Magnification is the enlargement of an object. Two lenses are used to magnify in a compound scope: an objective lens and an ocular (eyepiece) lens. The microscope shown here has lenses in four objectives: scanning (4 X), low power(10 X), high dry (40 X), and oil immersion (100 X). The ocular lens also magnifies ten times (10 X). Thus the total magnification equals the product of the objective and ocular magnification values. This means that an organism viewed through the ocular with the low power (10 X) objective locked in place would have a total magnification of 100 times (100 X). The oil immersion objective with its total magnification of 1000X is needed to magnify bacteria sufficiently.

Resolution (resolving power) is the ability of the lenses to distinguish between two points a specified distance apart. The brightfield microscope has a resolution of 0.2 micrometers. This means that the microscope can distinguish two points as separate objects if they are at least 0.2 micrometers apart. If two objects are any closer than 0.2 micrometers, they will be magnified as one object. 

The various parts of the microscope are identified in the images shown here. In addition to the objectives and the oculars, microscope parts include the following: 

 arm - supports the microscope and is used for carrying it
 base - support structure; used in carrying microscope
 body tube - holds the oculars and channels the light rays
 coarse adjustment knob- used to initially focus specimen
 condenser - collects the light rays into a single cone of light
 diaphraghm - provides contrast by regulating brightness of light
 fine adjustment knob- used for final focus of specimen
 mechanical stage - platform for holding slide; contains clips to hold 
                              slide; movement of stage is controlled by mechanical 
                              stage knobs
 revolving nosepiece - holds the objectives

Specimens are initially found on scanning or low power. (Bacteria are so small that the scanning power is of little use in locating them. Low power is much more effective.) The microscope shown here is parfocal. This means that a specimen in focus on one objective will be approximately in focus on other objectives. It is relatively simple to bring the organism into focus with high power and oil immersion if it has been focused on low power.

The oil immersion objective requires immersion oil to focus properly. Because the oil has the same refractive index as the glass, it keeps the light rays traveling through to the objective. In contrast, oil that is on the slide or the objective interferes with proper imaging of the scanning, low, and high dry objectives. 

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