Fig 002 Microscopes

Bright-field Microscopy

Bright-field microscopy is used with specimens stained with colored dyes.

A microscope magnifies the image of an object so smaller details can be seen by the eye. It forms a magnified image by focusing light through a series of glass lenses on a tissue section.

• Condenser lens - focuses light from the illumination source onto the specimen
• Objective lens - collects light that passes through the specimen, enlarges and projects an image onto the eyepiece
  • 4x, 10x, 20x, 40x, 60x, and 100x are widely used
  • Inverts the image of the specimen
• Eyepiece lens - further magnifies the image and projects it onto the retina
  • 10x eyepiece - most widely used
  • Inverts the image of the specimen

Total magnification is calculated by multiplying the magnifications of the objective lens and the eyepiece. With a 100x objective lens and a 10x eyepiece, the total useful magnification is 1,000x.

Because both the objective lens and the eye piece invert the image, the final image seen through the eyepiece has the same orientation as in the specimen.

The light path in a bright-field microscope is the opposite of what is shown here. The light source is beneath a stage that holds the specimen. Light passes upwards through the condenser, specimen, objective lens, and eyepiece.

Electron Microscopy

An electron microscope forms a magnified image of a specimen using electrons.

Transmission Electron Microscopy (TEM)

Ultrathin tissue sections (50 to 150 nm) are stained with heavy metals (such as osmium tetroxide, uranium or lead salts) to increase contrast.

TEM measures relative differences in transparency (i.e., contrast) of a specimen to electrons. The images are colorless and have a resolution of ~0.1 nm (50,000,000x, more than 1,000x better than a light microscope).

Scanning Electron Microscopy (SEM)

Specimens are coated with a thin film of metal (such as platinum).

SEM measures relative differences in the reflection of electrons to produce images of three-dimensional surfaces. The images are colorless and have a resolution of ~1.0 nm (1 to 2,000,000x).