An attachment (54) for a slit lamp that permits the operator to view the superimposition of an image of a patient's eye and a second image that may comprise a fluorescein angiogram (56) or other photographic image of the patient's eye, or an image formed by a CRT or other display system. The slit lamp includes objective means (46) for forming the first image directed along a first optical path, and viewing means (48) comprising an eyepiece (50,52), the viewing means (48) being attachable to the objective means (46) such that the eyepiece (50,52) is positioned along the first optical path. The attachment (54) includes a body (60), a beamsplitter (62), and an image forming system (64,66,68,70). The body (60) includes means (98,104) for attaching the body (60) between the objective means (46) and viewing means (48), and also includes a passage (76) through which the first image can pass along the first optical path to the viewing means (48). The beamsplitter (62) is mounted in the body (60), and positioned in the first optical path. The image forming system (64,66,68,70) projects the second image into the beamsplitter (62), such that a portion of the second image is reflected along the first optical path. The image forming system (64,66,68,70) may include a zoom lens system (64,66) for adjusting the size of the second image. The zoom lens system (64,66) may comprise a pair of lenses and positioning means including an inner tube (82) having a pair of helical slots (126) and a pair of outer tubes (140,142) having a pair of longitudinal slots (144,146). Beamsplitter positioning means (110,112) are also provided for permitting the operator to precisely position the first and second images with respect to one another.