Compound Microscope Parts, Labeled
Each part, where it sits, and what it actually does.
A compound microscope is usually described in three groups: the optical parts that form the image, the mechanical parts that hold and position things, and the illumination parts that light the specimen. Learning them in those groups is easier than memorising a list, because each group answers a different question.
Optical parts
| Part | Where | Function |
|---|---|---|
| Eyepiece (ocular) | Top of the body tube | Magnifies the objective's image for the eye. Usually 10×. |
| Body tube | Between eyepiece and nosepiece | Maintains the correct optical distance between the two lens systems. |
| Objective lenses | On the nosepiece | Gather light from the specimen and form the magnified real image. Typically 4×, 10×, 40× and 100× oil. |
| Condenser | Under the stage | Focuses light into a cone matched to the objective. |
Mechanical parts
| Part | Function |
|---|---|
| Arm | Supports the body tube and is the correct place to carry the instrument. |
| Base | Bears the weight and usually houses the illuminator. |
| Revolving nosepiece | Holds the objectives and rotates them into the light path. |
| Stage | The platform the slide rests on. |
| Stage clips or mechanical stage | Hold the slide; a mechanical stage moves it precisely on two axes. |
| Coarse adjustment | Large focus movements. Low power only. |
| Fine adjustment | Small movements for sharp focus at high power. |
Illumination parts
| Part | Function |
|---|---|
| Illuminator | The light source, usually an LED or tungsten lamp in the base. |
| Iris diaphragm | Adjustable aperture setting how wide the illuminating cone is. This is the contrast control. |
| Field diaphragm | On Köhler-capable stands, limits the lit area of the field. |
The mistake almost everyone makes
The iris diaphragm is not a brightness control. Closing it does dim the image, but what it is really doing is narrowing the illuminating cone, which raises contrast and reduces resolution. Use the lamp for brightness and the diaphragm for contrast. Students who dim the image by stopping down the iris lose the resolution the objective was bought for.
Total magnification
Multiply objective by eyepiece. A 10× eyepiece gives 40×, 100×, 400× and 1000× with the four standard objectives. Beyond roughly 1000× you get empty magnification: a larger image with no extra detail, because resolution has already hit its limit.