It has been well observed that the illumination of objects is quite as important as the glasses that are employed, and the most experienced microscopists have never done learning in this matter. Most microscopes are furnished with two mirrors beneath the stage, one plane and one concave. The first will throw a few parallel rays through any transparent object properly placed, and the latter causes a number of rays to converge, producing a more powerful effect. The first is usually used in daylight, when the instrument is near a window (one with a north aspect, out of direct sunlight, being the best); and the second is often useful when the source of illumination is a candle or a lamp. By varying the angle of the mirror the light is thrown through the object more or less obliquely, and its quantity should never be sufficient to pain the eye. Few objects are seen to the best advantage with a _large_ pencil of perfectly direct light, and the beginner should practise till the amount of inclination is obtained which produces the best effect. It is advisable that the hole in the stage of the microscope should be large--at least an inch and a half each way--so that the entrance of oblique rays is not obstructed, and it is desirable that the mirror, in addition to sliding up and down, should have an arm by which it can be thrown completely out of the perpendicular plane of the body of the instrument. This enables such oblique rays to be employed as to give a dark field, all the light which reaches the eye being _refracted_ by the object through which it is sent. The opticians sell special pieces of apparatus for this purpose, but though they are very useful, they do not render it less desirable to have the mirror mounted as described.