. Biophysical science. Biophysics. 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
. Biophysical science. Biophysics. 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 power2 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 Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the ori
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