The Specimen.

Specimens for the light microscope are traditionally prepared on glass slides measuring 3 x 1 inches (25mm x 75mm), and covered with a piece of thin glass of 0.17mm thickness called a coverglass. The slides are most commonly 1.0 to 1.2mm in thickness, and thinner slides at 0.8 to 1.0mm are also available. With the specimen placed on the stage of the microscope, these two thicknesses of glass become an important part of the microscope illumination and imaging system.
The thickness of the slide goes a small way toward correcting the large spherical errors of the Abbe condenser, and the thickness of the coverglass is essential to the spherical correction of the objective, especially those of x20 and higher powers. The x40 objective in particular will give very poor images if used with an uncovered specimen. A more detailed account of spherical aberration in microscope image formation and the means of correcting it is given in the advanced tutorial.

Quite apart from optical considerations, a coverglass is essential for examining specimens in water and other liquids to reduce sensitivity to vibration and to prevent contact with liquid or condensation of vapour on the frontlens of the objective. The thickness of the slide is an important consideration also when the substage condenser is immersed in oil for use with the oil immersion objective (a thicker slide requires less oil), and when the microscope is fitted with an achromatic condenser (see advanced tutorial).

Now that the microscope is correctly set up, a slight detour into the theory of microscope image formation is necessary to clarify terms before we proceed to apply the same principles to the use of the oil immersion objective.