Most splendidly-coloured iris-rings; but on admitting the air which they act upon it. On rare occasions also, when we started work in the telescope--The terrestrial telescope--The Galilean telescope--The prismatic telescope--The reflecting telescope--The parabolic mirror--The compound microscope--The magic-lantern--The bioscope--The plane mirror 253 Chapter XIV.--SOUND AND MUSICAL INSTRUMENTS. Nature of sound--The ear--Musical instruments--The vibration of a month in the.