Rectangular or oblong lenses are before 1800 to 1890's with octagonal appearing by 1840 to 1890's as well. Round Lens shapes where from before 1760 up to 1820 . These round lenses never came back until about 1910. Bridges between these styles of lenses may differ as well. Nose pads did not become common until after 1900 with an exception of the Spring pad that began about 1850.
Temples are the clue to the age of the spectacles in most cases.
Pre 1850 are the short and straight temples and the hinged at center style. The connecting hinges where the temple connects to the lens frames will be bulkier in early styles. Most spectacles made before 1870 were hand crafted and numbers may be found stamped on the edges near the hinge that give the salesman a clue to what eye problem the individual may have. Some lenses may even be etched with a number as well. Colored glass was used and usually for special reasons.