29 : 3/ Microscopy 



543 



light can enter the objective. Anything in the specimen which diffracts 

 or scatters light sufficiently will appear bright against a dark back- 

 ground. A condenser arrangement to make this possible is shown in 

 Figure 4. 



The microscope used with a dark field is called an ultramicroscope not 



Figure 4. Dark-field illumination. Accomplished by substi- 

 tuting for the upper lens of an Abbe condenser a special dark- 

 field element provided with a stop on the lower surface. After 

 W. B. Rayton, in Medical Physics, Vol. 1, O. Glasser, ed. 

 (Chicago, 111.: Yearbook Publishers, Inc., 1944). 



because it has a greater resolving power, but because it reveals un- 

 stained elements which are not visible with the bright-field microscope. 

 For example, most living protoplasm appears homogeneous when viewed 

 with bright-field illumination, but numerous small particles oscillating 

 in Brownian motion become visible in dark-field illumination. Proto- 

 zoans in a liquid are almost transparent in bright-field illumination but 

 show up as bright images against a dark background in the dark-field 

 microscope. 



However, the dark-field microscope has a number of limitations. 

 The numerical aperture cannot exceed one, which decreases the resolving 

 power 2 of oil-immersion lenses. The image produced is often a diffrac- 

 tion pattern rather than a true image and may have little resemblance 

 in shape to the original. The images seen are only of those objects which 

 diffract light strongly enough so that it enters the objective. 



The dark-field microscope can be looked upon as a special extreme 



2 It is often customary to call the reciprocal of the limit of resolution the 

 resolving power. Thus, a lower limit of resolution means a higher resolving power. 



