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Updated: April 30, 2025
*Summary.*—The nervous impulses that cause the sensation of sight are started by light waves falling upon a sensitized nervous surface, called the retina. By means of refractive agents, forming a part of the eyeball in front of the retina, light from different objects is focused and made to form images of the objects upon the surface.
Now the refractive index of Iceland spar is, for the extraordinary ray less, and for the ordinary greater, than for Canada balsam.
Faraday, you have been informed, endeavoured to improve the manufacture of glass for optical purposes. But though he produced a heavy glass of great refractive power, its value to optics did not repay him for the pains and labour bestowed on it. Now, however, we reach a result established by means of this same heavy glass, which made ample amends for all.
"Having made these observations, I first computed from them the refractive power of the glass, and found it measured by the ratio of the sines 20 to 31.
This is a rude contrivance compared with the iris, which by means of its muscular fibers can in a moment alter the size of the pupil, thus serving a similar purpose. The Refractive Media of the Eye. The eye is a closed chamber into which no light can pass but through the cornea.
We must get those books again. But the essential phase was to place the transparent object whose refractive index was to be lowered between two radiating centres of a sort of ethereal vibration, of which I will tell you more fully later. No, not those Roentgen vibrations I don't know that these others of mine have been described. Yet they are obvious enough.
We offer you, in the name of the people, your choice of any spot in this world. And we give you this!" The Scientist came forward. He had a disc-shaped plaque, perhaps three inches in diameter, made of a deep ruby-red metal. In the exact center was a green stone which seemed to shine of its own accord, with a pale, clear, green light; it was transparent and highly refractive.
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