Though antique frames reveal much of the maker, and can give you a window onto the skilled hand manufacturing techniques of a bygone age as well, you should not neglect the lenses when you are scrutinizing an old pair of spectacles, whether they are from the age of the Robber Barons, the Civil War Era, or even the Colonial period.
Eyeglasses were not manufactured with the same scale of power as is used today. The change occurred in 1876, when the modern diopter was introduced as a measure of optical power. Prior to that date, the pouce, the inch, or another measure was used instead. Although it requires some technical expertise, you can check the lenses contained in antique frames with a modern lensometer.
Those made prior to 1876 will not show whole number readings in diopters – except at the points where the systems overlap, at the whole numbers themselves. In other words, a 3 diopter pair of lenses could have been made before or after 1876, but fractional diopters (½, 2 ¾, etc.) were made in 1875 or earlier. Though this is a method of identification that lacks much granularity – in effect, it is no more than a “yes/no” answer to the question “was this lens ground after 1876?” – it is still another tool available to you for narrowing down date of production somewhat.
It is also invaluable for determining if someone put modern lenses into antique frames, thus ruining their value.
Dating the bifocals found in antique frames
Though they are a fairly rare breed, antique bifocals do exist, and in fact are more likely to appear in early American antique frames than in spectacles from Europe. This is because, unlike many other useful inventions – including eyeglasses themselves, which were made in the late 13th century by an Italian artisan from Pisa who ironically remains unknown because he wished to keep the invention entirely for himself – we know exactly who first made them, even if the precise date has been lost.
Benjamin Franklin is the first known creator of bifocals, and thus, for decades after their invention, they remained largely confined to New World spectacles. The eccentric inventor and politician produced bifocals by grinding two pairs of lenses, one considerably smaller than the other, and fusing the smaller lenses to the larger ones to produce a bifocal effect.
The traditional date for this is in the 1780s, shortly before his death, but Franklin's letters actually suggest he fashioned the first bifocals in the 1730s, when he was still a young man with a rather inordinate fondness for ladies.
These early bifocals were made in the same manner until the late 1840s or early 1850s, when German lens grinders fleeing the Revolutions of 1848 came to the United States. These expert technicians found the two-piece American bifocal lenses in the local antique frames and, seeing the potential of the idea, set about improving it. They possessed the knowledge and more advanced technology needed to grind two different focuses into a single piece of glass or rock crystal, producing the type of bifocal lens that is still made today.
Besides being interesting facts in the history of the American lenses fitted into antique frames, these pieces of information can help you determine the provenance of a pair of old bifocal spectacles you are examining. Regardless of whether you can tell where a pair of antique frames were fashioned, old bifocal lenses are very likely to be American, and are certainly no older than the 1730s.
Those with separate, smaller lenses fused onto the lower quadrant of larger spectacle lenses were made between 1730 and 1850, while those that include two focuses ground into the same piece of glass or crystal were made after 1848. This is just one of the many intriguing secrets that may be found within the lenses contained in antique frames by a careful observer.