Microscopes, together with telescopes, belong to the category of optical instruments known as compound, since they are composed of two coaxial lenses (or lens systems): the objective and the eyepiece.
The objective, which is in all cases convergent, gives a real image of the object observed and the eyepiece gives a virtual image of same in the most suitable position for observation.
In microscopes the objective, which has a very short focal length, and the eyepiece, which is also convergent, are at the extremities of a brass tube at a distance that is much greater than the sum of their focal lengths.
As is well known, the objective gives a real image, upside down and enlarged, of an object placed in front of its first focus; it falls just behind the first focus of the eyepiece, which gives a virtual image, right side up and enlarged.
In the one presented here, as in almost all microscopes, the slide holder is fixed and the observer, in order to have the image provided by the eyepiece form at the distance at which his or her eye is placed, he or she can bring the objective closer to the object by making the tube run inside a brass cylinder which holds it up to about half its total length. Between the objective and the real image it supplies, there is a third converging lens, known as the field lens which, with the eyepiece lens, constitutes what is known as the Campani eyepiece, invented by Giuseppe Campani (1636-1715). This system, with the same enlargement with respect to a microscope having the same objective and the same eyepiece lens but without the field lens, allows the doubling of the field and a great reduction of the chromatic aberration.