APPENDIX. 355 



Supposing the value of the divisions of the ocular 

 micrometer are known with any given arrangement 

 of the microscope, then the magnification of any 

 object drawn can easily be measured; suppose for 

 example that the drawing measures 5 mm. and the 

 object itself with the given arrangement of the 

 microscope occupies 5 divisions of the ocular micro- 

 meter, the value of each division being -^^ mm., 

 then in the drawing the object is magnified 

 100 times. 



The magnifying power of a microscope with any 

 given ocular, objective, and length of draw-tube, is 

 usually taken as the amount of magnification when 

 the image of the object is seen at a distance of 

 25 centimetres (10 inches) from the eye. To de- 

 termine this, the stage micrometer lines are drawn 

 at this distance and the distance between these 

 lines is measured ; thus if one division of the stage 

 micrometer equal y^j- mm., and the drawing of this 

 measures exactly 1 millimetre, then the magnifying 

 power is 100. In order to compare the tables given 

 by different instrument makers of the magnifying 

 powers of their microscopes, it is necessary to know 

 the method used in determining it; in all cases when 

 an object is drawn its magnification should be 

 determined by the method given above or by that 

 given in the text. 



OBSERVATION OF FRESH TISSUES. 



Sections of fresh tissues may be made with the aid 

 of a freezing microtome (cp. p. 368) ; if, however, 

 specimens can be prepared without freezing, it is 



232 



