ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



337 



' The second part of the apparatus (which in fig. 90 is raised ahove its 

 ordinary position for the purpose of rendering its parts more conspicuous), 

 consists of a plate E, fixed to the slide by the screw S and two pegs. 

 From E projects, over the ball-joint, the piece g, to which is applied the 

 bar m, fised by a screw u. Through m and g the screw-peg T passes to be 

 inserted in the centre of the ball-joint. The left end of m terminates in the 

 pointer W, and this is so situated that it points to the same division of the 

 scale K as P does. 



The pieces r n t m form a parallelogram, with movable joints at 

 d d' d'' d'". At the ends of n are the diopters X and Y, and the plane in 

 which they lie is parallel to the plane which passes through the centre of 

 the ball, the peg T, and the pointer W. 



Fig. 90. 



To the right of the lower part of the illustration is the piece y, which 

 is fixed to the object-carrier, and bears at its free end the knob Ji. This 

 piece and the knife are so arranged that the ball Ji touches the under sur- 

 face of the knife near its edge, throughout its length ; and when this has 

 been accurately effected, the division of the scale K, which the pointer P 

 indicates, is noted. 



A definite position or line is thereby obtained, so that the knife can be 

 raised or lowered without deviating from its correct position, and, in other 

 words, it may be said that the purpose of the foregoing apparatus is to 

 enable the microtomist to put the section line of the knife in a plane 

 parallel to the course of the slide, and also to render it possible to lower 

 the edge and raise the back without the plane which the knife-edge 



