MICROSCOPY 101 



is held in a clamp and moved mechanically over a very sharp blade where 

 a thin slice is cut off. On the next stroke, the embedded tissue is advanced 

 tow^ard the blade by a distance equal to the desired thickness of the tissue 

 slice. Each stroke thus advances the tissue, usually by means of a lead 

 screw, and then cuts off the amount of the advance. The sections of 

 tissue, surrounded by the embedding material, slide off the knife, one 

 after another, in a more-or-less continuous ribbon. If a series of sections 

 is prepared in this way, each slice can be studied separately, and the 

 whole series allows the biologist to assemble a three-dimensional picture. 

 The microtome is a rugged but precise instrument, permitting the slicing 

 of sections thinner than a single cell. 



Permanent prepared slides are made from sections cut on the micro- 

 tome. The section of biological material is attached to the slide, and the 

 embedding material is dissolved away. Further treatment involves soak- 

 ing in one or more stains or dyes, chemical treatment to make the 

 staining permanent (at least in certain parts of the cell), and washing 

 to remove excess dye. Dyes are selected on the basis of their affinity for 

 certain chemical substances in the cells. For example, one dye might be 

 attracted only to lignin and thus would collect in the cell walls of plant 

 cells, staining them, say, red. A second staining treatment might stain 

 cytoplasmic proteins green. The finished slide would show individual 

 cells with red cell walls and green cytoplasm, which certainly is more 

 contrast than the original material furnished. Once the staining is com- 

 pleted and chemical treatment has withdrawn all water, a cover glass is 

 cemented over the biological material with a natural or synthetic cement 

 of high refractive index. The whole preparation might require several 

 hours or days, but slides prepared in this manner last for years and can 

 be examined under the microscope at any time. If the slide has been well 

 made, there will be a minimum of disarrangement and swelling or shrink- 

 age of the parts of cells. 



Use of the microscope 



The compound microscope is a familiar instrument to anyone who 

 has taken a biology course, so only a few general pointers on the use of 

 the microscope will be presented here. 



The first steps are preparatory. Make sure that the lenses of the micro- 

 scope are clean, using nothing but specially prepared lens paper which 

 will not scratch the glass. Several commercial solvents are available for 



