The angles of reflexion and of refraction are those contained between the said perpendiculars and the reflected or refracted rays.

Rays are said to be parallel, converging, or diverging, according as the lines they describe are always at the same distance from each other, approach nearer together, or recede from each other in their progress. The point from which rays proceed, is called the radiant point.

The point to which they converge is denominated the focus.

The imaginary radiant point, is the point from whence the rays seem to diverge, when they have been reflected or refracted.

The imaginary focus, is the point to which the rays tended before refraction or reflexion, which turned them off before they arrived at it.

The divergency and convergency of rays is measured by the small angles, contained between the lines which they describe.

If rays proceed from a radiant point, or tend to a focus at an infinite distance, their divergency or convergency is considered as nothing, and they are said to be parallel.

A lens is a thin round piece of polished glass, which has both its sides spherical, or one side spherical and the other side plane. Of these, there are five sorts, viz. the plano-convex, double convex, plano-concave, double concave, and meniscus, or concavo-convex.

The axis of a lens is a line perpendicular to both its surfaces.

If the axis of a ray coincide with the axis of a lens, it is said to fall directly upon the lens, but if otherwise, obliquely.

A mirror is a polished surface of glass or metal, either convex, concave, or plane.

After premising these definitions, we are next to consider the nature and properties of the rays of light.

Light consists of very small particles of matter, very probably of different magnitudes, emitted in all directions from a luminous point, in right lines, with amazing velocity, and decreasing in density as the squares of the distances from the radiant point increase.

That light is propagated in right lines is evident, from what we have observed already concerning the shadows of bodies, being similar to the bodies which project them. And, indeed, were not this the case, they would bend round them, like the circular pulses of air, which convey sound, and prevent them from casting any shades at all: and it is equally evident, from the impossibility of seeing any object through a bended tube. The same consideration proves, that it consists of particles of matter, and that it is not an impulse of the luminous body communicated to a subtile elastic fluid; for, upon that supposition, it would pass as easily through a bended as through a straight tube; and, bending round an opposing obstacle, it would render the luminous body as visible as if nothing stood in the way: in the same manner that sound is heard, by the vibrations of the elastic air. And as there is no middle opinion, but light must be either a real body, or an impulse upon some subtle fluid; and as it cannot be the latter, being continually found to disagree with all the laws of such a motion, we conclude that it must consist of real particles of matter; especially when we reflect farther, that it is acted upon by the attractive force of matter, as well as other bodies. This is evident

from the inflexion and refraction of a ray of light pas sing by the edge of a knife, or any other body, and its change of direction upon entering a medium of different density from the medium in which it was moving.

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That light consists of separate particles is evident, from the reflexion of some of its rays, while others are transmitted by any object, on which they fall. And our being able to see all the objects that are before us, through the smallest pin-hole in a card, by means of the rays that come from every point of every one of the objects, through that hole, evidently proves the inconceivable smallness of the particles of light. particle of light weighed but the two hundredth part of a grain, it would strike the eye with a momentum equal to that of a cannon ball, of ten pounds' weight, when first discharged from the cannon. And if so small a particle would have such a prodigious momentum, on account of the velocity of the rays of light, to lessen this momentum, they must be inconceivably small indeed, for the tender coats of the eye to bear their continual impulses. You will wonder less at the extreme minuteness of the particles of light, when you reflect upon what we have said, concerning the infinite divisibility of matter, and the almost infinite number of luminous particles emitted from a lighted candle in a single instant, which are sufficient to render it visible in every point of a sphere of eight miles in diameter.

That the rays of light spend some time in their progress from one place to another, and are not propagated instantaneously, is a subtle discovery of the celebrated Reaumur, derived from his observations of the eclipses of the satellites of Jupiter. If from the known

period of one of his moons, the time of an eclipse were calculated for a spectator in the center of the sun, or at the earth at her mean distance from Jupiter; it would be found, as he found it, that when the earth was nearest to Jupiter, that is, in his oppositions, this eclipse would happen between seven and eight minutes before the calculated time; and that when the earth was at the greatest distance from Jupiter, or when the sun and Jupiter were in conjunction, the eclipse would happen as much later than the calculated time; which phenomenon is to be accounted for only on the supposition, that the rays of light employ between seven and eight minutes, in passing over the semidiameter of the earth's orbit, which is about ninety-five millions of miles. From hence it is plain, that the velocity of light is nearly twelve millions of miles in every minute.

That rays are propagated in all directions from every point of a visible body appears from the impossibility of placing an eye in any position, from whence right lines may be drawn to any part of the body, but a sufficient number of rays from it will reach the eye to make it visible: provided the distance be not so great, as to give them room to diverge so far as not to be dense enough to occasion vision, when they fall upon the eye. For,

The rays of light decrease in density, in an inverse ratio to the squares of the distances from the radiant point. The same number of particles that fall upon any given surface at a given distance must, from their right-lined direction, be disseminated over four times that surface at double the distance, and over nine times that surface at triple the distance; and therefore can have but one fourth or one ninth of their former density at these distances: as is evident from 19 E. 6.

Notwithstanding the inconceivable minuteness of the particles of light, yet we shall presently see, when we consider their separation by the prism, that they probably have different magnitudes among themselves.

If the rays of light be sufficiently condensed in the focus of a lens, or mirror, they will burn with an intensity or force, which is proportional to the area of the lens or mirror directly, and to the area of the circular spot inversely, into which they are collected, at the focus: that is, directly as the square of the diameter of the lens or mirror, and inversely as the square of the focal distance. Because, the larger that the area of the glass is, so many more rays are transmitted or reflected by it, and their power of burning must be proportional to their number: and farther, their power of burning will also be increased by their density, because in that condition, there are more rays collected in a given space, and therefore the less the circular spot is, into which a given number of rays is collected at the focus, the greater will be their density and power of burning. Therefore, as the power of burning depends upon both these circumstances, it will be directly as the area of the lens or mirror, or the square of its diameter, and inversely as the area of the spot into which the rays are collected. Now this circular spot is the image of the sun inverted, when the rays proceed from the sun. For the rays which proceed from one limb, cross those that proceed from the opposite limb, in the center of the glass, and paint the image of the sun, in the focus, in an inverted position. Therefore the image and the object will always appear under the same angle from the vertex of the glass. Hence the image of the sun, being always seen under the same