32 PRACTICAL HISTOLOGY [IV 



the lens of the eye-piece, (2) the lower angle of the 

 prism nearly touches the edge of the eye-piece, (3) the 

 anterior exposed surface of the prism is parallel with 

 the drawing-paper. Then turn the prism round the 

 vertical axis so as to uncover the eye-piece. 



Focus with a low power a stage micrometer. When 

 the lines are seen distinctly, turn the prism back again 

 into position. On looking down the microscope with the 

 eye close to a, the lines of the stage micrometer and the 

 point of a pencil placed on the drawing-paper should 

 be visible. If they are not, watch the lines carefully, 

 for the appearance of the pencil point over them as 

 the prism is turned slowly and slightly around the 

 horizontal axis, the point of the pencil at the same 

 time being moved; try .also the effect of shading the 

 drawing-paper. 



3. Draw the -^th micrometer lines. Then place 

 the drawing alongside a millimetre scale, and note the 

 length in millimetres of the scale drawn. If it mea- 

 sures 80 mm., then the scale drawn, which on the 

 micrometer was one millimetre, has been magnified 80 

 times. 



4. Draw similarly on another piece of paper the 

 lines of the stage micrometer with the high eye- 

 piece, and objective. It will be sufficient to draw the 

 lines indicating yj^th of a mm. Ascertain the magnifi- 

 cation of the drawing. If the -j^th of a mm. drawn 

 measures 38 mm., the magnification will obviously be 

 380. 



5. Substitute now a preparation of human blood 

 for the stage micrometer, and draw some red cor- 



