A black line in the eye may be caused by an obstruction caused by an eyelash or eyebrow falling on the cornea in front of the eye, or it may be caused by the remnants of the original vitreous tube or by a condition such as fiducial mosquitoes. There are more reasons for patients to feel a black line in the eye. When there is a thin substance in front of the eye such as eyebrow or eyelash attached to the optical center of the frame glasses or the surface of the corneal center, patients can feel a black line in the eye, which is usually easy to be found through face-to-face or mirror examination. Due to the incomplete degeneration of the primitive vitreous tube which remains in the vitreous cavity, the patient can also feel the black line in front of the eyes when looking at something, but this situation is rare in clinic. In addition to this, patients with fiducial disease may experience dark shadows floating in front of their eyes due to vitreous clouding. It is recommended that patients who experience dark shadows in front of their eyes that interfere with their vision should seek prompt medical attention to evaluate their condition and follow the doctor’s instructions for treatment.