Hence the eye-pieces require to be as carefully preserved from damp and dust as the object-glass, and to be more frequently cleaned.
The tube of the telescope should be light, but strong, and free from vibration.

Its quality in the last respect can be tested by lightly striking it when mounted; the sound given out should be dead or non-resonant.

The inside of the tube must absorb extraneous light, and should therefore be coloured a dull black; and stops of varying radius should be placed along its length with the same object.

Sliding tubes, rack-work, etc., should work closely, yet easily.
The telescope should be well balanced for vision with the small astronomical eye-pieces.

But as there is often occasion to use appliances which disturb the balance, it is well to have the means of at once restoring equilibrium.