APPENDIX. 291 



millimetre of the mixture. The counting is performed 

 with the aid of a squared ocular micrometer, the micro- 

 scope as before having been so arranged by observation 

 of a stage micrometer that the side of the square shnll 

 have the value of one of the lengths (400 ,, for example) 

 marked on the slide. The result of the counting gives 

 the number of corpuscles in a known quantity (y^.-g- cub. 

 mill.) of the mixture, and the number in the same volume 

 of blood can readily be deduced. 



Microtomes. When it is desired to obtain a number 

 of perfectly even, consecutive sections of an organ, of the 

 spinal cord for instance, a section-instrument or micro- 

 tome may be employed. One of the first of these to be 

 introduced was that devised by Mr. Stirling (Fig. 3t), 



Stirling's microtome. 



and most of the other instruments are modelled on the 

 same principle. It consists of a hollow brass cylinder 

 with a broad metal plate (p) fitted over the top, and a 

 long, finely cut screw (.s-) working in the lower end, and 

 serving to push a brass plug upwards in the tube. The 

 instrument is clamped at the edge of the table by the 

 screw cs. 



