354 APPENDIX. 



should be about equal, so that the point of a pencil 

 on the drawing surface is distinctly seen. 



Heasnrement of the size of microscopic objects. 



The method of doing, this which has been given in 

 the text (p. 37, § 3) involves making a drawing of the 

 object ; when this is not done the size of the object 

 may be measured by means of an ocular micrometer. 

 This is a flat piece of glass on which a scale is 

 scratched (e.g. 5 mm. divided into ten parts, the actual 

 distances of the lines are however not important), 

 this is dropped into the ocular so that it rests on a 

 ledge placed on a level with the focus of the upper 

 lens of the eye-piece. With this scale in the eye- 

 piece the stage micrometer (cp. p. 37, § 3) is ob- 

 served, and the number of divisions of the stage 

 micrometer which correspond to one division of the 

 ocular micrometer noted. Say that one division of 

 the one exactly covers one division of the other, 

 then if (as is usually the case) one division of the 

 stage micrometer is yJtt '^™' ''^® division of the 

 ocular micrometer, with the objective, ocular and 

 length of draw-tube used; corresponds to -^^ mm. of 

 the field of the microscope. Hence any object seen 

 with this arrangement of the microscope can be at 

 once measured by the ocular micrometer, an object 

 which exactly occupies say 5 divisions measures in 

 that direction exactly j^ luui. 



If another ocular or objective be used, or if the tube be 

 drawn out, the divisions of the ocular micrometer 

 will obviously correspond to a different number of 

 divisions of the stage micrometer, so that the value 

 of the divisions of the ocular micrometer must be 

 determined for each arrangement of the microscope. 



