In setting a surveyor's three-set screw level first see that the parallel plates are about parallel, and the screws just up to their work; set the legs open a convenient distance, and stand between two of them, with the left hand grasping the tripod head. Place the telescope across the direction of the leg at the right hand, and move the leg backward or forward to bring main bubble central. Then place the telescope in line with this leg, and move it in or out to bring the bubble again central. This is the leg adjustment common to all forms of level and theodolite, and should never be omitted. The fine adjustment for a three-screw level will then be as follows. Place the telescope parallel with two adjacent screws and bring the bubble central, by turning them " thumbs in " or " thumbs out," as the case may be. Then move the telescope round so that the object glass is central between these two screws, and the eyepiece over the third one, and adjust the third screw to bring the bubble central. The bubble ought now to remain in the centre of its run for any position of the telescope.

By mea s of a cross level on the end the leg may be set approximately true for both directions in one operation, but it is more useful on the four-screw instrument.