The pleasure arising from a just view of a microscopic object, the distinctness of vision, &c. depend on a due management of the light, and adapting the quantity of it to the nature of the object, and the focus of the magnifier; therefore, an object should always be viewed in various degrees of light. It is difficult to distinguish in some objects between a prominency and a depression, between a shadow and a black stain; and in colour, between a reflection and a whiteness; a truth which the reader will find fully exemplified in the examination of the eye of the libellula, and other flies, which will be found to appear exceedingly different in one position of the light from what they do in another.
The brightness of an object depends on the quantity of light; the distinctness of vision, on regulating the quantity to the object; for some will be lost and drowned, as it were, in a quantity of light that is scarce sufficient to render another visible, as a different portion of light under the same apparatus will often exhibit in perfection, or totally conceal an object in the substance to be examined. This is more particularly the case with the animalculæ infusoriæ, whose thin and transparent form blend as it were with the water in which they swim; the degree of light must therefore be suited to the object, which, if dark, will be seen best in a strong and full light, but if very transparent, it should be examined in a fainter.
A strong light may be thrown on an object various ways: first, by means of the sun and a convex lens; for this purpose, place the microscope about three feet from a southern window; take a deep convex lens, that is mounted in a semicircle and fixed on a stand, so that its position may be easily varied; place this lens between the object and the window, so that it may collect a considerable number of the solar rays, and refract them on the object, or the mirror of the microscope. If the light thus collected from the sun be too powerful, it may be tempered by placing a piece of oil paper, or a glass lightly greyed, between the object and the lens: by these means, a convenient degree of light may be obtained, and diffused in an equal manner over the whole surface of an object, a circumstance that should be particularly attended to; for if the light be thrown in an irregular manner, that is, larger portions of it on some parts than on others, it will not be distinctly exhibited.