A practical manual explains the history and invention of spectacles and basic optical principles, describes the physiology of the eye and common visual defects, and gives clear guidance on selecting and testing lenses, including how to determine focal lengths. It exposes deceptions practiced by itinerant and dishonest opticians and warns against gimmicks and false claims, offering rules for avoiding fraud. The author condenses technical knowledge into plain language, supplements advice with examples and correspondences with medical practitioners, and emphasizes empirical, scientifically based remedies to help readers make informed purchases and protect themselves from quackery.