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mounted in a shorter tube than any other kind of equal power. Should it be desired to have the most powerful telescope in the smallest space, this form should, therefore, be chosen. The length of focus secures a flatness of field and obviates the use of deep eye-pieces.

The adjustments of the ordinary form of Cassegrainian are precisely similar to those of the Gregorian, and it is generally focussed in the same way.

STANDS FOR DIRECT VISION REFLECTORS.

As a rule any stand which will carry a refractor will be found suitable for a direct-vis ion reflector, and therefore it will be unnecessary to describe them again, especially as the first form of reflector just described is seldom used at the present time, except of a very small size, and for terrestrial purposes. The stands for these small reflectors are, however, generally rather more solid in their construction.

INDIRECT-VISION REFLECTORS.

NASMYTH’S MODIFICATION OF THE CASSEGRAINIAN TELESCOPE.

Fig. 8.

The great objection to the ordinary form of the Cassegrainian telescope for astronomical purposes is the uneasy position of the observer when viewing zenith objects; but this defect has been entirely removed by Mr. Nasmyth, at the expense of a slight loss of light. Instead of allowing the rays from the small convex mirror to pass through the large speculum, he interposes a flat mirror which diverts them at right angles to an eye-piece, placed in the side of the body-tube. In his Alt-azimuth the eye-piece was in the vertical axis of the telescope, and therefore was. always in one position; but as this would be rather inconvenient with an equatorial stand, the eyepiece is placed close to the axis, and revolves with the body-tube : it can, therefore, be always brought to a horizontal position, and at such an altitude as to be accessible without the necessity of mounting a

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