An essay on visual glasses : (Vulgarly called spectacles) wherein it is shewn, from the principles of optics, and the nature of the eye, that the common structure of those glasses is contrary to the rules of art, to the nature of things, &c. and very prejudicial to the eyes; the nature of vision in the eye explained, and glasses of a new construction proposed. The whole illustrated by a large copper-plate print. By B. Martin.