96 The Microscope (chap. 9) 



Obviously every combination of oculars and objectives, also different 

 tube lengths, must each be calibrated in this fashion. 



Specialized Microscopy 



The image of a biological specimen can be formed and used in 

 several ways. The most common form, known in all laboratories, is 

 the use of color images, either natural color in the specimen or differ- 

 ential color applied to it. But, with the exception of a few vital stains, 

 the latter method cannot be used on live material. Most living material 

 is transparent to light; that is, the light wave passes through it with very 

 little loss of intensity, so other means for examining it must be em- 

 ployed. 



Dark Field Microscopy 



In dark field, the objects themselves turn the light into the microscope 

 by reflecting it or scattering it, and the object appears luminous on a 

 dark background. No light from an outside source reaches the eye. A 

 stop in the substage condenser blocks out the central part of the solid 

 cone of light formed in the condenser, and only oblique rays in a hollow 

 cone of light, striking from the sides, illuminate the object. 



The simplest form of dark field can be developed with a black central 

 patch stop inserted in the carrier under the condenser. Dark field 

 elements in a threaded mount can be used in place of the upper lens of 

 an Abbe condenser. This is practical with all objectives if the numeri- 

 cal aperture does not exceed 0.85 and immersion is used with the slide. 



Dark field can be modified with colored stops and outer rings below 

 the substage condenser, instead of black stops, to produce optical stain- 

 ing. The central stop determines the backgroimd color and the outer 

 ring the color of the object, giving optical coloring to unstained objects 

 and helping to reveal detail and structure. 



Dark field and optical staining can be useful for examining: 1. bac- 

 teria, yeasts, and molds; 2. body fluids, plant or animal; 3. colloids; 4. 

 living organisms in water; 5. foods, fibers, and pigments; 6. insects and 

 scales; 7. crystals; and 8. bone, plant, and rock sections. 



It is a common means of studying the results of microincineration, 

 the investigation of minerals present in different parts of tissues. Paraffin 

 sections of tissue fixed in a solution of formalin and alcohol are 

 mounted on slides and placed in an electric furnace. The temperature 



