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marine-glass should be employed, the power of a race-glass being greater than that of an opera-glass, and so on : a marine glass being the most powerful Galilean telescope now in common use. Glasses are now often made with revolving eye-pieces of different powers. Those intended for out-door use should always be provided with sliding shades to protect the object-glass from damp or to shield it from the sun’s glare. Glasses mounted in aluminium are only about one-third the weight of those in the ordinary metal, and are therefore much more portable, but are more expensive.

There are two important adjustments to be attended to before the best effect is obtained, the first is, of course, the correct focussing; and the second, adjustment to the width between the eyes. People who are obliged to wear powerful spectacles, often find they cannot focus an opera-glass :—a slight alteration in the curves of the eyepieces will enable any sight to be suited, or if only one eye is defective, only one eye-piece will require alteration. As opera-glasses are usually made to fit an average face, the eye-lenses are large enough to allow a majority of persons to see the two fields of view coincide. Should, however, the width between the pupils of the eyes be much over or under the average, a pair of glasses should be selected of suitable width, or one capable of adjustment to various faces.

The lenses will require no adjustment, having been carefully set by their maker. They will require cleaning, which should be done by wiping them carefully with a soft clean piece of wash-leather or a silk handkerchief. If at any time it should be necessary to remove the lenses from their mounts or cells, care should be taken that they are replaced as they were originally. All the above remarks on the opera-glass equally refer to similar instruments. The focussing screw sometimes gets out of order, but can be re-tightened to the frame with ease ; a screw has lately been invented which does not become so readily unloosed.

THE ORDINARY REFRACTING TELESCOPE.

Fig. 2.

The ordinary form of the refracting telescope differs from the Galilean in possessing a convex eye-piece. Should the use of this

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La langue de reconnaissance de l'OCR est l'Anglais.

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