The study examined the development of ophthalmology from ancient times to the 19th century and its role in the diagnosis of systemic diseases. In ancient Sumer in the 3rd millennium BC, people interpreted changes in the appearance of the eyes as signs of demonic possession of internal organs. Between 500 BC and 200 BC, the Greek Hippocratic Corpus, the Chinese Huang Di Nei Jing, and the Indian Ayurvedic writings replaced supernatural explanations with rational, if imprecise, models of disease. These frameworks enabled the advancement of empirical thought and led to later discoveries such as Ibn al-Haytham's knowledge of vision, Galvani's discovery of electrical conduction in nerves, and Harvey's demonstration of a closed circulatory system. The invention of the ophthalmoscope by Helmholtz in 1851 marked a decisive breakthrough, allowing doctors to directly image the retina and optic nerve in living patients for the first time. With the help of ophthalmoscopy, the eye was transformed into a real diagnostic window for systemic diseases, and thus the modern role of the eye in assessing the health of the whole organism was firmly established.