Page:History of the Royal Society.djvu/341

 He discours'd to them a Natural and easy Theory of Refraction, which exactly answer'd every Experiment. He fully demonstrated all Dioptrics in a few Propositions, shewing not only (as in Keplers Dioptrics) the common properties of Glasses, but the proportions by which the individual Rays cut the Axis, and each other; upon which the Charges (as they are usually called) of Telescopes, or the proportion of the Eye-glasses and Apertures are demonstrably discover'd.

He has made constant Observations on Saturn; and a Theory of that Planet, truly answering all Observations, before the printed Discourse of Hugonius on that Subject appear'd.

He has essay'd to make a true Selenography by measure; the World having nothing yet but Pictures, rather than Surveys and Maps of the Moon. He has stated the Theory of the Moon's Libration, as far as his Observations could carry him. He has compos'd a Lunar Globe, representing not only the Spots, and various degrees of whiteness upon the Surface, but the Hills, Eminencies, and Cavities moulded in solid Work. The Globe thus fashioned into a true Model of the Moon, as you turn it to the Light, represents all the Menstrual phases, with the variety of appearances that happen from the Shadows of the Mountains and Valleys. He has made Maps of the Pleiades, and other Telescopical Stars; and propos'd Methods to determine the great doubt of the Earths motion or rest, by the small Stars about the Pole to be seen in large Telescopes.

In order to Navigation he has carefully pursu'd many Magnetical Experiments; of which this is one of the noblest and most fruitful Speculation. A large Terella is plac'd in the midst of a Plane Board, with a hole Rh