ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 529 



rinsed in tap, and then in distilled water, and dried ; a large number can 

 thus be treated at one operation. The selected tubes are charged with 

 the blood to be examined, being preferably one-half to two-thirds filled, 

 and are carefully sealed by meltiDg their ends in a flame in the ordinary 

 way. In the laboratory the length of the column of blood between the 

 bases of the two menisci is first measured by means of a scale graduated 

 to half millimetres, assisted by a lens mounted to avoid parallax. The 

 end of the tube is then cut off by means of a fine file or writing diamond, 

 and its internal diameter is measured. This, the most important of all 

 the manipulations, is carried out in the following manner. 



The tube is supported in the axis of the Microscope by means of two 

 iris diaphragms carried respectively by the stage and by the substage. 

 These close concentrically, and support the tube, at the same time cut- 

 ting off all light reflected from the mirror. The opaque blood in the 

 interior of the tube in the same way cuts off all light that might pass 

 through the lumen of the tube. The only light that thus reaches the 

 optical system is that transmitted by the glass walls of the tube. In the 

 field of vision the lumen of the tube now appears as a black circle sur- 

 rounded by a brilliantly illuminated annulns. Owing to the extreme 

 sharpness of the image, the measurement of the diameter of the tube 

 can be carried out with great accuracy by means of an eye-piece micro- 

 meter arranged to give readings to the one-thousandth of a millimetre. 

 From the formula V = D 2 x L X 0-7854, where Y = volume of the 

 cylinder, D = diameter of the cylinder, L = the length of the cylinder, 

 the volume of the blood is calculated. If many measurements have to 

 be made, a Fuller's slide-rule greatly expedites the calculations. It is 

 of the utmost importance that the diameter of the tube be accurately 

 measured, and therefore the adjustment of the eye-piece micrometer 

 must be carried out with great care, and the scale by which it is set 

 should be one the accuracy of which is undoubted. For, considering the 

 formula (V = D 2 x L x • 7854), it will be seen that any error in the 

 measurement of D will be increased in a geometrical ratio in the final 

 result ; hence the comparative inefficiency of a capillary tube method in 

 which D is measured indirectly and with less accuracy. 



Having ascertained the volume of blood, it remains to carry out the 

 dilutions ; this is accomplished as follows. 



The capillary tube is placed in a vertical thick-walled glass tube of 

 slightly larger bore, the lower end of which is submerged in the re- 

 quired amount of the diluting medium (measured out by a graduated 

 pipette) contained in a hollo wed-out block of glass, such as is used in 

 histological and microscopical work. The capillary tube with its con- 

 tained blood is then crushed en masse by means of a soft iron plunger, 

 and the interior of the containing tube is washed out several times by 

 drawing up the diluting medium into its lumen, either with the mouth 

 or by means of an indiarubber teat. The powdered glass quickly falls 

 to the bottom, and the mixture may be employed for conducting the 

 serum reaction either by the microscopic or the macroscopic method. 



Molybdenum Method for Demonstrating the Neuro-fibrils and the 

 Golgi-network in the Central Nervous System.*— Herr A. Bethe de- 

 * Zeitschr. f. wiss. Mikr., xvii. (1900) pp. 13-35. 



