Research laboratory

Viewing instruments

Optical microscopeZoomZoom
© Catherine Vamos

Optical microscope

The specimen is deposited on a slide and can be covered with a cover-glass. An objective lens is chosen to focus the image using the macro and micrometric screws. A more powerful objective lens can then be used to increase the magnification. To obtain the total magnification, the magnification of the objective lens must be multiplied by the magnification of the ocular lens.

This microscope relies on a system of lenses capable of modifying the trajectory of the light beams traveling between the studied object and the eye of the observer. Most photon microscopes are capable of enlargements up to 1000X.

Electron microscopeZoomZoom
© Armand-Frappier Museum

Electron microscope

The sample is composed of a small round grid, 3 mm in diameter, on which a plastic membrane and some fine grains of carbon have been deposited. This base is used as a support for the specimen, which can be dried material or thin sections of cells.

The apparatus is powered at a voltage of between 25 and 125 kV and the tungsten filament is heated to more than 2200°C. This produces a beam of electrons controlled by a system of magnetic fields. High performance pumps create a vacuum inside the column. This vacuum must be between 10-4 and 10-7 TORR (1 mm Hg) to avoid collisions between the electrons and air molecules. The electrons are focused on the specimen using electromagnetic lenses.

Depending on the thickness, density, and chemical nature of the sample, the electrons are more or less absorbed and the electron beam produces a transparency image of the irradiated zone. This image is enlarged by the objective, intermediate, and projector lenses and reproduced on a phosphorescent screen, which gives a gray image on a green background.

"In electron microscopy, the lenses are made of metal; they are electromagnets capable of producing uniform magnetic fields of variable intensity depending on the strength of the exciting current*."

"In electron microscopy, the cathode rays emitted by the source (generally a tungsten filament heated to incandescence) are accelerated (about 80 kilovolts) and pass through the condenser, which regulates the intensity and the convergence of the rays illuminating the specimen. The objective lens then focuses the electron beam that has traveled through the object to give an enlarged intermediate image. Then the projector (the ocular in photonic microscopy) enlarges part of the intermediate image. Finally, a fluorescent screen (a zinc and cadmium sulfide alloy) allows the conversion of the invisible electron beam into a visible photonic image*."

*From Introduction à l’ultrastructure cellulaire by Paul-Émil Messier, Presses de l’Université de Montréal, 1971.

Parts of the electron microscope column

The main component of the microscope is the column that is divided into four parts:

1- The illumination system

  • a cathode (-) or electron canon containing a fine tungsten filament bent into a "V"
  • an anode (+)
  • one or a group of several lenses known as the "condenser"

2- The specimen chamber

3- The imaging system, composed of three series of lenses

  • the objective lens
  • the intermediate lens
  • the projector lens

4- The image-capture system

  • a phosphorescent or fluorescent screen
  • binoculars
  • standard film (35 mm, 70 mm, plates) camera
  • a television camera or a digital camera hooked up to a computer

Examples of techniques used

Thin sections: fixation, dehydration, coating in an epoxy resin, cutting into thin sections using a diamond knife, and staining (positive staining). Preparation time: 4 days before examination under the electron microscope.

Negative staining: deposition of liquid material on the grid, drying and impregnation with a solution of phosphotungstic acid for 1 minute. Preparation time: 15-30 minutes before examination under the microscope.