What are the Parts of a Microscope?
For the purposes of understanding how microscopes function, we will "dissect" a compound light microscope into its component parts.
1. Ocular Lens or Eyepiece
This lens sits directly in front of the observer's eye and does not typically change in magnification power. Older-style microscopes were "monocular," meaning they had only one eyepiece, but in recent years microscopes have been made "binocular" in order relieve eyestrain and allow for more natural viewing. Frequently one of the ocular lenses can be adjusted slightly to match the needs of those who require corrective vision.
2. Objectives (organized on a turret)
The objective lenses combine with the power of the ocular lens to create the magnified image. Frequently one starts on the lowest magnification to find focus and then increases magnification one objective at a time. This is facilitated by a rotating turret that allows the objectives to be interchanged smoothly.
3. Objective Lens
Objective lenses come in several varieties. In a light microscope, the lowest power objective (frequently 4x or 10x) is referred to as the "scanning" objective, as it allows the observer to find an area of interest before increasing magnification. As magnification increases, the amount of light collected from the sample decreases. For this reason, the highest objective on a light microscope (such as the 100x) requires mineral oil to be applied to the gap between lens and sample. The oil has a greater refractive index and bends more light into the objective.
4. Coarse Adjustment Knob
Both knobs change the distance between objective and viewing sample. The coarse adjustment knob is primarily used with the scanning objective. It allows the user to bring the scope into approximately the right plane of focus. Once this has occurred, it is preferential to leave the coarse adjustment alone and use only the fine adjustment.
5. Fine Adjustment Knob
The fine adjustment knob helps clarify a partially focussed image. It requires many more turns than the course adjustment to do the same amount of work. This allows the observer to take great care in clarifying the image.
The stage, usually adorned with stage clips of some sort, is where the specimen rests. After being centered over a hole in the middle and secured with the clips, the subject can be raised or lowered by the course and fine adjustment knobs. Also, the stage has its own controls for moving the subject left, right, back, and forward on the horizontal plane.
7. Light Source
The light source sits at the base of the microscope (usually) and either consists of a radiant bulb or a mirror for redirecting light. Light is shined up through the hole in the stage to illuminate the sample.
8. Diaphragm and Condenser
Before the light enters the stage opening, it passes through the diaphragm and the condenser. The diaphragm, which has an opening or "aperature" in the form of an iris, only allows light to pass through its center. By increasing or shrinking the size of the iris, you can control how much light passes through to the sample.
The condenser, on the other hand, does not prevent light from passing, but condenses the existing light. This is important not only to prevent the scattering of light, but also to create the most efficient condenser setting for each objective.
These two components are by far the most frequently overlooked and misunderstood by students, but they can often bring a new level of clarity to an otherwise weak image.
9. Structural Components of the Apparatus
The "body" of the microscope is divided into two parts: the base and the arm. The base is heavy and prevents the scope from slipping on the bench top or tipping over. The arm holds all of the apparatus in place above the light source. A student microscope should ALWAYS be carried by both the base and the arm.