This is the first full-length study of spectacles in the Victorian period. It examines how the Victorians shaped our understanding of functional visual capacity and the concept of 20:20 vision. Demonstrating how this unique assistive device can connect the histories of medicine, technology and disability, it charts how technology has influenced our understanding of sensory perception, both through the diagnostic methods used to measure visual impairment and the utility of spectacles to ameliorate its effects. Taking a material culture approach, the book assesses how the design of spectacles thwarted ophthalmologists attempts to medicalise their distribution and use, as well as creating a mainstream marketable device on the high street.