224 Drs. J. W. W. Stephens and H. B. Fantham. [Mar. 28, 



(1910) in our original plate and are well shown in the accompanying coloured 

 plate, for which we are indebted to Lady Bruce. 



Methods. 



The blood-films used were quickly dried, fixed in absolute alcohol, and 

 stained with a modified Eomanowsky solution. Films of this nature contain 

 trypanosomes most nearly approximating to the natural size. The flagellates 

 suffer shrinkage in films fixed with sublimate-alcohol. 



One thousand specimens of the trypanosome have been measured after 

 the manner introduced by Sir David Bruce for the differentiation of various 

 trypanosomes. In this method the length of the median longitudinal axis, 

 including the free flagellum, is determined as accurately as possible. We 

 found it advisable to modify Bruce's method in some respects : — 



(1) Instead of drawing the trypanosomes with a camera lucida, it is 

 much easier to project them on a screen, using a photomicrographic 

 apparatus in a dark room, and then to trace them in outline with a finely 

 pointed pencil. The magnification is verified by projecting a millimetre scale 

 in the same manner. The magnification adopted was 2500 diameters, using a 

 2-mm. apochromatic objective and an 8 compensating ocular. This method 

 not only saves much eyestrain in drawing, but is also much quicker. 



(2) A more important modification consists in the actual mode of 

 measuring the trypanosomes drawn on paper. Sir David Bruce uses for this 

 purpose a pair of compasses, set at a fixed distance of 2 mm., his trypano- 

 somes being magnified 2000 times. There are, however, two objections to 

 this method : — 



(a) It cannot and does not give an accurate measurement, because the 

 compass makes a series of " jumps " and theoretically and actually the 

 measurements given are always less than the true ones. 



We can illustrate our objections perhaps by supposing that we have to 

 measure the outline made by the teeth of a saw. If the teeth are equal and 

 the distance between the compass-points is equal to the depth of a tooth, 

 then the course can be measured. If the depths of the teeth are unequal, 

 then it will be impossible to get an accurate measurement by the compass 

 method, though this can be accurately done by the " tangent line " method. 

 Although the curves of a trypanosome do not change their direction so 

 acutely as the outline of a saw, yet the curves often do change their direction 

 to some extent and the principle of the objection remains. We therefore 

 used the method which we call the " tangent line " method. 



The requirements are : — (1) a piece of tracing paper on which a straight 

 line is drawn in ink, (2) a pin, (3) a millimetre scale. The tracing paper is 



