Persons not experienced in the use of the microscope will not be displeased that I here insert some remarks which will be useful to them, if they repeat the following experiments, or make new ones. We should give the preference to double microscopes, in which we see objects perpendicularly, from their having a plain or concave mirror, which shews the objects clear; the concave mirror is the most preferable when the observations are made with the strongest lens. Leeuwenhoek, who undoubtedly has been the greatest and most indefatigable of all microscopical observators, is said to have only made use of simple microscopes, with which he viewed objects horizontally. If this is true, it is necessary to remark, that most of the plates given by Leeuwenhoek of microscopical objects, especially spermatic animals, represents them much thicker and longer than he really saw them, which renders the microscopes we speak of preferable to the horizontal, as they are more stable; the motion of the hand, with which the microscope is held, producing a little trembling, which causes the object to appear wavering, and never presents the same part for any time. Besides, there is always a motion in the liquors caused by the agitation of the external air, at least, if we do not put the liquor between two plates of glass, or even fine talc, which diminishes somewhat of its transparency, and greatly lengthens the experiment; but the horizontal microscope, whose tables are vertical, has the still greater inconvenience, that the most ponderous parts of the drop of liquor fall to the bottom; consequently there are three motions, that of the trembling of the hand, the agitation of the fluid by the action of the air, and also that of the parts of the liquor falling to the bottom: from the combination of which, certain small globules, which we see in these liquors, may appear to move by their own motion and powers, while they only obey the compounded power of those three causes.