There are, however, objects which require a fully illuminated field, when the lamp should be turned round and the Herschel lens condenser (shown in section, Fig. 148) should be used to collect the light and throw it upon the mirror. For moderate powers, as a four-tenth or one-fifth, the condenser should be used a little below the focus to give an even illumination over the whole field. Moreover, as to the use of the condenser for defining general objects, it must be borne in mind that to show different kinds of structure different apertures in the iris diaphragm are necessary, and that whereas some objects show their structure better with a large angle of light cut down in intensity by the use of blue glass, others show better with a small pencil of direct rays. For the resolution of diatoms it is often necessary to use oblique light only, and for this purpose diaphragms with central patches are used, the iris diaphragm being opened to its full extent. An annular ring of oblique light emerges from the condenser upon the object, and it is in this manner also that dark-ground illumination is obtained with moderate and low powers. THE DIAPHRAGM. [Illustration: Fig. 150.--The Diaphragm.] The early form of diaphragm in use was that shown in Fig. 150. [Illustration: Fig. 151.--Shutter Diaphragm.] It consists simply of a circular brass plate with a series of circular openings of different sizes, arranged to revolve upon another plate by a central pin or axis, the last being also provided with an opening as large as the largest in the diaphragm-plate, and corresponding in situation to the axis of the microscope body. The holes in the diaphragm-plate are centred and retained in place by a bent spring in the second plate, which rubs against the edge of the diaphragm-plate and catches in a notch. The blank space shuts off the light from the mirror when condensed light is about to be used. It is usually made to fit in under the stage of the microscope. This has been almost superseded by the iris diaphragm, originally designed by Wales, of America. It was made by this optician for his working students’ microscope. An early form of the iris diaphragm is seen in Fig. 151. By pressing upon the lever handle at the side the aperture gradually closes up, and without for a moment losing sight of the object under examination. The Mirror.