A display device is provided for generating an image which is perceivable, by a viewer (M) of an object located in an object plane (O), as being superimposed on the object, said device comprising an image generating device (1) for generating the image in an image plane (B) and a superimposing unit (7), wherein the image generating device (1) and the superimposing unit (7) are mounted on a support device (12), which is placeable on the head of the viewer (M), and the image generating device (1) comprises a focusing unit (3) by which the distance from the image plane (B) to the support device (12) is changeable and which includes a lens (4) having a variable refractive index as well as a control unit (6) for adjusting the refractive index of the lens (4), wherein, when the support device (12) is placed on the head of the viewer (M), the superimposing unit (7) effects superposition of the generated image on the object for the viewer (M) and the control unit (6) adjusts the refractive index of the lens (4) such that the object plane (O) coincides with the image plane (B).