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Updated: June 7, 2025
Only experience and actual trial will teach the observer to choose the particular eyepiece suitable for a given night or a given object.
The light, after passing through the lenses, is brought to a focus at the lower end of the tube, where the image is examined visually with an eyepiece, or photographed upon a sensitive plate. The largest instruments of this type are the 36-inch Lick telescope and the 40-inch refractor of the Yerkes Observatory.
Just north of Aquila is the little constellation Sagitta, containing several interesting doubles and many fine star fields, which may be discovered by sweeping over it with a low-power eyepiece. The star zeta is double, magnitudes five and nine, distance 8.6", p. 312°. The larger star is itself double, but far too close to be split, except with very large telescopes.
If the oblong image does not turn round with the eye, but does turn when the eyepiece is twisted round, then the astigmatism is in the latter. If, finally, it does not follow either the eye or the eyepiece, it is the objective that is at fault. But instead of being oblong, the image and the rings may be misshapen in some other way.
Opticians make glasses with three ranges, and write upon a little bar which shifts their eyepieces, 'Theatre, 'Field, 'Marine. Which of the three is your glass set to? The turn of a button determines its range. You can either look at the things close at hand, or, if you set the eyepiece right and use the strongest, you can see the stars. Which is it to be?
If, however, the supposed plane surface proves to be convex, the image will not be sharply defined in the telescope until the eyepiece is moved away from the object glass; while if the converse is the case, and the supposed plane is concave, the eyepiece must now be moved toward the objective in order to obtain a sharp image, and the amount of convexity or concavity may be known by the change in the focal plane.
There is a special instrument for centering and illuminating. On the stage of the instrument, the Bacterium with its flagellum in distinct focus is placed. Instead of the simple eyepiece, camera lucida is placed upon it.
The work to be done was in Metcalf's hands and brain, and, considering its potentiality, it was a most undramatic performance. He waited until the leading flag-ship was within half a mile of being abreast; then, turning on a hanging electric bulb, he held it close to the eyepiece of the periscope, knowing that the light would go up the tube through the lenses and be visible to the fleet.
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.
From the wood's edge to the wall he crawled with the speed and silence of a Houssa scout, and, once in shelter of the stones, was not long in finding a crevice roughly funnel-shaped, which gave him, with small eyepiece, a wide outlook.
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