The re-publication of the original directions is given with the view of showing what a clear conception Gillett had of the value of his invention. The careful directions given for centring must be regarded with interest, although nearly superseded by the centring screw arrangement in connection with the sub-stage. The best results, he goes on to say, will be secured by using the plain mirror and focussing the window-bar on the object, while a white-cloud illuminator will afford as much light as may be required. It is a mistake to suppose that direct light is more critical than indirect. As a rule, the student is given to over-illuminate the object. These questions will, however, be discussed further on. Very many modifications of Gillett’s condenser have, since 1850, become known to microscopists. Ross’s present improved form (Fig. 127) is made to drop into the sub-stage of the microscope, and when adjusted, is an extremely efficient instrument. The optical part is similar to a 4/10-inch objective. It has two sets of revolving diaphragms, with apertures and stops for showing surface markings in a perfect manner. Abbe’s Condenser. The essential feature of this condenser is its short focus, which collects the light reflected by the mirror, so as to form a cone of rays of very large aperture, having its focus in the plane of the object. [Illustration: Fig. 128.--The Iris Diaphragm, and carrier for Stops.] The full aperture of the illuminating cone should only be used when finely granular and deeply stained particles (protoplasm, bacteria, &c.) are being examined with objectives of large aperture. In all cases the cone must be suitably reduced, either by an iris, or other form of diaphragm (_central illumination_). By placing the diaphragm excentrically, by means of rack-work attached to the carrier, the central rays are excluded and a certain extra-axial portion of the illuminating pencil falls upon the object (_oblique illumination_). When the diaphragm is thus excentrically placed, this oblique pencil can be directed from all sides by rotating the carrier round the optic axis. The central stop diaphragm shuts off all the axial and transmits only the marginal rays, thus producing _dark-ground illumination_. The iris diaphragm (Fig. 128) is so shaped that the edge of its smallest opening closely approximates the object-slide on the stage. [Illustration: Fig. 129.--The Abbe Condenser, detached from the Sub-stage of the Microscope.] The Abbe condenser is the most popular form in use, for all purposes. Owing to the large aperture of the cone of light which it projects, it can be employed with the highest powers; by removing the top lens it can also be used with low powers. Dark ground illumination may be obtained with it up to a 1/4-inch objective. [Illustration: Fig. 130.--Optical Arrangement of Abbe Illuminator, 1·2 N.A.] [Illustration: Fig. 131.--Optical Arrangement of Abbe Illuminator, 1·4 N.A.]