CRITICAL ILLUMINATION 121 



It follows that a condenser must be selected to suit the 

 objective in use. A low-power objective requires a 

 condenser of correspondingly long focal length. 



9. Numerical Aperture of the Condenser and Objective. 

 Focus the image of the light across the object by means 



of the back combination of the condenser, and fit the 

 \" objective. Examine the Ramsden circle ; it will be 

 found impossible to fill the back lens of the objective 

 under these conditions, and the light employed is not 

 critical. It may be that no inferiority in the image to 

 that obtained with critical light can be detected ; it is 

 only a worker of experience examining special objects 

 who can detect the depreciation, and for a great deal of 

 photomicrography critical light is not essential, but the 

 experiment points to the necessity of having a condenser 

 whose N.A. is adequate for the objective in use. It is 

 generally not desirable to use the entire N.A. of the 

 objective, as very few lenses work quite satisfactorily 

 without a small restriction, but whatever aperture can be 

 used, or is best for the particular object under examina- 

 tion, should be evenly filled with light. 



10. Diffraction Effects and N.A. Examine carefully 

 an object with well-defined lines or dots an insect with 

 hairs, for instance first with the iris open, and then 

 gradually closing it down. When the iris is wide open 

 the image appears drowned in light and indistinct, as the 

 aperture is closed to f or f the clearest image is formed, 

 and as the iris is further closed to J or J aperture " dif- 

 fraction effects " are set up, and appear as white margins 

 to lines or dots or as double images (Plate 12). As a 

 rule, these become apparent in a photomicrograph 

 before the iris is closed far enough to make them very 

 noticeable visually, and great care must always be taken 

 to avoid them. Different objects require a varying 

 restriction of aperture. Stained bacteria are best shown 

 with as little curtailment of the aperture as possible, 



