Using Eyeglasses to Correct Refractory Errors
The strength of reading specs was originally based on the patient’s age. Hence a man of about 30 would be given lenses with 2 degrees of strength and a person of about 70, given, roughly, 4 degrees. It wasn’t until the middle of the 19th century that eyeglasses started to be issued by people who knew some basics about optics.
There are still a few luddites who ignore the technological advances of the 21st century and get their reading specs from the local 7-11.
Astigmatic correction was not generally available until late in the nineteenth century, despite Airy’s cylindrical correcting lens (1827) and Donders’ comprehensive book on refraction (1864). A great many problems have been solved due to necessity. Bifocal lenses were actually invented by Benjamin Franklin. He needed some for himself and did a bit of a Heath Robinson Job, by cutting each of his lenses in half and sticking them together, so that he could see into the distance using the top half and read with the bottom half of his lenses.
In the early 1800s Hawkins tried to launch trifocal lenses to the spectacle wearing population. Smoothly increasing multifocal lenses, were then brought in about 100 years later. Sadly, these innovative lenses didn’t work well in practice as the range of clear vision was too small to make their use practical.
|If presbyopia is just emerging, then multifocal lenses may have a role, whilst the person learns to tolerate the increasing strengths.
You may find the history of spectacles interesting as I do. I would advise you to really study glasses and their beginnings to help you better appreciate what a wonderful device they truly are. If you are going to buy specs online, please ensure that you buy them from somebody who is sufficiently trained and interested in the subject and your eyes.
Mail this post
Tags: eyeglasses, specs, spectacles
March 31st, 2009 at 7:22 pm
[...] the distance. Squinting the eye will actually make the object clearer. Vision can be corrected with prescription eyeglasses with either a bifocal or trifocal [...]