An accessory optic is described which enables a non-immersed, long working distance microscope objective to be used as a higher-power immersion objective. This is particularly useful for biological imaging of specimens in an index-matching clarification medium. The accessory approach is the opposite of conventional unitized immersion objective design and presents several advantages over conventional design. The front lens element of the accessory may be selected to have precisely the refractive index of the specimen immersion medium and the accessory may be designed for use with immersion media that are incompatible with conventional immersion objectives. A dual accessory enables two such modified objectives to be optimized for light-sheet microscopy.