The apparatus shows the paths of light rays in a terrestrial telescope.
From the ends of the object viewed, represented by an arrow placed on the bottom of the wooden board, depart coloured threads which simulate light rays, reaching the opposite side of the board where they enter the eye. The lenses are represented by the transparent discs mounted in black wooden circles; the holes made in the discs guide the threads along their paths from the objective to the eyepiece through the lenses; the first disc in front of the arrow represents the objective, and the eyepiece is the last one in front of the eye.
The paths of the light rays show that the objective creates an inverted and reduced image of the object viewed, just as occurs in a telescope. In order to right the image it is necessary to add a system of two lenses of suitable focal lengths, between which the light rays cross. The image received by the eyepiece is thus upright and enlarged.

Instrument on display.