what passes upon the surface, as what is seen in the body of the liquor. There are often bubbles on the surface which have irregular motions produced by the contact of the air. We can see much better with the light of two short candles, than in the brightest day, provided this light is not agitated, which is avoided by putting a small shade on the table, inclosing the three sides of the lights and the microscope. It will often appear as though dark and opaque bodies become transparent, and even take different colours, or form concentrical and coloured rings, or a kind of rainbow on the surface; and other matters, which are seen at first sight transparent and clouded, become black and obscure; these changes are not real, but only depend on the obliquity the sight falls on the body with, and the height of the plain in which they are found.