MICROSCOPICAL TECHNIQUE. 399 



ferent values for each combination of lenses used and for every 

 variation in the length of the microscope-tube. The unit for micro- 

 scopical measurements is one-thousandth of a millimeter, or one- 

 millionth of a meter ; it is called a " micrometer/ 7 and is desig- 

 nated by the Greek letter //. One division of the micrometer-slide 

 mentioned above would, therefore, equal 10 {*. From these data it 

 is possible to calculate the value of each division of the eye-piece 

 micrometer-scale in terms of fj. for each combination of lenses, the 

 length of the microscope-tube being fixed. (Most Continental 

 stands and many American stands have graduated tubes, and the 

 objectives are constructed for a standard tube-length of 160 milli- 

 meters.) 



It is well for the student to get into the habit of estimating the 

 sizes of the objects he examines. A good standard for mental com- 

 parison is the diameter of the unaltered red blood -corpuscle, which 

 is about 7.5 /*. 



MICROSCOPICAL TECHNIQUE. 



Useful preparations for study under the microscope may be pre- 

 pared from tissues in one of three ways : 1, simple scrapings of the 

 tissues may be mounted on a slide in the fluids derived from the 

 tissues themselves, or in a neutral solution e. g., 0.75 per cent, salt 

 solution ; 2, the tissue-elements, cells, and intercellular fibres, etc., 

 may be separated from each other by treatment with some macerat- 

 ing-fluid e. g., very weak chromic acid (1 : 10,000), 36 per cent, 

 caustic potash, ^ alcohol ; 3, sections of the tissue may be prepared 

 either while they are fresh, with a razor or a freezing-microtome, or 

 after hardening. 



The first method has a limited application. It is serviceable 

 only when the tissue-elements are so loosely held together that they 

 readily separate from each other and can be examined in an isolated 

 condition. This is the case with a considerable number of tumors, 

 the superficial tissues of mucous membranes, the spleen, etc. If 

 the inside of the cheek be scraped with the finger-nail, and the 

 material thus removed be diluted with saliva, placed upon a slide, 

 and covered with a cover-glass, the squamous epithelial cells lining 

 the cavity of the mouth will be readily seen in an isolated state. 

 An appropriate dye may now be introduced under the cover, and by 

 its means the nuclei of the cells stained, thus bringing them into 

 clearer view. 



