51�Թ�

Ramsden’s dioptric micrometer consists of a divided lens placed in the conjugate focus of the innermost lens of the erecting eye-tube of a terrestrial telescope.

In order to observe the latter we therefore employ what is called a terrestrial telescope, which is merely a refractor with some extra lenses added in the eye portion for the purpose of turning the inverted image the right way up again.

The simple microscope—Use of the simple microscope 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.

The lack of atmosphere made it possible to use a power of magnification that no terrestrial telescope may use.

Whether the white colour of the island, perceptible even in a large Terrestrial telescope, is in any degree due to the whiteness of the birds, their nests, and leavings, or wholly to reflection from the bright spar-like surface of the rock itself, and especially of the flat table-like summit, I will not pretend to say.