The latter are in fact little microscopes carrying a vernier etched on glass, in lieu of a filar micrometer.
The light is finally received in a Galilean telescope, containing an analyser and carried at the centre of a circular plate, that is graduated on its rim and can be turned in front of a vernier by means of a rack and pinion.
When the plates are of equal thickness, their combined effect is nil, but by adjusting the second, a rotation in the one or the other direction may be introduced, a scale attached to one prism and a vernier to the other giving the thickness of the resultant quartz plate.
A micrometer drum reads to 2', while the vernier reads to single minutes so that very fine adjustments can be made.
A slight rotatory motion of the telescope E on its axis enables the vernier of the declination circle to be read through prism 1.