In the microscope the _parabolic reflector_ fits into the cylindrical fitting under the stage, and the adjustment of its focus upon the object is made by giving it a spiral motion when fitted in--that is, carefully pushing it up or down at the same time that it is turned round by the milled edge B B. It must then be focussed by the rack and pinion motion. As the rays of light must be parallel when they enter it, a _flat mirror_, which in this case should be added to the instrument, is generally used; daylight will then require only direct reflection, but the rays from an artificial source will have to be made parallel by placing a side condenser between the light and the mirror, about 1-3/4 inch from the former and 4-1/2 inches from the latter. Nearly the whole surface of the mirror should be equally illuminated; this may be tested by temporarily placing upon it a card or piece of white paper. Parallel rays can also be obtained from the concave mirror, if the light is placed about 2-1/2 inches from it. Dark-ground illumination is not suitable for very transparent objects--that is, unless there is a considerable difference in their index of refraction, or they are pervaded by air-cells. [Illustration: Fig. 145.--Parabolic Reflector.] One very remarkable example of this may be seen in the tracheal system of insects. If any of the transparent larvæ of the various kinds of gnat be mounted in gelatine and glycerine jelly, slightly warmed but not enough to kill the insect outright, about the third day the fluids circulating in the body will be absorbed and replaced by air. Illuminated by the parabolic condenser, and viewed with a binocular microscope, and a low power, the gnat-larva becomes a superb object. The body of the insect is but faintly visible, and in its place is displayed a marvellous tracheal skeleton, with the tubes standing out in perspective, shining brilliantly, like a structure of burnished silver. Unfortunately, such objects are not permanent, for when the whole of the water dries up, the tracheal tubes either collapse or become refilled with fluid. As to the blackness of field, and luminosity of the object, this depends upon excess of light from the paraboloid received beyond the angle of aperture of the object-glass. It is found in practice that more and more of the inner annulus of rays from the paraboloid has to be stopped off, until at last, with high-angled objectives, it is scarcely possible to obtain a black field. The light, on the whole, most suitable for this method of illumination is lamp, the rays of which should in all cases be rendered more parallel by means of a large plano-convex lens, or condenser. [Illustration: Fig. 146.--Wenham’s Hemispherical Lens.]