CM. II] 



LIGHTING AND FOCUSING 



5i 



of stiff, black paper. Care must be taken, however, to make the 

 opening exactly central. This is best accomplished by putting the 

 paper disc over the iris or metal diaphragms and then making the hole 

 in the center of the small circle uncovered by the metal diaphragm 

 For the hole a fine needle is best), l^ight well and lower the objec- 

 tive so that it is at about its working-distance from the top of the 

 condenser. If now the condenser is lowered or racked away from 

 the objective the image of the diaphragm will appear. If the open- 

 ing is not central it should be made so by using the centering screws 

 of the condenser. 



A better plan than to lower the condenser to focus the image of 

 the diaphragm, is to raise the body of the microscope slowly with 

 the coarse adjustment. It is almost impossible to make apparatus 

 so accurate that two parts like the body of the microscope and the 

 substage, each working on different sliding surfaces, shall continue 

 in exactly the same plane. So one will find that if the condenser be 

 accurately centered with the condenser lowered, and then the con- 

 denser be racked up close to the stage and the image of the dia- 

 phragm opening brought again into focus by racking up the body of 

 the microscope, it will not be accurately centered in most cases. 

 For this reason it is advised that the condenser be left in position . 

 close to the stage and the tube of the microscope be used to focus 

 the diaphragm exactly as in ordinary work. 



Fig. 48. Shows that the optic 

 axis of the condenser does not coin- 

 cide with that of the microscope. (D). 

 Image of the diaphragm of the con- 

 denser shown at one side of the field 

 of view. 



Fig. 49. Shows the image of the 

 diaphragm (Z>) in the center of the 

 field of the microscope, and thus the 

 coincidence of the axis of the con- 

 denser with that of the microscope. 



Fig. 49 



§ 93. Centering the Image of the Source of Illumination. — 

 For the best results it is not only necessary that the condenser be 

 properly centered, but that the object to be studied should be in the 

 image of the source of illumination and that this should also be cen- 

 tered (Figs. 50, .51). After the condenser itself is centered the iris 

 diaphragm is opened to its full extent or the diaphragm carrier 



