Lens reversal technique, a very popular close-up technique in the good old days.

The effectiveness of this technique can be proved easily: if a lens of focal length X (in mm) is reversely mounted on a lens (on camera) of focal length Y, the combo will have a magnification of Y/X. Thus, if a 50mm (resp., 25mm) lens is reversely mounted on a 200mm (on camera) lens, the combo will yield a magnification of 4X = 200/50 (resp., 8X = 200/25). This is the easiest way to increase magnification.