IV] MEASUREMENT OF OBJECTS 33 



puscles under the high eye-piece and object-glass; 

 taking care that the drawing-board and paper are in 

 the same positions as in 2. Fold the scale drawn 

 in 2 at right angles to, and through the lines ; and 

 measure with it the diameter of the corpuscles you have 

 drawn. One division of your scale is T ^ mm. i.e. 10 p. 

 (fj, = micron, y^- mm.) The red corpuscle drawn should 

 occupy rather less than one division of your scale since 

 the red corpuscle is ordinarily 7 to 8 p in diameter. 



6. Write on each scale drawn, the eye-piece and object-glass 

 with which it was drawn, and the position of the drawing-board. 

 Later, in drawing microscopic objects, draw them in the condition 

 of one of the two scales. Thus the scale will allow the diameters 

 of the object to be at once measured. 



The phrase magnifying power of the microscope is used for 

 the magnification of the scale, when it is drawn on a level with 

 the stage of the microscope. 



7. Measurement of size of objects by means of an ocular 

 micrometer. The ocular micrometer consists of a disc of glass 

 with a scale engraved on it. A ledge to receive it is placed in 

 the eye-piece on a level with the focus of the upper lens. 



Take the high eye-piece and objective, place the ocular 

 micrometer face downwards in the ocular. Note the number of 

 divisions of the ocular and of the stage micrometer which corre- 

 spond exactly with one another. Reckon from this how many 

 of the smaller divisions of the stage micrometer, each of which 

 is 10 p. one division of the ocular micrometer corresponds to. 

 Thus if 5 ocular divisions cover exactly 4 stage divisions ; 

 1 ocular division = ^ x 10 /* 8/i, and a human red blood corpuscle 

 would almost exactly be covered by one division of the ocular 

 micrometer. If the microscope has a draw tube, draw it out to 

 its full length, and redetermine the value of the ocular divisions. 

 Determine similarly the value of one division of the ocular micro- 

 meter with the low ocular and objective. 



L. 3 



