SIZE 13 



stagnant water, or in solutions containing decayed 

 organic matter. 



2. Size. Bacteria are among the smallest of all 

 living things. Their dimensions are usually stated in 

 thousandths of a millimetre. One thousandth of a 

 millimetre (.001 mm.), known as a micron, is denoted 

 by the sign I ,a, and is equal to about 2TljoT> of an inch 

 (.00004 inch). Cocci range from less than .5 //, up 

 to 2 p in diameter ; bacteria and bacilli from .5 //, 

 or less in breadth to 10 ^ in length; the majority, 

 however, are much shorter than the latter figure. On 

 account of their minute size they are, when dry, readily 

 blown about by the slightest breeze, and find their way 

 into any crevices wherever dust can penetrate. They 

 are carried in streams of water, and are to be found 

 in and about all foods, milk and other liquids, in 

 houses and stables and in the soil, on the skin and 

 in the stomach and intestines of all animals. They are, 

 however, never met with in the blood or tissues of 

 healthy animals or plants, and are rare in water which 

 has come from deep wells or springs, or has been filtered 

 through great depths of soil and rocks. 



Ex. 10. To Measure the Size of a Bacterium or other 

 Organism. 



(a) A simple method is by means of a stage micrometer and 

 camera lucida. 



The stage micrometer consists of a scale of tenths and 

 hundredths of a millimetre accurately ruled in fine lines on a 

 glass plate. 



Place it on the stage of the microscope, and make drawings 

 of its scale on a sheet of paper with the camera lucida, using in 

 turn the f in., Jin., in., and yg- in. objectives, with a No. 3, 4, 

 or other eyepiece. 



