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Enough has now, perhaps, been said about the respective properties of object glasses and mirrors, but a word should be added concerning eyepieces. Without a good eyepiece the best telescope will not perform well. The simplest of all eyepieces is a single double-convex lens.
These remarks have been introduced here as being important for those who may not understand the principles of enlarging or reducing pictures in copying. If we hold a double-convex lens opposite any object, we find that an inverted image of that object will be formed on a paper held behind it.
A term applied to vessels and nerves which encircle parts, as the coronary arteries of the heart. Like a crow's beak; thus the coronoid process of the ulna. A cartilage of the larynx resembling a seal ring in shape. One of the humors of the eye; a double-convex body situated in the front part of the eyeball. Cumulative.
William Hyde Wollaston, one of the greatest and most versatile, and, since the death of Cavendish, by far the most eccentric of English natural philosophers. This was the suggestion to use two plano-convex lenses, placed at a prescribed distance apart, in lieu of the single double-convex lens generally used.
"At the earliest period of the employment of the camera obscura, a double-convex lens was used to produce the image; but this form was soon abandoned, on account of the spherical aberration so caused. Lenses for the photographic camera are now always ground of a concavo-convex form, or meniscus, which corresponds more nearly to the accompanying diagram."
Now, as has been said before, the dissimilar rays having an unequal degree of refrangibility, it will be impossible to obtain a focus by the light passing through a double-convex lens without its being fringed with color. Its effect will be readily understood by reference to the accompanying cut.
The old-fashioned double-convex lens used in telescopes became so heavy as its size grew, that it bent perceptibly from its own weight, when pointed at the zenith, distorting the vision; while when it was used upon a star near the horizon, though the glass on edge kept its shape, there was too much atmosphere between it and the observed object for successful study.
The reader may readily comprehend the phenomena of refraction, by means of light passing through lenses of different curves, by reference to the following diagrams: By these it is seen that a double-convex lens tends to condense the rays of light to a focus, a double-concave to scatter them, and a concavo-convex combines both powers.
Some are arranged in a ring round the margin of the pupil; others radiate from it like the spokes of a wheel. When the circular fibers contract, the pupil is made smaller, but if these fibers relax, the radiating fibers cause the pupil to dilate more or less widely. The Crystalline Lens. Just behind the pupil and close to the iris is a semi-solid, double-convex body, called the crystalline lens.
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