Microscopes help scientists see objects invisible to the naked eye. A common type known as a compound light microscope uses lenses and light to magnify an image. It has two systems of lenses: the eyepiece (or ocular lens) into which an observer looks, and objective lenses located directly above the object being observed. The objective lenses have different powers of magnification. Quality compound microscopes have achromatic, parcentered, parfocal lenses.
An objective lens is actually multiple lenses encased in a metal cylinder called the objective barrel that measures a few centimeters in height and approximately one centimeter in diameter. At the top of the barrel is a screw that attaches the objective lens to a revolving objective drum on the microscope. The revolving drum holds three or four lenses of varying magnifications and allows the user to conveniently switch from one lens to another. Specifications engraved on the side of the cylinder provide information such as the type, magnification and resolution of each objective lens.
A microscope typically has three or four objective lenses; each such lens provides a different power of magnification -- commonly 4X, 10X, 40X and 100X -- indicated on its barrel by both a number and a color-coded band. The total magnification achieved using a particular lens is determined by multiplying the power of the ocular lens, usually 10X, by that of the objective lens being used. For example, the total magnification achieved using a 4X objective lens is 10X times 4X, or 40X. The 4X, 10X, 40X and 100X objectives have red, yellow, blue and white bands, respectively. The higher the power of magnification provided by the lens, the longer its length.
The 4X objective is also called the scanning objective. Although this lens provides the least magnification, it offers the greatest field of view and allows one to search for objects that might go unnoticed at higher levels of magnification. The 10X, or low-power objective, allows observation of an object's general structure. The 40 X, or high-power objective lens shows objects in great detail. Since high-power objectives rest in close proximity to the objects being examined, they are susceptible to damage caused by contact with the glass specimen slide; for this reason, high-power objectives have spring-loaded, retractable lenses. The 100X objective is called the oil immersion lens. Placing a drop of immersion oil on top of a prepared slide eliminates the air space between the specimen and lens and increases image resolution; when used in conjunction with appropriate staining techniques, individual cell characteristics are visible.
Objective lenses differ in their ability to correct optical flaws, called aberrations, that decrease image clarity. White light is made up of different colors that refract at different angles when passing through the lens. The inability of the lens to bring all the colors into common focus causes colored fringes around the image. Achromatic lenses are designed to compensate for such color aberrations. Parcentered and parfocal lenses allow the microscope user to transition from one lens to another without having to re-center or refocus the object, respectively.