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frozen zone, i. e. from 66§ degrees north longitude to the pole,the sun in the northern summer is never seen to set, owingto the obliquity of the globe, and is never seen there,only by the effect of refraction, during the winter half-year.
The doctrine of refraction was discovered by Snellius ,professor of mathematics at Leyden, and the cause of itmore fully explained by his contemporaries Descartes andLeibnitz . Grimaldi, an Italian painter, first observed thatthe coloured image of the son refracted through a prism isalways oblong, or perhaps, elliptical, and that coloursproceed from refraction. The complete illustration of thissubject is said to have been left for the genius of Newton,who explained it, with the different refrangibility of the raysof light, to which his attention is said to have been directedwhilst in the action of grinding optical glasses, in 1666.
The rays of light being thrown back by any opposingbody, are said to be reflected; the bodies capable of suchreflexion are opaque substances, and generally called re-flecting surfaces. When they are absorbed by penetratingthe surface of any body, they are then said to be lost, andsometimes they are extinguished.
Further, the rays of light are, like other small bodies,subject to the laws of attraction: e. g. if a stream of lightbe admitted by a small hole into a dark room, and the edgeof a knife be exposed out of that line formed by the ray,the column of light will be directed from its natural course,and inflected towards the edge of the knife, llefraction issaid to consist in this, that the rays are more attracted by adense than a rare medium ; by one colour more than an-other. Thus, light coloured bodies reflect, whilst dark onesabsorb, the solar influence. Dr. Benjamin Franklin provedthis, by laying various bits of cloth of several colours uponsnow, when he discovered the dark shades attracted thesolar rays in various proportionate degrees much more thanthose of a light nature, and black the most of any.
Rays proceeding from the same point as a centre, andcontinually receding from each other, are said to be diver-gent ; those rays moving in the contrary direction are saidto be convergent: their point of concourse is called thefocus; the point whence they proceed is called the radiantpoint.
Parallel rays passing out of one medium into another ofdifferent density, and separated by a plane surface, will alsobe parallel after refraction, as may be seen in the instanceof our perceiving the natural form of a coin at the bottomof a vessel of clear water, when the eye is placed as before