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Updated: June 9, 2025
If in looking he makes a mistake of half the thickness of one of these threads, the observation is good for nothing; judge what his uneasiness must be; at the critical moment, a puff of wind occasioning a vibration in the artificial light adapted to his telescope, the threads become almost invisible; the star itself, whose rays reach the eye through atmospheric strata of various density, temperature, and refrangibility, will appear to oscillate so much as to render the true position of it almost unassignable; at the very moment when extremely good definition of the object becomes indispensable to insure correctness of measures, all becomes confused, either because the eye-piece gets steamed with vapour, or that the vicinity of the very cold metal occasions an abundant secretion of tears in the eye applied to the telescope; the poor observer is then exposed to the alternative of abandoning to some other more fortunate person than himself, the ascertaining a phenomenon that will not recur during his lifetime, or introducing into the science results of problematical correctness.
Of course I kept my eye glued almost uninterruptedly to the eye-piece of the instrument, merely withdrawing it for a minute or so occasionally to give the visual organ a rest.
To avoid that, I put the little mirror in the middle, near the top, just at the right slant, so that the rays are turned off at right angles into the eye-piece, and so we are able to look without interrupting the light." "Oh, I see now," said Tom thoughtfully. "It's very clear." "Yes," said Uncle Richard. "Sir Isaac Newton, who contrived that way, was a clever man.
To search with ease, and in the best manner, the observer should have what among astronomers is familiarly known as a "broken-backed telescope." This instrument has the eye-piece on the end of the axis, where one would never think of looking for it.
"Well, Jupiter's moons, Saturn's ring, and the markings on Mars are all invisible to the naked eye. So are the craters in the moon; so we use the big speculum to gather the light, and then look at the spot where all the rays of light come to their narrowest point, with an eye-piece which really is a microscope." "But I don't understand now," said Tom uneasily. "I wish I was not so "
He stood back and helped Daisy to take a kneeling position in her chair, so that her eye could reach the eye-piece of the microscope. Daisy looked, took her eye away to give a wondering glance of inquiry at her friend's face, and then applied it to the microscope again; a pink hue of delight actually spreading over her poor little pale cheeks. It was so beautiful, so wonderful.
Although the construction of the eye-piece is much easier than that of the objective, since the same accuracy in adjusting the curves is not necessary, yet the price is lower in a yet greater degree, so that the amateur will find it better to buy than to make his eye-piece, unless he is anxious to test his mechanical powers.
Opposite to it a plain diagonal mirror was placed, to transmit the image to the eye. The whole was mounted on a turn-table, having a seat opposite to the eye-piece, as will be seen in the engraving on the other side. The observer, when seated, could direct the telescope to any part of the heavens without moving from his seat.
In the joint of the angle was a reflector which threw the image upon a mirror immediately under the eye-piece. "What's that for?" the boy asked. "So that we can look at the reefs at their own level," was the reply. "No matter how much you allow for refraction and foreshortening, you'll find it almost impossible to get correct values by studying a reef from the top only.
"She's a full-rigged ship, lying broadside-on to us, Mr Delamere," announced young Dundas. "So I perceive," I returned somewhat dryly. "And I notice, also, that she has swung with her head to the southward." "She's a big lump of a craft, not very far short of 900 tons, I should say," commented Henderson, with his eye still glued to the eye-piece of the schooner's glass.
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