2. =Spirochaeta Pallida (Treponema Pallidum).= This organism is best examined in the living condition by means of the dark-field illumination (dark-field condenser). A very simple method of dark-field illumination consists of the use of India ink. The suspected discharge or serum is placed on a slide and an equal quantity of ink (Gunther’s or Higgin’s) added. The serum and ink are rapidly mixed and spread over the slide to dry in a pale brown smear. The oil for the immersion is placed directly on the smear. The spirochætes appear as white spirals against a brownish-black field. The best results are obtained with serum; the presence of mucus or fibrin interferes with the clearness of the picture obtained. Smears of serum from syphilitic lesions may be dried in the air and fixed in absolute alcohol or equal parts of absolute alcohol and ether for 15-20 minutes. They may then be stained by Giemsa’s (old formula) stain (azur II-eosin 3 grms., azur II 0.8 grm., glycerin [Merck’s chemically pure] 250 grms., methyl-alcohol [Kahlbaum I] 250 grms.). This solution can be obtained from Grübler. Ten drops of the stain are mixed with 10 cc. of distilled water immediately before the staining. The fixed preparation is covered with the diluted staining fluid and warmed over the flame until a slight steam arises. It is then allowed to cool for about 15 seconds, when the stain is poured off and replaced by fresh, and the process repeated four or five times, when the preparation is washed, dried and mounted in balsam. Spirochætes are dark red. Slide or cover-glass and forceps must be absolutely clean. Smears may also be fixed and stained by _Wright’s blood-stain_. For the demonstration of the treponema in sections the method of _Levaditi_ gives the most satisfactory results:— 1. Fix thin pieces of tissue 24 hours or longer in 10 per cent formol. (Formol-Müller’s and alcohol-fixation may also be used.) 2. 24 hours in 96 per cent alcohol. 3. Transfer to distilled water until tissue sinks. 4. Impregnation for 3 days in incubator, in a 1.5-3 per cent silver nitrate solution. 5. Wash for a short time in water. 6. Reduce for 48 hours, in the dark, at room-temperature, in pyrogallic acid 4 grms., 40 per cent formol 5 cc., distilled water 100 cc. 7. Wash in water. Cut on freezing-microtome, or imbed in celloidin or paraffin. Toluidin-blue or safranin may be used as a contrast-stain. The spirochætes are dark brown to black. Silver precipitates occur chiefly in the outer portions of the tissue. The reticulum is brown; other parts of the tissue are yellowish. Levaditi’s more recent modification of this method does not give so good results as the original. _Schmorl’s Staining of Sections with Giemsa’s Stain._ 1. Fix in 10 per cent formol. Cut very thin sections on freezing-microtome.