The direct method of employing the mirror, that more generally resorted to, is by reflecting rays from the concave surface; the plane surface is preferred when the condenser is used. Whichever is employed, it should not be forgotten that the _optic axis_ must be preserved throughout, and so brought to the centre of the open tube of the microscope. Another method is to interpose a bull’s-eye lens, and in this way supply the mirror with a beam of parallel rays of light. The plane side of the bull’s-eye lens should be turned towards the lamp, so that lamp, bull’s-eye, sub-stage condenser, and objective, are brought into an exact line, the bull’s-eye being set at right-angles to the line. A piece of thin white paper held across the bottom of the sub-stage will serve to show whether the rays of light are fairly parallel. The next care is to focus the object on the stage, and then the sub-stage condenser on the slide; further correction should be made by means of the centring screws of the sub-stage, or by moving the bull’s-eye lens or lamp slightly, thus perfecting the arrangements for working with parallel rays of light. [Illustration: Fig. 153.--Rectangular Prism.] Accessories of the Microscope. The accessories and appliances of the microscope have become so very numerous, that any attempt to describe them and explain the uses to which they are put would demand more space than I find myself in a position to bestow upon them. I must therefore confine my remarks to those accessories in more general use. [Illustration: Fig. 154.--The Lieberkühn.] Having described the method of employing transmitted light, I have a few words to add with regard to the illumination of opaque objects by reflected light. A very early and efficient form of opaque illumination is the well-known _Lieberkühn_. This has not been entirely surpassed by more recent inventions. The concave speculum termed a Lieberkühn, so named after its celebrated inventor, directly reflects down upon the object the light received either from the mirror or bull’s-eye lens. It consists of a silver cap, which slides over the objective (Fig. 154), _a_ indicating the lower part of the compound body, and _b_ the objective over which slides the Lieberkühn, _c_; the rays of light are collected to a focus upon the object at _d_. The object may either be mounted on a slip of glass, or held by the stage-forceps, _f_; if very small, or transparent, it may be gummed to the dark well, _e_, or mounted on a Beck’s opaque disc-revolver. [Illustration: Fig. 155.--Stage Forceps, for holding objects while under examination.] [Illustration: Fig. 156.--Beck’s Disc-holder.]