Possibly a scratch on the lens element. 

The Leitz Wetzlar 5x/0.09 Microscope Objective Lens is a high-quality scientific instrument manufactured in Germany. This lens is designed for microscopy and offers a magnification of 5x with a numerical aperture of 0.09, providing clear and detailed images of samples being studied. Its precision engineering and craftsmanship make it a valuable tool for researchers and professionals in the field of science and medicine.