ACCESSORIES TO THE TELESCOPE.

EYE-PIECES.

In order to successfully observe, it is of the greatest importance that a suitable eye-piece is employed. To an inexperienced observer the use of very high powers will be both disappointing and useless; he must remember that as the magnifying power of the instrument increases, so, apparently, does any tremor or vibration in the stand, and any unfavourable condition of the atmosphere. He will also lose much of the pleasure felt by those who possess instruments unprovided with clockwork motion, when they allow an object to slowly pass across the field of view, as with high powers this will be small. As experience increases, high powers may be used on favourable occasions and suitable objects.

The lowest power should have the largest field possible, as it will be employed for the observation of nebulse, clusters, and comets, many of which occupy a comparatively large space in the heavens. No. i in the list of Aplanatic eye-pieces at the end of this book, magnifying 60 diameters on a 6 feet focus, exactly answers this requirement, and the observer cannot do better than select it as the lowest of his series. This power also gives a very beautiful view of the moon, the whole disc being contained in the field; but when the study of individual craters is desired, higher powers must be employed, say from 120 to 250. These latter eye-pieces will also be generally useful for planetary and stellar work. It will thus be seen that the three eyepieces most suitable for a beginner are 60, 120 and 250. As the eye becomes trained to delicate observation, one or two of the highest powers should be added to the foregoing series—their use will be for splitting the closest double stars, and observing the minute craters on the moon.

The eye-pieces in the list are classed under the following heads: Huyghenian, Ramsden’s, Aplanatic, and Kellner’s. The first named is that most generally employed, as it is slightly more achromatic than the second form; but the Ramsden has the great advantage of giving a flatter field, and when each of its lenses is corrected for colour, no better eye-piece can be desired, especially on planets.

The Aplanatic construction is a modification of the Kellner, and possesses the same advantage—a very large field; they are, therefore, especially useful on nebulse, &c.

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