Abstract: The invention contemplates a multiple-element microscope objective assembly which is removably mountable to a microscope body and which includes a front element adapted for immersion contact with a liquid medium through which a microscopic object is to be observed. The invention features an externally accessible control adjustment for selectively axially positioning one of the optical elements of the objective with respect to the rest of the assembly such that a corrective optical adjustment is made for such change in refractive index as may characterize the use of one vs. another liquid-immersion substance, thereby enabling the microscope to preserve its normal focusing action consistently for use of the objective with a plurality of different immersion substances having different refractive indices.