Visus illustratus; or, The sight rendered clear and distinct [electronic resource] : Being an enquiry or examination into the cause of the inefficacy or defect of the present mode of constructing spectacles for the relief of the presbytes, or weak-sighted eyes: and to illustrated the use and superior advantage of spectacles formed upon a new construction, for the remedy of such defect; for the making and vending of which His Majesty has been graciousy pleased to grant the author his royal letters patent; the whole being the result of experimental proofs, to elucidate defective vision in spectacles. By Addison Smith.