The degree of perfection in the construction of the optical part of a microscope, whether simple or compound, is judged of by the dis tinctness and comfort (by which we mean the freedom from strain or effort on the part of the observer) with which it exhibits certain objects, the details of which can only be made visible by combinations of lenses of high magnifying power and a near approach to correctness. Such are called test objects. They are of various degrees of difficulty. For testing the penetrating powers of a microscope, the lined scales on the wings and bodies of certain in sects are commonly employed. The scales from the wings of many Lepidoptera are so coarsely marked, that a good ordinary com pound microscope, or deep single lens, will make the lines apparent. But there are many others, which, under such magnifying powers, only show a flat unmarked surface, requiring lenses of large angular aperture to make their lines visible. One of the most beautiful of these, and at the same time most easily re solved, is the scale of the Menelaus butterfly ; its longitudinal stria may be seen in the best ordinary microscope, but they require a cor rected object-glass to be well made out; and its transverse markings are only to be seen distinctly with a superior instrument. A scale with more delicate lines than these is the larger of those of the Lyeama Argus; the smal ler one will be presently noticed. The most difficult of the test-scales, however, is that of the Podura plumbea, or common spring-tail ; and a microscope which will distinctly exhibit its markings may be regarded as, for this class of objects, of the highest order. For defining power, however, another class of objects is needed. Among these one of the best is the small scale of the Lyerna Argus (orbattledore), which, with an inferior instrument, appears covered with coarse longitudinal lines; but these, when more perfectly defined, are found to be resolvable into separate circular or oval dots, arranged in a linear manner. The curi ous hair of the Dermestes, and that of the Bat, require a microscope of good defining power to represent their forms with clearness and accuracy. We have ourselves been accus tomed to employ the branching hairs of the common bee as tests of the correctness of a microscope of moderate power; for they have a remarkable tendency to produce fringes of colour, under most of the ordinary modes of illumination, unless there is a perfect freedom from chromatic aberration. The spiral fibres
lining the trachea: of many insects, also, will be found good tests of the defining power, and freedom from aberration, of a microscope. For still lower powers, we consider the glan dular dots in the woody fibre of the resinous trees (especially those of the order Conifetee) as very advantageous tests. They ought to be rendered distinctly visible with an object-glass of half an inch focus; and the depression in the centre should be clearly made out, with a perfect freedom from colour. We have seen objectives of English construction (in which a large aperture for showing opaque objects was the chief point aimed at) so defective in this respect, as to be far inferior in their exhibition of these and similar objects to French acro matics of much smaller aperture. We may here repeat the general rule, that those micro scopes are, ceteris paribus, the best, which will show the most with the lowest magnifying power. It will sometimes happen that, al though the details of an object may he made out with tolerable clearness, there is a sort of thin fog or mist over the whole field. This fault may proceed from the too great enlarge ment of the aperture of the objective, or from a faulty mode of illumination; or it may result from the imperfect extinction of the rays re flected within the body of the microscope from the surfaces of the lenses of the eye-piece, and from the interior of the tube itself,*— a fault which may be obviated by carefully coating the inner surface with a black covering, adapted to absorb all the false light. Black velvet may be advantageously used for this purpose. If the aperture be too great, the fault may generally be corrected by the use of stops beneath the stage, by which it may be diminished as required. This plan, which will be presently described more in detail, will be found very much to increase the applicabi lity of low-power achromatic lenses of that large aperture which is desirable for opaque objects.