Baker’s Nelson Condenser, shown in Fig. 140, is intended for use with their medium instruments. It has, however, many pieces of apparatus essential to those of a higher class. It is applicable, indeed, to all instruments having sufficient depth beneath the stage to receive it. It comprises an achromatic combination of 90° aperture, available with all powers up to 1/8-inch tinted glass for neutralising the yellow rays of artificial light, focussing adjustment, dark-ground illuminator, large diaphragm with rotating tube to carry oblique light stops, small wheel of apertures, polarising prism with two selenite films, clear aperture, and oblique light-shutter for low powers. Baker’s Students’ Condenser (Fig. 141) is designed to take the place of Abbe’s, and costs much less. It transmits a larger aplanatic cone of light, and can be used either with high or low powers by removing the front lens. It is equally useful for photo-micrographic work. Mr. J. Mayall’s semi-cylinder or prism for oblique illumination (Fig. 142) is a convenient form, as it permits of the semi-cylinder being tilted and placed excentrically; in this manner, without immersion contact, and by suitable adjustment, a dry object can be viewed with any colour of monochromatic light. If placed in immersion contact with the slide, the utmost obliquity of incident light can be obtained. Objects in fluid may be placed on the plane-surface of the semi-cylinder, and illuminated by ordinary transmitted light, or rendered “self-luminous” in a dark field, as with the hemispherical illuminator or Wenham’s immersion paraboloid. A concave mirror with a double arm is quite sufficient to direct the illuminating pencil. This semi-cylinder was originally made by Tolles, of Boston, for measuring apertures, but, at Mr. Mayall’s suggestion, Messrs. Ross mounted it as an illuminator. [Illustration: Fig. 141.--Optical Arrangement of Baker’s Abbe Condenser.] The spiral slot should be fixed close beneath the larger lens of the condenser, and when properly arranged will be found a convenient mode of obtaining oblique light. [Illustration: Fig. 142.--Mayall’s Semi-Cylinder Illuminator and Spiral Diaphragms.] _The Webster-Collins Universal Condenser_ (Fig. 143) is so well known that it scarcely calls for any lengthy description. It is an inexpensive form of condenser, designed in the first instance for use with the students’ microscope. It is fitted into the sub-stage; has an iris diaphragm as well as a series of revolving diaphragms moved by a milled head screw arrangement. [Illustration: Fig. 143.--The Webster-Collins Universal Condenser.] Oblique Illumination.