The Gauss eyepiece is an eyepiece in the tube of which, in correspondence to a hole in the side, a glass plate tilted 45 degrees with respect to the optical axis is placed.
A beam of light that enters through the hole strikes the glass plate perpendicularly to the axis and is reflected in the direction of the axis and, after striking the reticule, exits from the telescope through the lens.
When there are no objects quite far away, to focus the telescope at infinity it is possible to use the Gauss eyepiece by placing in front of the telescope and normal to its axis a flat mirror that reflects the beam of light that is projected through the hole in the eyepiece; at that point, on looking through the eyepiece we see the reticule and its image reflected by the mirror. If the telescope is focused on infinity the reticule and its image coincide when no parallax effect is noted for slight shifts of the eye in front of the eyepiece. Otherwise it is necessary to adjust the lens in the telescope tube so that the coincidence is perfect.
It is understandable that the Gauss eyepiece can also be used to measure the angle of refractivity of a prism with the method of the fixed telescope and the turning prism, without the need for a collimator. It is sufficient to place the prism with the corner normal to the telescope axis on the platform, keeping the graduated disk in a fixed position, and directing the beam of light from the auxiliary source placed in front of the hole in the side of the eyepiece first on one face and then on the other and measuring the angle by which it was rotated.
The Gauss eyepiece can also be used to position the normal axis of the telescope and the corner of the prism parallel to the axis of the instrument.
Bussetti (1967), p. 423
Persico (1932), p. 134