1.2.2: Identifying the Components of the Compound Microscope

The microscopes you will be using in lab are bright-field compound microscopes. Compound microscopes use two sets of lenses, ocular lens and objective lens, to magnify a specimen. Bright-field microscopy refers to the use of white light to visualize the specimen. The specimen absorbs the light causing the specimen to be darker than the surrounding background. Biological specimens usually lack the contrast (the ability to distinguish the object from the background) to observe details, and thus are generally stained before observation.

Microscopes allow the magnification of a specimen by refracting light rays as they pass through the magnifying lenses. Magnification alone will not produce a quality image. It is important that resolution and contrast be achieved in addition to magnification to generate a quality image. Resolution refers to the ability to distinguish two objects from one another. Contrast refers to the difference in light intensity of the specimen and the background.

You will need to have knowledge of the parts of the microscope to be able to follow instructions during future laboratory exercises. You will identify each of the following components on the microscope and describe the function of the component.

• Abbe Condenser – collects the light from the light source and focuses it onto the specimen
• Arm – connects the head of the microscope to the base; used to assist in carrying the microscope
• Base – bottom of the microscope that provides support for the microscope and houses the light source
• Coarse Focus (Adjustment) Knob – used to focus on the microscope
  • ***Note*** Should only be used with the 4x (scanning objective lens)
• Condenser Focus Knob – moves the condenser up and down to control the focus of the light on the specimen
• Eyepiece – houses the ocular lens; the component you look through at the top of the microscope
• Fine Focus (Adjustment) Knob – used to focus the microscope; it allows for fine tuning the focus; used with higher power objective lenses
  • ***Note*** Using only this focus knob with higher power objective lenses will help prevent driving the objective lens into the slide, possibly damaging the objective lens of breaking a slide
• Iris Diaphragm – controls the amount of light reaching the specimen
• Iris Diaphragm Aperture Lever – adjusts the aperture (opening) of the iris diaphragm to allow more or less light to reach the specimen
  • ***Note*** As you increase in objective magnification power you will need more light; you will need to adjust the amount of light reaching your specimen using this lever to ensure you have adequate contrast between your specimen and the background
• Mechanical Stage – platform where a microscope slide containing the specimen is placed; may be moved utilizing mechanical stage control knobs to move the slide around
• Objective Lenses – primary source of magnification; (scanning, low, high, and oil immersion). The oil immersion objective lens requires the use of immersion oil, which prevents the loss of light due to refraction, thereby preserving resolution (level of clarity/detail observed). A drop of immersion oil is added to the slide before rotating to the immersion oil lens. The scanning, low, and high objective lenses are dry objective lenses, meaning they do not require the addition of oil to maintain good resolution. ***Note*** when using the microscope to view a specimen you begin with the lowest power objective lens and then work to higher powers
• Ocular Lens – lens located within the eyepiece
• Revolving Nose-piece – houses the objective lenses; rotates to select for the different objective lenses
• Stage Control Knobs – moves the mechanical stage or stage clips to move the specimen; these knobs allow you to locate items not in the field of view or center a specimen within the field of view

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