[3] It seems generally admitted that, within a certain range of power not exceeding that of a lens of 1/20th of an inch focus, the beauty and truth of the vision given by the new compound microscopes cannot be equalled by that of any single instrument, at least of glass. It is no less true, however, that the _picture_ of the compounds, however perfect, is not like a real object, will not admit of amplification beyond a certain point with advantage. Under the action of very deep eye-glasses, the image of opaque objects especially, first loses its strong, well-determined outline—then grows soft and nebulous, and finally melts away in shadowy confusion. Let the experiment be made of raising the power of a compound up to that of a 1/60th inch lens—then try it against the single microscope of that power (having, of course, the utmost opening the nature of the object viewed will permit). The observer, if open to conviction, will soon be taught the superior efficacy of the latter—for it will show the lines on the dust of Menelaus with such force and vivacity, that they will always be apparent _without any particular management of the light—nor can their image be extinguished by causing the illumination to be directed truly through the axis of the lens (as it always may in the compounds)_. A due consideration of the teeth and inequalities on the surface of a human hair, together with the _transverse connecting fibres between the lines on the scales of the curculio imperialis_, viewed as opaque objects, will suffice to complete the illustration of the subject; though the last object is not to be well seen by that kind of light which is given by silver cups—and a single lens of 1/60th inch focus can of course have no other. The effectiveness and penetrating faculties of simple magnifiers are invariably increased by an accession of power however great—that of compounds seems to be deteriorated beyond certain limits. An opinion may be hazarded that the achromatics and reflectors yet made _do not really surpass the efficacy of equivalent single lenses, even of glass, when their power exceeds that of a_ 1/20th lens, from 1/20th to 1/40th the vision may be about equal—but from 1/40th upwards infinitely inferior.