Cleland • Cytology: the Study of the Cell 



229 



gins to lose definition, to become more 

 and more fuzzy. 



Various improvements have been 

 made from time to time on the hght 

 microscope. Quartz lenses have per- 

 mitted the "use of ultraviolet light 

 which, because of its shorter wave 

 lengths, permits a higher resolving 

 power than does visible light. An ultra- 

 violet microscope, however, can be 

 used onlv photographically, since ultra- 

 violet light is invisible to the human 

 eye. 



Dark-field microscopy has provided 

 another useful supplement to the light 

 microscope. It depends upon visible 

 light which is introduced obliquely into 

 the material being studied and is re- 

 flected of! the surface of particles to the 

 eye of the observer. When light from 

 below is cut off and only the light re- 

 flected from the surfaces of particles 

 is seen, the presence of particles too 

 small to be resolved by the ordinary 

 miscroscope is revealed. 



In recent years, other mechanical 

 developments have been added to the 

 cvtologist's arsenal of weapons. One of 

 the most serious sources of error in cy- 

 tological study stems from the fact that 

 protoplasm in its living state is so 

 transparent that one can look right 

 through it with the ordinary micro- 

 scope and fail to see most of the struc- 

 tures which it contains. This has made 

 it necessary to use dyes which will stain 

 different structures differently or stain 

 particular structures and leave others 

 unstained. Such dyes, however, will 

 function as a rule only after the cell has 

 been killed. The cytologist is com- 

 pelled, therefore, to kill the cell first, 

 then stain it. This necessity introduces 

 into the work an element of uncer- 

 tainty, for one is confronted with the 

 possibility that the killing process has 

 altered the fine structure of the cell or 

 has rendered parts of the cell soluble 

 in the reagents that must be used in 

 the staining and mounting process. He 



now has two new kinds of microscopes, 

 the phase-contrast microscope and the 

 interference microscope, which utilize 

 certain optical principles hitherto un- 

 used in microscopy to make many of 

 the constituent parts of the cell stand 

 out from their surroundings when the 

 protoplasm is still living, so that they 

 can be studied in respect to both their 

 structure and behavior. 



Perhaps the most spectacular me- 

 chanical development which in recent 

 years has become available to the cy- 

 tologist is the electron microscope. 

 This instrument is capable of providing 

 sharplv resolved images at magnifica- 

 tions iO times that of the light micro- 

 scope. Although there are certain 

 drawbacks to its use, the greatly in- 

 creased magnification which it affords 

 has brought it into great prominence 

 as a cytological tool. The drawbacks to 

 its use, beside the great cost of the in- 

 strument, are the fact that it must be 

 used primarily as a photographic in- 

 strument; the fact that the material 

 studied must be ver\' thin, since elec- 

 trons will not pass through thick la\ers 

 of matter; and the requirement that 

 material must be studied in a vacuum, 

 since electrons will not penetrate air. 

 Tlie use of the electron microscope as 

 a photographic instrument is in one 

 respect an asset; for it is possible to 

 take pictures at magnification of 15,- 

 000 diameters or more on fine grained 

 film, from which enlargements can be 

 made which retain sharp definition up 

 to magnifications of 50,000 or more. 

 The electron microscope is making 

 as drastic a revolution in cytological 

 knowledge as did the light microscope 

 in the 1870's and 1880's. It is revealing 

 an amazing intricacy of structure in the 

 cell beyond anything dreamed of a 

 few years ago. Resolving powers as low 

 as 8 Angstroms are being achieved, i.e., 

 it is possible to reveal particles as small 

 as one 30 millionth of an inch in diam- 

 eter. 



