The negative eye-piece, being achromatic, is commonly employed in all
observations requiring distinct vision only. But as it is clearly unfit
for observations requiring micrometrical measurement, or reference to
fixed lines at the focus of the object-glass, the positive eye-piece is
used for these purposes.

For observing objects at great elevations the diagonal eye-tube is
often convenient. Its construction is shown in fig. 7. ABC is a totally
reflecting prism of glass. The rays from the object-glass fall on the
face AB, are totally reflected on the face BC, and emerge through the
face AC. In using this eye-piece, it must be remembered that it
lengthens the sliding eye-tube, which must therefore be thrust further
in, or the object will not be seen in focus. There is an arrangement by
which the change of direction is made to take place between the two
glasses of the eye-piece. With this arrangement (known as the _diagonal
eye-piece_) no adjustment of the eye-tube is required. However, for
amateurs' telescopes the more convenient arrangement is the diagonal
eye-tube, since it enables the observer to apply any eye-piece he
chooses, just as with the simple sliding eye-tube.

[Illustration: _Fig. 7._]

We come next to the important question of the _mounting_ of our
telescope.

The best known, and, in some respects, the simplest method of
mounting a telescope for general observation is that known as the
_altitude-and-azimuth_ mounting. In this method the telescope is
pointed towards an object by two motions,--one giving the tube the