As I suspected, the gist of the joke is contained within the definition of the subject matter. I had to look it up but I’ll save you the trouble: (Wikipedia) Diplopia is the simultaneous perception of two images of a single object that may be displaced horizontally, vertically, diagonally (i.e., both vertically and horizontally), or rotationally in relation to each other.
As I suspected, the gist of the joke is contained within the definition of the subject matter. I had to look it up but I’ll save you the trouble: (Wikipedia) Diplopia is the simultaneous perception of two images of a single object that may be displaced horizontally, vertically, diagonally (i.e., both vertically and horizontally), or rotationally in relation to each other.