Theodolite Quotes

In order to receive in the eye all the light diffracted through a narrow opening, and to see the phenomena strongly magnified; still more in order to directly measure the inflection of the light, I placed in front of the objective of a theodolite-telescope a screen in which there was a narrow vertical opening which could be made wider or narrower by means of a screw. By means of a heliostat I threw sunlight into a darkened room through a narrow slit so that it fell upon this screen, through whose opening the light was therefore diffracted. I could then observe through the telescope the phenomena produced by the diffraction, magnified, and yet seen with sufficient brightness; and at the same time I could measure the angles of inflection of the light by means of the theodolite. (Joseph von Fraunhofer)
Since the violet rays through the objective of the theodolite telescope have a shorter focal length than the red rays, it is evident why the eye-piece must be displaced in order to see plainly the lines in the different colors. View quote
Joseph von Fraunhofer