A new Parallel Plate Comparator

Abstract

The "Parallel Plate Comparator" is best adapted for measuring the separation of two parallel lines (eg. spectral lines) whose structures are practically identical and which are on the same negative and only a few millimeters apart. It consists of two plane parallel plates of glass attached to a mechanism such that they can be set at equal and opposite angles with the plane of the negative. The two equal and opposite angles between the plates and the negative must lie in a plane normal to the lines whose separation is to be measured. The lines are viewed through the inclined glass plates with a long focus microscope having a sufficiently large field to include both plates and both lines. The lines are brought into apparent coincidence in the field of the microscope by varying the obliquity of the glass plates to the line of sight. This obliquity is then a measure of the separation of the lines. If the lines are of identical structure and intensity, even though they are not symmetrical in structure, much greater precision is possible by bringing them into apparent coincidence than by attempting to set a crosshair first on one line and and then on the other. (The lines should be photographed or blocked off so that the upper half of one and the lower half of the other only is visible. The slightest fault in the apparent coincidence of the ends of the two half lines is then glaringly evident through the microscope. When coincidence is obtained, the two half lines give a good photometric match across their juncture and appear as one. In a properly designed instrument, the crack separating the two plane parallel glass plates is invisible because it is not in the focal plan of the objective.)