620 



EFFECT OF APERTURE IN PROJECTION 



[Cn. XIV 



the substage condenser at d 2 (fig. 170, 349). The effect of the 

 diaphragm at d\, in front of the large condenser, and at dz, where the 

 crater image is formed, is just opposite in the two cases. With the 

 usual arrangement (fig. 348) the diaphragm A, will limit the aper- 

 ture of the objective while the diaphragm D 2 , will limit the size of 

 the field illuminated. With the Kcehler arrangement, however, 

 (fig. 349) the diaphragm DI, limits the size of field illuminated, while 

 the diaphragm D 2 , limits the aperture of the objective used. 



<& 



FIG. 349. KOEHLER'S METHOD OF ILLUMINATIM; MICROSCOPIC Si-i-x IMKNS. 

 L Light source. 

 C, Condenser. 

 D l First diaphragm. 

 D 3 Second diaphragm. 



L' Image of light source on second diaphragm. 

 C a Substage condenser. 

 5 Specimen. 



D' Image -of diaphragm D lt on the specimen. 

 Objective. 

 D\ L* Image of diaphragm D 2 and light source. 



865. Advantages and disadvantages of Koehler method. 



The Kcehler method has the advantage that it enables an easy 

 control of the size of the field illuminated and of the aperture of the 

 objective used. If a larger field than necessary is illuminated, 

 there may be undu heating of the specimen and the best results 

 are obtained only when just the right objective aperture is used. 

 On the other hand, the use of a substage condenser precludes the 

 use of a cooling stage, (fig. 134), except a very thin form. It 

 limits the size of field which may be used, and transmits only 70% 

 of the incident light and reduces the general flexibility and ease of 

 handling the apparatus. 



