Optical System
The primary function of the optical system is to form a magnified image of the specimen on the observer's retina. This system consists of two main elements secured within the body tube.
Objective lens: Located at the lower end of the body tube, facing the specimen directly. This is a complex lens system divided into two types:
- Dry objectives: Operate in an air medium. Standard magnifications include $\times4$, $\times8$, $\times10$, $\times20$, and $\times40$.
- Immersion objectives: Designed for high-power magnification ($\times90$). To operate properly, the lens must be immersed in a specialized medium—a drop of cedar oil placed on the coverslip.
Eyepiece (Ocular): Located at the upper end of the body tube, facing the observer's eye. Eyepieces are interchangeable components with magnifications of $\times7$, $\times10$, or $\times15$.
To calculate the total magnification, multiply the objective power by the eyepiece power. For example, using a $\times20$ objective and a $\times10$ eyepiece yields a total magnification of 200×.
Illumination System
The illumination system directs light through the specimen. Its components are arranged strictly along the path of the light beam.
- Light source: Can be built-in or external (desk lamp, natural window light).
- Mirror: Directs light rays upward toward the specimen. It features two working surfaces. The flat surface is used in natural daylight since sunlight provides parallel rays. The concave surface is used with artificial light (from a lamp) to gather diverging rays. Modern models with built-in illumination may lack a mirror.
- Diaphragm: Regulates the light flux. It consists of opaque leaves forming an aperture of variable diameter. At low magnification, the aperture is narrowed to reduce spherical aberration; at high magnification, it is opened wider.
- Condenser: A system of lenses that gathers and focuses light rays precisely onto the specimen. The focus can be adjusted by raising or lowering the condenser via a dedicated adjustment knob. This element is often structurally integrated with the diaphragm.
Mechanical System
The mechanical system acts as the structural framework. It is designed to secure the optics and manipulate the specimen.
Key structural components include the arm and pillar. Optical elements are housed in the body tube, while the specimen rests on the stage.
Focusing mechanisms—adjustment knobs—are used to achieve a sharp image. Connected to the pillar, they allow the arm and body tube to move vertically:
- Coarse adjustment knob (macrometer): Used for rough focusing and exclusively with low-power objectives.
- Fine adjustment knob (micrometer): Required for precise focusing under high-power magnification.
The mechanical system also includes stage translation controls that allow horizontal movement of the specimen for precise positioning beneath the objective lens.
Microscopy Principles and Light Path
Because examination relies on transmitted light, histological specimens must be extremely thin and transparent. Additionally, the optics produce an inverted image.
Light travels along a strictly defined path from the source to the observer's eye:
- Light source.
- Mirror (directs the light upward).
- Diaphragm and condenser (restrict and focus rays onto the specimen).
- Specimen.
- Objective lens (forms the primary magnified image).
- Body tube (transmits the image).
- Eyepiece (delivers the image for visual perception).