objective. With the medium and higher power (i6mm 

 and above) objectives the minimized image of the mirror 

 with its mounting will be seen. Swing the mirror bar to 

 the median line and as nearly as possible arrange the mir- 

 ror so that its mounting will be concentric with the per- 

 iphery of the back lens of the objective. All of the better 

 microscopes are made with a ''center stop," indicating 

 when the mirror bar is in a line parallel with the optical 

 axis of the microscope. This is done because central, or 

 axial, light gives a symmetrical illumination, which is 

 best for observing the large proportion of transparent 

 objects. This in itself does not insure axial light. The 

 mirror must be so turned that the rays of light, or the 

 axis of the cone of light, reflected from it enter the objec- 

 tive parallel to its axis.. This cannot always be done. 

 Other considerations are more important than exact 

 central light. In working with daylight, reflections from 

 trees, window sash, etc., are apt to be seen on the mirror. 

 If the whole microscope cannot be so shifted as to clear 

 the mirror of these reflections the mirror itself should be 

 turned so that, if possible, no images will appear upon it. 



If artificial light is used the mirror should be so turned 

 that the image of the light is seen in the center of it. 

 The more nearly this image covers the mirror, the better. 

 If a bulKs eye condenser is at hand, so place it between 

 the light source and the object that a sharp image of 

 the light source will be seen in the center of the back 

 lens of the objective. 



If the above rules are followed it will be found upon 

 replacing the eyepiece that the field is evenly illumi- 

 nated. It may be necessary to vary the width of the cone 

 and the quantity of light by use of the diaphragm which 

 is always placed on all the better microscopes as nearly 

 as possible even with trie top surface of the stage. 

 "When no condenser is used the size of the opening in 

 17 



