DARK-GROUND ILLUMINATION WITH ORDINARY MICROSCOPE EQUIPMENT

The interest in the Spirochæta pallida has brought the methods of dark-ground illumination into prominent use for practical purposes. The following is a method of securing this form of illumination, without the aid of special apparatus. The only things needed are a microscope-stand fitted with an Abbé condenser of 1.20 N. A. or 1.40 N. A.—focusing screw for the condenser—a swing out ring below the condenser (such as is fitted to most microscopes, for holding a blue glass disc), 16-millimeter and 4-millimeter objectives, the latter preferably of low aperture (0.65 N. A.), although the one with a higher aperture will do. The illuminant may be a simple one-inch flat-wick oil lamp, although a high power single glower nerust lamp is very much better. Dark-ground illumination is easily obtained with low power objectives, therefore it is best to practice with the 16-millimeter lens before using the higher power necessary to distinguish