Thomas Young's exploration of the eye begins as a careful inquiry into how vision adapts to distance, but quickly widens into a sweeping study of optics, anatomy, and the mathematics of light. He revisits earlier theories of accommodation, argues that the crystalline lens changes shape, and sets out to explain vision by moving from general principles to exact calculations, experiments, and comparisons with other senses, especially hearing. The result is a rigorous portrait of the eye as both a biological organ and a finely engineered instrument. Across dense geometric optics, Young develops the laws of refraction, image formation, focal points, and spherical lenses, then applies them to a practical optometer for measuring the eye's focal distance and selecting lenses for myopia and presbyopia. He provides calibration tables, detailed instructions, and experimental methods for testing visual defects. The work then turns inward, measuring corneal curvature, pupil size, lens power, the blind spot, and even the author's own asymmetrical vision. Along the way, Young reveals the eye's brilliance, its hidden irregularities, and its subtle chromatic imperfections. This edition has been fully restored with modern typesetting, custom cover design.