About the Model
The main model was made out of copper bands and wire taken from salvaged computer parts and clothes hangers. And overall it serves as an ample example of the original design of the bifocal first conceived by Franklin. It was wrapped with an insane amount of electrical as the solder that was holding it fell apart. The dimensions of the model are one and three twentieths larger than a small pair of bifocals. The measurements were all done within millimeters to preserve accuracy and were almost completely replicated in the fabricated model. The slight variations lie in the added size of the tape.
Calculations and Conics
A conic is considered to be a section of a double cone, essentially two inverted cones touching tips at a point. The lens within a pair of eyeglasses can be considered a type of conic section as it intersects the cones of light hitting the eye to refract and redirect them.The type of conic section is specifically the parabola, which has a focus point around which the parabola curves till it reaches through the vertex and arches out at a gentle angle.
The curves of the lenses act as the actual form of the parabola and force the refraction of light to correct the focus point of light in the eye.The actual focus point of the conic sections vary depending on the type of lens. The concave lens has a focal point outside of the glass and a vertex at the center and thinnest point in the glass. The convex lens has a focal point at the center inside of the glass and a vertex at its thickest point. There are two parabolas within each lens, one for each curved and refracting face.
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The cornea acts a second lens that refracts incoming light to project the image into the retina at the back of the eye. People with farsighted vision see only objects that are far away, while people who are nearsighted only see objects clearly that are close to the eye.
(Above)This is a version of farsighted vision with a Convex lens. The conic
is not visible as the focus point is behind the eye.(Epstein, 1983)
is not visible as the focus point is behind the eye.(Epstein, 1983)
Corrective lenses are used to move the focus point of light going through the cornea either from a virtual point behind the eye or from a point in front of the retina inside the eye. A pair of Bifocals acts on both of these complications of eyesight by having the lens in them separated between convex and concave. In this way a person may look down and read through their convex bottom half lenses and look up through their concave top half lenses to look at a painting hanging on the wall.
Scaling
Three pairs of true bifocals were broken down into segmented parts, whose dementions were averaged. The averages were then scaled up on a scale of 1:1.15 mm. The resulting sizes are illustrated in the table, and represent a pair of bifocals just a bit larger than a small pair of bottle bottom glasses.