The early form of Ross’s condenser consists of two small brass tubes made to slide one in the other. To the outer one is attached a flat brass plate which slides underneath the stage of the microscope, and by means of a screw the adjustment of the axis of the illuminator is effected. The upper portion of the apparatus carries the achromatic combination, which by a rack and pinion movement is brought nearer to, or removed further from the object on the stage. The several parts of the illuminator unscrew, so that the lenses may be used either combined for high powers, or separated for low powers. [Illustration: Fig. 126.--Original form of Gillett’s Achromatic Condenser.] Messrs. Smith & Beck greatly improved upon Ross’s condenser by adding another achromatic lens to the combination, three being employed when used with high-power objectives and two or even one with the lower, the adjustment and focussing being made by rack and pinion arrangement beneath the stage. Some further changes for the better were made in the condenser by Powell, and in 1850 an amateur microscopist, Mr. Gillett, fully grasping the value of controlling the cone of rays passing into the microscope, devised a new form of condenser, in connection with which a revolving series of diaphragms of different values were made to pass between the achromatic lenses and the source of light. Andrew Ross constructed the first condenser on Gillett’s principle, and this proved to be one of the most successful pieces of apparatus contrived. _Gillett’s Condenser_ consists of an achromatic lens _c_, about equal to an object-glass of one quarter of an inch focal length, with an aperture of 80°. This lens is screwed into the top of a brass tube, and intersecting which, at an angle of about 25°, is a circular rotating brass plate _a b_, provided with a conical diaphragm, having a series of circular apertures of different sizes _h g_, each of which in succession, as the diaphragm is rotated, proportionally limits the light transmitted through the illuminating lens. The circular plate in which the conical diaphragm is fixed is provided with a spring and catch _e f_, the latter indicating when an aperture is central with the illuminating lens, also the number of the aperture as marked on the graduated circular plate. Three of these apertures have central discs for circularly oblique illumination, allowing only the passage of a hollow cone of light to illuminate the object. The illuminator above described is placed in the secondary stage _i i_, which is situated below the general stage of the microscope, and consists of a cylindrical tube having a rotatory motion, also a rectangular adjustment, which is effected by means of two screws _l m_, one in front, and the other on the left side of its frame. This tube receives and supports all the various illuminating and polarising apparatus, and other auxiliaries.