The latter are in fact little microscopes carrying a vernier etched on glass, in lieu of a filar micrometer.
A micrometer drum reads to 2', while the vernier reads to single minutes so that very fine adjustments can be made.
If we now attach to the polar axis a graduated circle D D, called the" hour circle,"of which the microscope or vernier R reads o h when the declination axis is horizontal, we can obviously read off the hour angle from the meridian of any star to which the telescope may be directed at the instant of observation.
The observer's eye is applied to the small telescope E, which (by means of prisms numbered I, 2, 3, 4) views the vernier attached to the cross-head simultaneously with the hour circle attached to the upper end of the polar axis.
A slight rotatory motion of the telescope E on its axis enables the vernier of the declination circle to be read through prism 1.
The polarizing prism is fixed at the centre of a circular disk, that has a scale on its circumference, which with a fixed vernier determines the positions of the polarizer, for which the bands disappear at the assigned point of the field.
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.
7) is given in Bontemps's Guide du vernier, pp. 602-612.