Bifocals

[박대선]

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Bifocals with separate lenses

Bifocals are eyeglasses with two distinct optical powers. Bifocals are commonly prescribed to people with presbyopia who also require a correction for myopia, hyperopia, and/or astigmatism.

Contents

 [hide] 

• 1 History
• 2 Problems
• 3 Future
• 4 Bifocals in the animal world
• 5 References
• 6 Sources
• 7 External links

History[edit]

Benjamin Franklin is generally credited with the invention of bifocals. Historians have produced some evidence to suggest that others may have come before him in the invention; however, a correspondence between George Whatley and John Fenno, editor of The Gazette of the United States, suggested that Franklin had indeed invented bifocals, and perhaps 50 years earlier than had been originally thought.^[1] Since many inventions are developed independently by more than one person, it is possible that the invention of bifocals may have been such a case. Nonetheless, Benjamin Franklin was among the first to wear bifocal lenses, and Franklin's letters of correspondence suggest that he invented them independently, regardless of whether he was the first to invent them.^[1]

John Isaac Hawkins, the inventor of trifocal lenses, coined the term bifocals in 1824 and credited Dr. Franklin.

In 1955, Irving Rips of Younger Optics created the first seamless or "invisible" bifocal, a precursor to all progressive lenses.^[2]

Construction

Original bifocals were designed with the most convex lenses (for close viewing) in the lower half of the frame and the least convex lenses on the upper. Up until the beginning of the 20th century two separate lenses were cut in half and combined together in the rim of the frame. The mounting of two half lenses into a single frame led to a number of early complications and rendered such spectacles quite fragile. A method for fusing the sections of the lenses together was developed by Louis de Wecker at the end of the 19th century and patented by Dr. John L. Borsch, Jr. in 1908. Today most bifocals are created by molding a reading segment into a primary lens and are available with the reading segments in a variety of shapes and sizes.The most popular is the D-segment, 28 mm wide[citation needed]. While the D-segment bifocal offers superior optics, an increasing number of people opt for progressive bifocal lenses.

Problems[edit]

Bifocals can cause headaches and even dizziness in some users. Acclimation to the small field of view offered by the reading segment of bifocals can take some time, as the user learns to move either the head or the reading material rather than the eyes. Computer monitors are generally placed directly in front of users and can lead to muscle fatigue due to the unusual straight and constant movement of the head. This trouble is mitigated by the use of trifocal lenses or by the use of monofocal lenses for computer users.

In an interesting legal case reported in the UK in 1969, plaintiff's ability to use bifocals was impaired by accident.^[3]

Future[edit]

Research continues in an attempt to eliminate the limited field of vision in current bifocals. New materials and technologies may provide a method which can selectively adjust the optical power of a lens. Researchers have constructed such a lens using a liquid crystal layer sandwiched between two glass substrates.

Bifocals in the animal world[edit]

The aquatic larval stage of the diving beetle Thermonectus marmoratus has, in its principal eyes, two retinas and two distinct focal planes that are substantially separated (in the manner of bifocals) to switch their vision from up-close to distance, for easy and efficient capture of their prey, mostly mosquito larvae. This is the first ever recorded use of bifocal technology in the animal world.^[4]

References[edit]

1. ^ Jump up to: ^{a b} The College of Optometrists. "The 'Inventor' of Bifocals?". 
2. Jump up ^ http://www.visionmonday.com/Default.aspx?TabId=211&Parent_content_id=11079&content_id=11095&SkinSrc=%5BL%5DSkins/VM2007/PagePrint&ContainerSrc=%5BL%5DContainers/VM2007/SimpleContainer&dnnprintmode=true
3. Jump up ^ Agarwal, R.K. (1984), Plaintiff's ability to use bifocals impaired by accident, The Ophthalmic Optician, 24 (25), page 898 (the title of this journal was changed to Optometry Today in 1985, published by the Association of Optometrists, London, England).
4. Jump up ^ Dawn Fuller (duly edited) (24 August 2010). "Bug With Bifocals Baffles Biologists". ScienceDaily. ScienceDaily LLC. Archived from the original on 25 August 2010. Retrieved 25 August 2010. 

Sources[edit]

• Letocha, Charles E., M.D. (1990). "The Invention and Early Manufacture of Bifocals". Survey of Ophthalmology 35 (3): 226–235. doi:10.1016/0039-6257(90)90092-A. PMID 2274850.  CS1 maint: Multiple names: authors list (link)
• G. Li; et al. (April 2006). "Switchable electro-optic diffractive lens with high efficiency for ophthalmic applications". Proc. Natl. Acad. Sci. USA 103 (16): 6100–6104. doi:10.1073/pnas.0600850103. PMC 1458838. PMID 16597675. 

External links[edit]

• Franklin's letters to Whatley concerning double spectacles.

[hide] v t e Benjamin Franklin
January 6, 1706 – April 17, 1790 President of Pennsylvania (1785–1788), Ambassador to France (1779–1785) Second Continental Congress (1775–1776)
Founding of the United States	'Join, or Die' (1754 political cartoon) Albany Plan of Union Albany Congress Hutchinson Letters Affair Committee of Secret Correspondence Committee of Five Declaration of Independence Model Treaty Franco-American alliance Treaty of Amity and Commerce Treaty of Alliance Staten Island Peace Conference Treaty of Paris, 1783 Delegate, 1787 Constitutional Convention Pennsylvania Provincial Assembly Postmaster General Founding Fathers
Inventions, other events	Franklin's electrostatic machine Bifocals Franklin stove Glass armonica Gulf Stream exploration, naming, and chart Lightning rod Kite experiment Pay it forward Associators 111th Infantry Regiment Junto club American Philosophical Society Library Company of Philadelphia Pennsylvania Hospital The Academy and College of Philadelphia University of Pennsylvania Philadelphia Contributionship Union Fire Company Early American currency United States Postal Service President, Pennsylvania Abolition Society Master, Les Neuf Sœurs Other social contributions and studies Gravesite
Writings	Silence Dogood letters (1722) A Dissertation on Liberty and Necessity, Pleasure and Pain (1725) Pennsylvania Gazette (1729–1790) Poor Richard's Almanack (1732–1758) "Advice to a Friend on Choosing a Mistress" (1745) "The Speech of Polly Baker" (1747) Observations Concerning the Increase of Mankind, Peopling of Countries, etc. (1751) Experiments and Observations on Electricity (1751) Birch letters (1755) The Way to Wealth (1758) Pennsylvania Chronicle (1767) Rules by Which a Great Empire May Be Reduced to a Small One (1773) Proposed alliance with the Iroquois (1775) A Letter To A Royal Academy (1781) Remarks Concerning the Savages of North America (1784) The Morals of Chess (1786) An Address to the Public (1789) A Plan for Improving the Condition of the Free Blacks (1789) The Autobiography of Benjamin Franklin (1771–90, pub. 1791) Bagatelles and Satires (pub. 1845) Franklin as a journalist
Legacy	Franklin Court Benjamin Franklin House Benjamin Franklin Institute of Technology Benjamin Franklin National Memorial Franklin Institute Benjamin Franklin Medal Depicted in The Apotheosis of Washington Benjamin Franklin statue, Washington D.C. In popular culture Ben and Me (1953 short) Ben Franklin in Paris (1964 musical play) 1776 (1969 musical 1972 film) Benjamin Franklin (1974 miniseries) Liberty! (1997 documentary series) Liberty's Kids (2002 animated series) Benjamin Franklin (2002 documentary series) John Adams (2008 miniseries) Sons of Liberty (2015 miniseries) Sons of Ben (supporters group for the Philadelphia Union soccer club Franklin half dollar One-hundred dollar bill Washington-Franklin stamps other stamps Cities, counties, schools named for Franklin Franklin Field State of Franklin Ships named USS Franklin Ben Franklin effect
Related	Age of Enlightenment American Enlightenment The New-England Courant The American Museum magazine American Revolution patriots Syng inkstand
Family	Deborah Read (wife) Sarah Franklin Bache (daughter) Francis Franklin (son) William Franklin (son) Richard Bache Jr. (grandson) Benjamin F. Bache (grandson) Louis F. Bache (grandson) William Franklin (grandson) Andrew Harwood (great-grandson) Alexander Bache (great-grandson) Josiah Franklin (father) Jane Mecom (sister) James Franklin (brother) Mary Morrill (grandmother) Peter Foulger (grandfather) Richard Bache (son-in-law) Ann Smith Franklin (sister-in-law)

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• Corrective lenses
• Inventions by Benjamin Franklin
• American inventions