44 LIGHTING AND FOCUSING [CH. II 



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. 42 . Shows that the optic axis of 

 the condenser does not coincide with that 

 of the microscope. (D). Image of the 

 diaphragm of the condenser shown at 

 one side of the field of view . 



Fig. 43. Shows the image of the 

 diaphragm (D) in the center of the field 

 of the microscope, and thus the coin- 

 Exc c cidence of the axis of the condenser with 



that of the microscope. 



Fig. 42 Fig. 43 



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

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

 perl} 7 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. A transparent specimen like the fly's wing is put under the 

 microscope and focused. The condenser is then turned up and down 

 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 position of the lamp or of the bull's eye 



(Fig- 53)- 



§83. Proper Numerical Aperture of the Condenser. — As 



stated above, the aperture - of the condenser should have a range by 

 means of properly selected diaphragms to meet the requirements of all 

 objectives from the lowest to those of the highest aperture. It is 

 found in practice that for diatoms, etc., the best images are obtained 

 when the object is lighted with a cone which fills about three- fourths 

 of the diameter of the back lens of the objective with light but for 

 histological and other preparations of lower refractive power only one- 

 half or one-third the aperture often gives the most satisfactory images 

 (§34)- 



