8 FUNDAMENTALS OF CYTOLOGY 



power, i.e., the ability to render fine detail in an image. The photo- 

 microscopes used for direct visual observation by cytologists reach the 

 limit of their resolving power somewhere between 0.2 and 0.1 ju/ two 

 minute objects closer together than this appearing as one. A single 

 particle somewhat below this limit may appear as a dark or light spot, 

 depending upon the method of illumination, but its true form is not fully 

 revealed. When ultraviolet light, quartz lenses, and photographic plates 

 are used, the resolving power is about twice as great, for it varies with 

 the frequency of the radiation employed. With the electron microscope, 

 however, the resolving power is increased 20 to 50 times. The material 

 to be examined must be dry for the best results, the interior of the instru- 

 ment where it is placed for observation is occupied by a vacuum, and the 

 image obtained is a shadow picture only; hence there are great difficulties 

 attending the study of biological objects. Some of the results already 

 obtained, such as those with viruses, together with the prospect of 

 ingenious improvements, warrant the hope that the instrument will lead 

 to further discoveries of immense importance. 



Some of the fixing and staining techniques devised half a centmy ago 

 still survive in the laboratory \x\i\\ little modification, some have been 

 greatly altered and improved, and some have been replaced by new ones 

 of greater value. Advances in physical chemistry and microchemistry 

 have enabled the technician to go about his tasks with an increased 

 understanding of what is going on in the tissues before him, even though 

 much is still there that has not been fully explained. The cause of one 

 of his chief perplexities has been very largely removed by the standardiza- 

 tion of biological stains, an accomplishment of a cooperative group of 

 biologists, manufacturers, chemists, and other interested workers. 



Cytologists as well as investigators in other fields are now making 

 increasing use of statistical methods in the evaluation of their data. 

 Many valuable contribu^ons have been made in past years with little 

 conscious attention to the mathematical aspects of the problems in hand, 

 but it is now more widely recognized that inferences drawn from observa- 

 tions may stand or fall with the results of mathematical analysis and 

 that the true significance of what has been observed may not appear 

 unless such methods are employed. 



By far the most significant and encouraging development in twentieth- 

 century cytolog.y has l^een its more definite alliance with neighboring 

 fields of biology. For many decades the various fields had given promise 

 of becoming more mutually helpful, and now that promise is being ful- 

 filled: the nineteenth century's subdivision of biology is being succeeded 



^ The symbol ix stands for micron. This equals 0.001 millimeter, or 1/25,400 

 inch, and is the unit of length most often used in cytologj'. The angstrom unit (A), 

 also used at times, equals 0.0001^1. 



