Friday, June 18, 2010

Myopia

Section 14.12 in the 4th edition of Intermediate Physics for Medicine and Biology discusses the physics of the eye. One topic related to vision that I have always found fascinating is myopia:
“In nearsightedness or myopia, parallel rays come to a focus in front of the retina. The eye is slightly too long for the shape of the cornea […] The total converging power of the eye is too great, and the relaxed eye focuses at some closer distance, from which the rays are diverging. Accommodation can only increase the converging power of the eye, not decrease it, so the unassisted myopic eye cannot focus on distant objects. Myopia can be corrected by placing a diverging spectacle or contact lens in front of the eye, so that incoming parallel rays are diverging when the strike the cornea.”
The interesting thing about myopia is that, in contrast to far-sightedness (hypermetropy), you cannot correct it by accommodation. Before the invention of eye glasses in the late Middle Ages, if you were born with myopia then distant objects would always be a blur.

When teaching Biological Physics (PHY 325) at Oakland University, I often end my discussion of myopia with a quote from David McCullough’s wonderful biography of Theodore Roosevelt, Mornings on Horseback. Roosevelt suffered from myopia and didn’t get his first glasses until he was a teenager. McCullough tells the story:
“Then, at a stroke, the summer of 1872, he was given a gun and a large pair of spectacles and nothing had prepared him for the shock, for the infinite difference in his entire perception of the world about him or his place in it.

The gun was a gift from Papa—a 12-gauge, double-barreled French-made (Lefaucheux) shotgun with a lot of kick and of such simple, rugged design that it could be hammered open with a brick if need be, the ideal gun for an awkward, frequently absent-minded thirteen-year-old. But blasting away with it in the woods near Dobbs Ferry he found he had trouble hitting anything. More puzzling, his friends were constantly shooting at things he never even saw. This and the fact that they could also read words on billboards that he could barely see, he reported to his father, and it was thus, at summer’s end, that the spectacles were obtained.

They transformed everything. They ‘literally opened an entirely new world to me,’ he wrote years afterward, remembering the moment. His range of vision until then had been about thirty feet, perhaps less. Everything beyond was a blur. Yet neither he nor the family had sensed how handicapped he was. ‘I had no idea how beautiful the world was…I could not see, and yet was wholly ignorant that I was not seeing.’

How such a condition could possibly have gone undetected for so long is something of a mystery, but once discovered it did much to explain his awkwardness and the characteristic detached look he had, those large blue eyes ‘not looking at anything present.’ "
I am a lover of history, and a big fan of David McCullough. A couple of his books with a scientific or engineering bent are Path Between the Seas: The Creations of the Panama Canal and The Great Bridge: The Epic Story of the Building of the Brooklyn Bridge. His purely historical books, such as 1776 and John Adams, are also excellent.

To learn more, see the information about myopia on the website for the American Optometric Association. An modern option for correcting myopia that was not available in Roosevelt’s time is laser surgery to reshape the cornea.

2 comments: