an operatory chair , such as a dental chair , having an upper supporting member to which the seat and back unit of an operatory chair may be attached , particularly pertaining to the mechanism by which said supporting member for the seat and back is raised and lowered . the preferred construction comprises a pair of sets of pivotally connected links arranged in lazy tong manner , said links being operated by linkage mechanism actuated by a rotatable screw and follower in such manner that the movement of the supporting member for the seat and back is smooth and relatively slow , particularly at the commencement of elevating movement so as not to include sudden and rapid upward movement as is conventional with typical lazy tong structures used in jacks and the like .