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ADDENDUM,

[The four pages following were accidentally omitted in the regular course of printing. The reader is requested to read them after the fifth line of p. 37, as the introduction to the second paragraph).

Light. Those men who were first distinguished by the appellation of philosophers "doubted whether objects became visible by means of any thing proceeding from them or from the eye of the spectator.” The fallacy of this scepticism is apparent, otherwise we could see as well by night as by day. Empedocles and Plato qualified it by maintaining that “vision was occasioned by particles continually flying off from the surface of bodies, which met with others proceeding from the eye;" but Pythagoras ascribed it solely " to the particles proceeding from the external objects and entering the pupil of the eye.”

Among the modern philosopliers there have been two celebrated opinions, viz. the Cartesian and Newtonian. According to the former, light is an invisible fluid, pre

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sent at all times and in all places, but which requires to be set in motion by an ignited or otherwise qualified body, in order to make objects visible to us. The Newtonians maintain, that light is not a fluid per se, but consists of a vast number of exceedingly small particles, shaken off in all directions from the luminous body with inconceivable velocity, by a repulsive power, and which, most probably, never return again to the body from which they were emitted. These particles are also said to be emitted in right lines by the body from whence they proceed; and this rectilinear direction they preserve until they are turned out of their original path by the attraction of some other body near which they pass, and which is called inflection; by passing through a medium of different density, which is called refraction, or by being thrown obliquely or directly forward by some body which opposes their passage, and which is called reflection; or, lastly, till they are totally stopped by the substance of any body into which they penetrate, and which is called their extinction. A succession of these particles following one another in an exactly straight line is called a ray of light; and this ray, in whatever manner it has its direction changed, whether by refraction, reflection, or inflec

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