42 



LIGHTING AND FOCUSING. 



[CH. II. 



the small circle uncovered by the metal diaphragm. For the hole a fine 

 needle is best). If now the condenser is lowered or racked away from 

 the objective the image of the diaphragm will appear. If the opening 

 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 work- 

 ing 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 condenser be racked up close to the 

 stage and the image of the diaphragm opening brought again into focus 

 by racking up the body of the microscope, it will not be found accu- 

 rately 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 mi- 

 croscope be used to focus the diaphragm exactly as in ordinary work. 



Fig. 42. Shows that the optic axis of 

 the condenser does not coincide with 

 that of the microscope. (D). Dia- 

 phragm of the condenser shown at one 

 side of the field of the microscope. 



Fig. 43 Shows the diaphragm (D) 

 in the center of the field of the micro- 

 scope, and thus the coincidence of the 

 axis of the condenser with that of the 

 microscope. 



§ 78. 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 centered (Figs. 

 44, 45). After the condenser itself is centered the iris diaphragm is 

 opened to its full extent or the diaphragm carrier turned wholly aside. 

 The condenser is then racked up toward the objective until the image of 

 the flame is apparently on the specimen. If this cannot be accomplished 

 the relative position of the lamp and condenser is not correct and should 

 be so changed that the image of the edge of the flame is sharply defined. 

 This image must also be centered. This is easily accomplished by 

 manipulation of the mirror or, if a lamp is used, by changing the posi- 

 tion of the lamp or of the bull's eye (Figs. 34, 52). 



