ILLUMINATING APPARATUS AND METHODS OF ILLUMINATION The methods of illumination commonly used are very varied; each method requires the proper apparatus for its convenient ap plication. The principles involved in some of the most impor tant of these, together with the apparatus needed, are here dis cussed.
For obtaining an illuminating cone of larger angle than the con cave mirror can give, a condenser is interposed between the (plane) mirror and the object. The condenser is a system of lenses capable of concentrating the rays reflected from the plane mirror so that they converge on to the object as a cone of wide angle (see fig. 22). A diaphragm, usually of the "iris" type, is fitted below the condenser, in order that the angle of the cone of illumination may be varied conveniently at will. By focussing the condenser so as to form an image of the source, or of a bull's eye lens in front of the source, on to the object, intense illumina tion of the object is obtained.
Provision should be made for "centering" the condenser in order that the illuminating beam may be truly axial. Dry condensers capable of filling an object-glass having a numerical aperture of 0.95 are obtainable, well corrected for chromatic and spherical
aberrations. Immersion condensers are made with numerical apertures as high as 1.4, similarly corrected for chromatic and spherical aberrations and capable of being used with microscope slides of thicknesses up to about 1.5 mm. Immersion condensers are brought up close to the underside of the microscope slide, and are put into "immersion contact" with this by filling the space between the underside of the slide and the top surface of the condenser with cedar-wood oil. Immersion condensers are used when it is desired to fill the aperture of an immersion object glass.
Ordinary dry and immersion condensers can be used to give dark-ground illumination if pro vided with central opaque stops.
For work with object-glasses of apertures above o.65, immersion condensers fitted with central opaque stops can be used, but more satisfactory results are ob tained with special immersion dark-ground illuminators. The reflecting systems used in these illuminators give less stray light and at the same time produce a blacker ground.