The major difference between the electron microscope and the ordi- 

 nary light microscope is the physical design of the two instruments. 

 Because electrons will travel appreciable distances only in a vacuum, 

 the entire microscope unit of the electron microscope is enclosed in a 

 vacuum-tight column. The similarity in arrangement and function of the 

 optical systems of the two microscopes can be seen from examination 

 of Figure 11-9. In the electron microscope, the illuminating source is 

 a cathode filament (C) which emits a narrow beam of electrons. This 

 electron beam is collected and focused upon the specimen by an electro- 

 magnetic condenser lens (Lc). After passing through the specimen, the 

 electrons are collected by the electromagnetic objective lens (Lo) which 

 forms an enlarged image of the specimen. An electromagnetic projector 

 or "eyepiece" lens (Lp) further magnifies the image of the specimen and 

 projects it on to a fluorescent viewing screen (S). 



The electron image is focused by varying the magnetic field of the 

 objective lens (Lq). Magnification of the image can be increased or de- 

 creased by varying the current in the projector lens (Lp). Since the 

 objective and projector lenses of the electron microscope contribute 

 more or less equally to the magnification of the image, variation of the 

 magnetic field of the objective lens permits a range of from 1000 to 

 60,000 X to be obtained. In the fight microscope, magnification is varied 

 by changing the objective (L„) and/or eyepiece lens (Lg). The maxi- 

 mum magnification of an oil-immersion objective is about lOOx, while 

 the magnification contributed by the eyepiece generally never exceeds 

 15x. In practice, the highest magnification possible with reasonable 

 resolution, with the ordinary light microscope, is about 1500X, 



Phase-Contrast Microscopy 

 Most living cells are essentially transparent to visible light. This 

 means that light passing through such objects must suffer almost no loss 

 of intensity as the result of absorption by cell components. Because the 

 eye is sensitive only to differences in intensity and/or color, the living 

 cell wiU show almost no contrast when viewed with the conventional 

 optical microscope. Light transmitted through the living cell, however, 

 does encounter regions of different refractive index and thickness which 

 alter its velocity and direction. In Figure 11-10 are represented two 

 adjacent regions of the cell, A and B, of different thickness, ti and to, 

 and of different refractive index, «i and no. This variation in thickness 

 and refractive index produces a difference in the optical path (product 

 of refractive index and thickness) of light transmitted by the two regions, 

 i.e., it takes longer for light to pass through region B of higher refractive 



SURVEY OF CYTOLOGICAL TECHNIQUES / 227 



