MICROSCOPY 103 



cover glass and the lens. At this point, finish focusing with the fine 

 adjustment. Increase the light if necessary. The slide can still be moved 

 around but, so that the oil connection between the cover glass and the 

 lens won't be broken, only to a limited extent. When finished, be sure 

 to clean the oil off the lens and the slide. Oil-immersion microscopy is 

 not very satisfactory with fresh mounts in water. Focusing tends to move 

 the cover glass, which often moves the specimen. 



The microscope, a rugged instrument, will stand some abuse but 

 will never perform ideally afterward. As a precision instrument it de- 

 serves the very best of care, partly by way of appreciation of the lens- 

 makers' art. The practical limits and the theoretical limits of the micro- 

 scope almost coincide; probably no other instrument has reached such a 

 high level of development. The microscope is rarely called upon to de- 

 liver its full potentialities. 



Practice is necessary, but almost anyone can learn to use the ordinary 

 microscope effectively. The beginner may get lost behind great black 

 shapes of objects too thick for light to penetrate, or he may spend some 

 time focusing on the lovely air bubbles that are common in fresh mounts 

 in water. Plant cells usually are more readily recognized than animal 

 cells because of the prominent cell wall in plants. A little experience, 

 however, enables one to navigate among even the most difficult of ma- 

 terials. 



In the following sections, several microscopic methods are described 

 which demand too much for the casual operator. Considerable training 

 and skill is required, both to use the microscope and to interpret what is 

 seen. Only a very general description will be given. 



The phase contrast microscope 



Some biological materials and some parts of cells, especially when 

 alive, offer so little contrast in density, refractive index, or color that they 

 are difficult to study with an ordinary microscope. Some of these can be 

 studied more effectively through the techniques of phase microscopy. 



In ordinary "bright field" microscopy, high resolution depends upon 

 diffraction of light and the fringes that appear on the edges of structures. 

 The diffracted light must be caught by the objective lens, and the image 

 is formed from the interference pattern of the direct or axial light and 

 the diffracted light. If there is sufficient contrast (difference in absorp- 

 tion, reflection, or refraction) at these edges, the image is sharp and ob- 



