52 



LIGHTING AND FOCUSING 



\_CH. II 



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 manip- 

 ulation of the mirror or, if a lamp is used, by changing the position 

 of the lamp or of the bull's eye (Fig. 60). 



§ 94. 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 (§ 40). 



Fig. 50. Shows the image of 

 the flame {Fl.) in the center (C) 

 of the field of the microscope and 

 illuminating the object. 



Fig. 51. Shows the image of 

 the flame {Fl.) at one side of the 

 center {Exc.) and not properly il- 

 luminating the object. 



To determine this in any case focus upon some very transparent 

 object, take out the ocular, look down the tube at the back lens. If 

 less than three-fourths of the back lens is lighted, increase the open- 

 ing in the diaphragm — if more than three-fourths diminish it. For 

 some objects it is advantageous to use less than three-fourths of the 

 aperture. Experience will teach the best lighting for special cases. 



§ 95. Aperture of the Illuminating Cone and the Field. — 

 It is to be remarked that with a very small source of light the entire 

 aperture of the objective may be filled if a proper illuminator or 

 condenser is used. The aperture depends on the diaphragm used 



