O. Lummer and E. Gehrcke, exploiting the fact that in the passage from a more refracting means to one that is less refracting, if the incidence is close to the limit value, the reflecting coefficient is close to unity, invented and produced in 1903 the plate that bears their name, which is a multiple beam interferometer.
It consists of a long plate of glass (or quartz crystal) with perfectly smooth and parallel faces, in which the extremity transverse to the dimension of greater length is cut obliquely, or one of the faces has a supplementary prism on it. The slope of the face thus obtained is such that a parallel beam almost normally incident to it then strikes the two faces of the plate near the limit angle from the inside and is almost completely reflected.
As with any dispersive apparatus, the Lummer-Gehrcke interferometer plate is used in spectroscopy by placing it between a collimator and a telescope, obviously with the input face parallel to the slit of the collimator.
When a parallel beam of monochromatic light coming from the collimator strikes the plate in the conditions described above, with every reflection on its face only a small amount of light comes out and there is a succession of parallel beams having more or less the same intensity. ...continues