MEASUREMENTS 35 



numbers, rather than very large numbers or extremely small fractions. 

 The centimeter and millimeter follow the prefix system described in 

 Table 4-1, but smaller units have names of their own. One-thousandth 

 of a millimeter (10~^ m) is called a micron and is given the symbol /a. 

 One-thousandth of a micron (10~'' m) is the millimicron (m/^), and a 

 tenth of this (10"^*' m) is the Angstrom unit (A or A). 



Measurements of length are among the most familiar types of meas- 

 urement. Differences between parts of organisms, or living cells them- 

 selves, frequendy fall in a size range smaller than is convenient for the 

 human senses. One method of measuring small objects is to magnify both 

 the object and the scale with which it is compared. Measurement with 

 a microscope is possible in this manner, but more commonly we use an 

 "ocular micrometer," a scale engraved on a transparent disk which fits 

 into the eyepiece of the microscope. The optics are arranged so that the 

 scale and the object are seen at the same time. The ocular micrometer 

 is calibrated; that is, definite values are assigned to the divisions of the 

 scale by comparison with an accurately and finely divided scale engraved 

 on a microscope slide. Almost all measurements of the sizes of cells or parts 

 of cells are accomplished in this manner. Measurements of even smaller 

 units are exceedingly difficult indirect measurements and frequently are 

 calculated from the known geometry of an optical system. 



Area and volume are derived directly from length, two or three meas- 

 urements of which enable us to calculate these quantities. Several special 

 units of area exist, such as the acre and the are or hectare, but only 

 square centimeters (cm^) and similarly derived units are common in the 

 experimental laboratory. Volume can be expressed in terms of length 

 units cubed (cm^) if the volume is calculated geometrically from the 

 dimensions. The liter, the volume (about 1000 cm^) occupied by 1 kg 

 of water under certain specified conditions, is the basic metric unit of 

 fluid volume, while the gallon (231 in.^ in the U. S.) is the commercial 

 unit. Fractions of liters generally follow the system of Table 4-1. The 

 microliter (/u.1) has sometimes been designated by the symbol lambda (X). 

 This practice seems to be less common now, fA being more common, but 

 the symbol A. does exist in the literature and should be interpreted as 

 one microliter. 



Mass: Mass is a measure of the amount of material, a concept derived 

 from Newton's second law of motion which says that force is directly 

 proportional to acceleration. Expressed in this way, mass becomes a 

 quantitative unit related to the qualitative idea of inertia. Weight is 

 easily confused with mass, but weight is the force which gives a body 

 the acceleration of gravity and thus should be expressed in units of force. 



