Bifocals are eyeglasses

Glasses

Glasses, also known as eyeglasses , spectacles or simply specs , are frames bearing lenses worn in front of the eyes. They are normally used for vision correction or eye protection. Safety glasses are a kind of eye protection against flying debris or against visible and near visible light or...

 with two distinct optical power

Optical power

Optical power is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the reciprocal of the focal length of the device. The dioptre is the most common unit of measurement of optical power...

s. Bifocals are most commonly prescribed to people with presbyopia

Presbyopia

Presbyopia is a condition where the eye exhibits a progressively diminished ability to focus on near objects with age. Presbyopia’s exact mechanisms are not known with certainty; the research evidence most strongly supports a loss of elasticity of the crystalline lens, although changes in the...

 who also require a correction for myopia

Myopia

Myopia , "shortsightedness" ) is a refractive defect of the eye in which collimated light produces image focus in front of the retina under conditions of accommodation. In simpler terms, myopia is a condition of the eye where the light that comes in does not directly focus on the retina but in...

, hyperopia

Hyperopia

Hyperopia, also known as farsightedness, longsightedness or hypermetropia, is a defect of vision caused by an imperfection in the eye , causing difficulty focusing on near objects, and in extreme cases causing a sufferer to be unable to focus on objects at any distance...

, and/or astigmatism

Astigmatism (eye)

Astigmatism is an optical defect in which vision is blurred due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina. This may be due to an irregular or toric curvature of the cornea or lens. There are two types of astigmatism: regular and...

.
Benjamin Franklin

Benjamin Franklin

Dr. Benjamin Franklin was one of the Founding Fathers of the United States. A noted polymath, Franklin was a leading author, printer, political theorist, politician, postmaster, scientist, musician, inventor, satirist, civic activist, statesman, and diplomat...

 is generally credited with the invention of bifocals. Serious historians have, from time to time, produced evidence to suggest that others may have preceded him in the invention; however, a correspondence between George Whatley

George Whatley

George Whatley, Esq, was a contemporary, friend and correspondent of Benjamin Franklin. He was also Vice-President and Treasurer of the Foundling Hospital in London....

 and John Fenno

John Fenno

John Fenno , was a Federalist Party editor and major figure in the history of American newspapers. His Gazette of the United States played a major role in shaping the beginnings of party politics in the United States in the 1790s.-Biography:Fenno was born in Boston, the son of Ephraim 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.
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.