For high-power work, such as bacteriology or even the examination of sections of plants, etc., even the best concave mirror will not give a sufficiently powerful illumination; accordingly an instrument, known as the condenser, is fixed below the stage, between the mirror and the object. The condenser, as its name implies, condenses the rays of light reflected to it by the mirror. It consists of a series of lenses so arranged that they will throw a very powerful cone of light. Provision is made for focussing the rays from the condenser on to the object. Sometimes, for special forms of illumination, it is necessary to cut off some of the rays of light passing through the condenser. It may be that we desire to dispense with the outer rays of the cone of light or, when delicate details are being studied, we may wish to impede the central rays. In either case diaphragms, popularly called “stops” are used. Our diagrams show A the outer rays of a cone of light cut off and B the central rays similarly treated. [Illustration] In old pattern microscopes and in many instruments not provided with condensers, the diaphragm used for the purpose of cutting off the outer rays of the cone of light, consists of a blackened circular metal plate, perforated with a number of different sized circular holes. This plate is fixed below the stage in such a manner that, as it is revolved, holes of various diameters are brought one by one within the cone of light. It need hardy be remarked that the smaller the hole in the diaphragm the more light is cut off and the less reaches the object. In more modern instruments and in practically all which are fitted with a condenser, an Iris diaphragm is fitted. A diaphragm of this nature consists of a number of thin, blackened, metal leaves, fastened to a metal ring in such a manner that, when the ring is revolved, the leaves close together, making the opening in the centre smaller and smaller. The Iris diaphragm has many advantages over the old perforated metal plate. At will, we can have any opening from full to the merest pin-point or we can cut off the light rays altogether, should we wish to do so; we are not confined to a definite number of stops. As we cut off these outer rays of light we shall find that, up to a certain point, though the illumination becomes less and less the object becomes more and more clear, or, to use the correct expression, its definition is improved. When it is necessary to cut off some of the central rays of the light cone, either a circle of glass with an opaque centre is dropped into a metal holder below the stage, or a circular metal plate, held in the centre of a metal ring by three arms, is used in the same manner. The effect of cutting off the central rays of the light cone is, of course, to reduce the illumination and to show up delicate detail to advantage. No direct rays of light reach the objective, such as do pass into the microscope are all diffused from the edges of the object.