The Use of the Microscope 27 



copy when everyone was trying to make a point source of light but this 

 is now obsolete. 



The contemporary happy compromise is known as Kohler illumina- 

 tion. In this system light from a compact source— a lamp with small, 

 closely coiled filaments— is gathered by a large lens that projects it as 

 a fairly well corrected beam. An iris diaphragm immediately in front 

 of this lens is used as the actual light source and an image of this iris 

 is focused on the slide by the substage condenser. 



USE FIELD CONDENSER 

 TO FOCUS IMAGE OF 

 LAMP FILAMENTS ON 

 SUBSTAGE IRIS 



RAYS FORMING 

 IMAGE OF 

 FILAMENT ON 

 SUBSTAGE IRIS 



LAMP 



FIELD 

 CONDENSER' 



FIELD 

 IRIS 



MIRROR 



Fig. 26. Diagram to show the four stages in setting up Kohler illumination 



Reference should now be made to Fig. 26, which shows, in diagram- 

 matic form, the setup photographed in Fig. 25. 



To produce Kohler illumination in practice the student must be con- 

 scious of the necessity of forming two images. The first is an image of 

 the lamp filament thrown by the field condenser roughly into the plane 

 of the substage iris. The second is an image of the field iris thrown 

 exactly into the plane of the object by the substage condenser. These 

 images are produced, and the system centered, by the following routine 

 (Figs. 27 to 34). 



1. (Fig. 27.) Swivel the turret to bring the XlO objective into use. 

 Turn on the lamp and adjust the mirror until enough light is obtained 

 roughly to focus a slide on the stage. 



2. ( Fig. 28. ) Open up both substage and field iris. Place a dark, neutral 

 density filter in front of the lamp and focus the slide accurately. 



