8 



STUDENTS HISTOLOGY 



in describing such phenomena only to amplification in a single 

 direction. The diameter of the object under examination has been 

 increased ten times and would be expressed by prefixing the sign 

 of multiplication: e.g., X 10. 



A convenient unit of approximate measurement for the histolo- 

 gist is the apparent size of a human red blood -corpuscle with a 

 given objective. Thirty -two hundred corpuscles, placed side by 

 side, would measure one inch; or, we say, the diameter of a single 

 corpuscle is the thirty -two -hundredth of an inch. After consider- 

 able practice, you will become accustomed to the apparent size of 

 this object with a certain objective and eye -piece. This will aid in 

 an approximate measurement of objects by comparison, and will 

 further give the magnifying power of the microscope. If a cor- 

 puscle appears magnified to one inch in diameter, it is evident that 

 the instrument magnifies thirty -two hundred times. Should the 



3 



10 



4567 



Centimeters. 

 FIG. 3. ENGLISH AND METRIC SCALES. 



diameter appear one -quarter of an inch, the power is eight hun- 

 dred ; one-eighth of an inch, four hundred, etc. The instrument 

 which we have heretofore described, with the high- power in use and 

 the tube withdrawn, will present the corpuscle as averaging very 

 nearly one -eighth of an inch in diameter X 400. While this 

 gives a gross idea of amplification, the method will often prove 

 to be inaccurate because of individual errors in the estimation of 

 proportions. 



Measurements made with the microscope are usually based on 

 the metric system. The unit taken is one -thousandth part of a 

 millimeter, or a micro -millimeter, called also a micron, for which 

 the Greek letter p has been taken as the symbol. Roughly, 1 p 

 equals 25000 inch. A convenient instrument for measuring is an 

 eye -piece micrometer, a ruled disk of glass, which may be placed 

 within the eye -piece, and the diameters of objects read off by 

 means of the ruled scale. 



