132 CLELAND 



various stimuli, was the work of 19th-century investigators. The 1870's 

 and 1880's were especially significant in the history of cytology. It was dur- 

 ing this period that chromosomes were discovered, that the details of cell 

 division were worked out, that the foundations for the study of the struc- 

 ture, chemical composition, and behavior of protoplasm were laid. 



If one were to ask why it took so long after the cell was first seen for the 

 science of cytology to be born, the answer is easy. Cytology, dealing as it 

 does with units of microscopic size, had to await the development of magnify- 

 ing apparatus of sufficient strength and resolving power to enable objects as 

 small as a cell to be studied in detail. Such instruments were not developed 

 until the latter third of the 19th century. As soon as they became available, 

 an entirely new field of study was opened up. 



Ever since its birth, cytology has been limited in its achievements by the 

 instruments and methods at its disposal. On the observational side, it has 

 been limited by the ability of available equipment to magnify and resolve. 

 The microscope which uses ordinary light can only magnify in a really 

 satisfactory manner up to about 1500 diameters, and this result has only been 

 achieved by the use of special types of optical materials, such as fluorite. At 

 higher magnifications, the image begins to lose definition, to become more 

 and more fuzzy. Many of the secrets of structure and behavior which cytol- 

 ogists are seeking to uncover depend upon the analysis of structures which 

 lie beyond the resolving power of the ordinary microscope. 



Various improvements have been made from time to time on the light 

 microscope. Quartz lenses have permitted the use of ultraviolet light which, 

 because of its shorter wave lengths, permits a higher resolving power than 

 does visible light. An ultraviolet microscope, however, can be used only 

 photographically, since ultraviolet light is invisible to the human eye. One 

 must take photographs and study these, rather than look at the material 

 directly, and this creates a barrier to rapid and convenient study which, 

 together with the excessive cost of a quartz optical system, has greatly 

 limited the use of the ultraviolet microscope. 



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 reflected off the surface of particles to the 

 eye of the observer. When light from below is cut off and only the light 

 reflected from the surfaces of particles is seen, the presence of particles too 

 small to be resolved by the ordinary microscope is revealed. 



In recent years, other mechanical developments have been added to the 

 cytologist's arsenal of weapons. One of the most serious sources of error in 

 cytological study stems from the fact that protoplasm in its living state is so 

 transparent that one can look right through it with the ordinary microscope 

 and fail to see most of the structures which it contains. This has made it 

 necessary to use dyes which will stain different structures differently or stain 



