The regulation of the cylinder iris diaphragm is accomplished by means of a lever. Before the condenser is swung back into position the cylinder iris should be opened to its fullest extent. The lower iris diaphragm is for use in connection with the condenser only. It is regulated by means of a small knob. A horizontal rack and pinion arrangement permits oblique illumination, and a vertical rack and pinion permits adjustment of the whole sub-stage. By these means a most perfect control of the illumination of the object is assured. Ground or colored glass discs may be inserted above the iris diaphragm if modification of the light is desired. =$30.--= _This illuminating apparatus may also be fitted to Stands Ia and Ib._ [Illustration: Abbe's Illuminating Apparatus for Stand A and I. Swing-out Condenser. Cylinder Iris Diaphragm.] =b. Illuminating Apparatus of Stands Ia and Ib= (p. 30, 32, 34). [Illustration] The following are the parts of this apparatus: Cylinder diaphragm with 3 stops of different apertures. Condenser. Diaphragm carrier with iris diaphragm. Plane and concave mirror. To substitute the condenser for the cylinder diaphragm the diaphragm carrier is turned to the side and the cylinder diaphragm is drawn out of the sleeve which holds it in position and into which the condenser readily fits. The diaphragm carrier is then turned back into position and the amount of light regulated by manipulating the knob controlling the size of the iris. The whole sub-stage may be raised or lowered in the optical axis by means of a rack and pinion, and a similar arrangement permits lateral illumination by moving the iris diaphragm in the horizontal plane. Ground glass and colored plates may be inserted above the iris diaphragm. =$24.--= =c. Illuminating Apparatus for Stand II and IIa= (p. 36 & 38). The condenser and iris diaphragm form one piece, giving central but no oblique illumination. The whole apparatus may be raised and lowered in the optical axis by means of a screw. The cylinder diaphragm may be inserted in place of the condenser. A ring below the iris diaphragm permits the insertion of glass discs for the purpose of modifying the light. =$12.--= =d. Illuminating apparatus for Stand IIb= (p. 40). The same as for II and IIa fitted in a fixed sleeve; without the side-screw. =$10.--= =e. Small Illuminating Apparatus.= [Illustration: Small Illuminating Apparatus.] The small illuminating apparatus consists of a somewhat smaller condenser with iris diaphragm and can be adjusted to the Stand IV, when it has the cylinder diaphragm. =$8.--= [Illustration: Cylinder iris diaphragm.]