NATURAL PHILOSOPHY.

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asils rapid transition, being calculated from observationsmade upon the satellites of Jupiter to proceed ninety-fivemillions of miles in 1\ minutes!

The rays of light always proceed in straight lines, unlessthey are diverted and turned by some intervening objectinto some other direction.

Those rays, passing from one medium to another, may beinflected or diverted from a rectilinear course ; they are sus-ceptible of division also; and are absorbed by various bodies,when they are said to be refracted: to illustrate this, athin cloud is itself all illuminated by the solar rays, and inits turn, becomes a faintly luminous body; those rays, how-ever, which it emits are sensibly divided, and consequentlyweakened solar rays. This property of light is also calledits ref Tangibility.

The ancients knew that a ray of light was bent, when itpassed from one medium to another. They were led to this,from observing the appearance of a straight stick whenimmersed in water, the stick appearing to be bent; thecurve commencing at the precise point of the aquaticsurface, on which Archimedes is reported to have writtena book; also from a small coin or any similar substancebecoming visible at the bottom of a bowl or any vessel ofclear water, w r hen the eye is placed in such a situation that itcould not have been possibly seen, admitting water had beenabsent.

It should seem, Ptolemy , was well acquainted with re-fraction ; for he says, “ The light of the sun, moon, or“ stars, falling obliquely upon the gross atmosphere which“ surrounds the earth, are turned out of their rectilinear" course, which causes these luminaries to appear to rise" sooner than they do in reality.” He also ascribes theremarkably larger appearance of the moon, when near thehorizon, to the same caus^ and also the idea we conceiveof their distance, from the vast multitude of interveningobjects ; also, it is evident, that it is to this power in light,we have to ascribe the partial light the world enjoys in theabsence of the solar luminary, called twilight, the occasionof which is ascribed to the reflexion and refraction of thesolar rays upon the corpuscules or atomic particles of airand water found floating in the atmosphere; as the bodyof the globe is not so completely intervening as to preventthe passage of those rays whilst the sun is only 18i° belowthe horizon of the terrestrial surface, and for near the wholeof the summer half-year, in the northern latitudes, he is inthat situation. In the space or zone called the arctic circle, or