ZOOLOGY AND. BOTANY, MICROSCOPY, ETC. 



117 



negatives by immersing the plates in a formalin bath (4 ccm. formalin 

 and 30 ccm. water.) In 1898 Trillat devised a method for the estima- 

 tion of gelatin in gums and food-stufFs which was based on this 

 property of formalin. A 4 per cent, solution of formalin is employed 

 with success fur preserving and hardening microscopical objects, while 

 in the bacteriological laboratory it has many uses, among which may 

 be mentioned that of inhibiting the growth of cultures. The vapour of 

 formalin acts very detrimentally on the pigment of chromogenic bacteria 

 and on the fluorescence of micro-organisms. It would be of great value 

 if the exact strength of formalin, which was harmless to the growth and 

 development of microbes, and which did not affect the good properties 

 of gelatin, could be determined. 



Useful Caliper Gauge. — This convenient little out-and-in caliper 

 gauge (fig. 33), which may be purchased for 3s. or 4s. at any watch- 

 makers' tool shop, is a very cheap and handy gauge 

 for a microscopist's outfit. Among other uses it is 

 convenient for measuring the thickness of cover- 

 glasses. It reads by a vernier to O'l mm. It is 

 graduated both ways, and the points A and B form 

 an end gauge. For low-power work the scale may 

 be placed on the stage of a Microscope, and the 

 ■constant of an eye-piece micrometer found by com- 

 parison with the mm. divisions. 



Methods for Examining Trypanosoma Lewisi.* 

 — A. Laveran and F. Mesnil remark that observa- 

 tions on Trypanosoma in fresh blood are quite easy. 

 The blood obtained by puncturing the tip of the 

 tail of an infected rat is spread on a slide and 

 covered with a slip. For prolonged observations 

 the hanging drop is recommended. The blood may 

 be mixed with physiological salt solution, then 

 defibrinated with citrate solution to prevent coagula- 

 tion, or mixed with rat serum. The length of time 

 T. Lewisi can be preserved depends greatly on the 

 temperature : in summer rarely beyond four days ; Fig. 33. 



in winter as long as eighteen days; in a glaciarium 

 ■(5° to 7° C. above 0°) they will keep for four to seven weeks. For 

 studying the structure of Trypanosoma it is necessary to use stained 

 preparations, and the following procedure gave the best results. A thin 

 film of blood is spread on a slide, dried quickly, and fixed in absolute 

 alcohol (ten to fifteen minutes). For staining, three solutions are 

 (necessary : — (1) Borrel's blue. To make this, place some crystals of 

 nitrate of silver in a bottle capable of holding 150 ccm. and 50 to GO ccm. 

 of distilled water. When the crystals are dissolved the bottle is filled 

 with soda solution. The black oxide of silver thus formed is washed 

 several times with distilled water to remove the soda. Over the silver 

 oxide is then poured a saturated aqueous solution of methylen-blue 

 (Hochst). The mixture is allowed to stand for fifteen days, being shaken 

 up frequently the while. (2) A one per thousand aqueous solution of 

 eosin. (3) A 5 per cent, solution of tannin. 



• Ann. Inst. Pasteur, xv. (1001) pp. 678-82 (2 pis.). 



