ather for observation, there
are instants of distinct vision (with moderate powers) during which the
careful observer may catch sight of important details; and, similarly,
in the best observing weather, there are moments of unusually distinct
vision well worth patient waiting for, since in such weather alone the
full powers of the telescope can be employed.

The telescopist should not be deterred from observation by the presence
of fog or haze, since with a hazy sky definition is often singularly
good.

The observer must not expect distinct vision of objects near the
horizon. Objects near the eastern horizon during the time of morning
twilight are especially confused by atmospheric undulations; in fact,
early morning is a very unfavourable time for the observation of all
objects.

The same rules which we have been applying to refractors, serve for
reflectors. The performance of a reflector will be found to differ in
some respects, however, from that of a refractor. Mr. Dawes is, we
believe, now engaged in testing reflectors, and his unequalled
experience of refractors will enable him to pronounce decisively on the
relative merits of the two classes of telescopes.

We have little to say respecting the construction of telescopes. Whether
it is advisable or not for an amateur observer to attempt the
construction of his own telescope is a question depending entirely on
his mechanical ability and ingenuity. My own experience of telescope
construction is confined to the conversion of a 3-feet into a 5-1/2-feet
telescope. This operation involved some difficulties, since the aperture
had to be increased by about an inch. I found a tubing made of alternate
layers of card and calico well pasted together, to be both light and
strong. But for the full length of tube I think a core of metal is
wanted. A learned and ingenious friend, Mr. Sharp, Fellow of St. John's
College, informs me that a tube of tin, covered with layers of brown
paper, well pasted and thicker near the middle of the tube, forms a
light and strong telescope-tube, almost wholly free from vibration.

Suffer no inexperienced person to deal with your object-glass. I knew a
valuable glass ruined by the proceedings of a workman who had been told
to attach three pieces of brass round the cell of the double lens. What
he had done remained unknown, but ever after a wretched glare of light
surrounded all objects of any brilliancy.

One word about the inversion of obje