fhorter now, than it would have been a century ago, and yet I hope it is much larger than there will be any necefiity of making it a century hence; as it may be prefumed that, by that time, a connection will be traced between many facts, which now appear to be unconnected and independent of one another, and therefore require to be recited feparately.

To be as concife as poffible in delivering the elementary principles of the doctrine concerning light, I fhall purpofely omit the application of them to any of the phenomena of nature, though that be the chief object in all philofophical enquiries; it being my bufinefs at prefent, barely to recite the knowledge we have acquired of the laws of nature, as difcovered by an attention to thofe appear

ances.

The obfervations that were made in the first part of the laft period of this history will authorize us to take it for granted, that light confifts of very minute particles of matter, emitted from luminous bodies. Some of thefe particles, falling upon other bodies, are reflected from them, in an angle equal to that of their incidence, while other particles enter the bodies; being either bent towards or from a perpendicular to the furface of the new medium, if the incidence be oblique to it. In general, rays of light, falling obliquely on any medium, are bent as if they were attracted by it, when it has a greater degree of density, or contains more of the inflammable principle, than the medium through which it was tranfmitted to it. More of the rays are reflected when they fall upon a body with a fmall degree of

obliquity to its furface, and more of them are tranfmitted, or enter the body, when their incidence is nearer to the perpendicular.

The velocity with which light is emitted and reflected is the fame; and fo great, that it palles from the fun to the earth in the space of about eight minutes and twelve feconds. The velocity of light is fuppofed to be increafed or diminished by refraction, in proportion to the degree in which the angle of refraction is lefs or larger than the angle of incidence.

Rays of light, emitted or reflected from bodies, enter the pupil of the eye, and are so refracted by the humours of it, as to be united, accurately, or nearly fo, at the furface of the retina, or choroides, and fo make images of objects, by means of which they

are visible to us.

When a beam of light is bent out of its courfe by refraction, all the rays of which it confifts are not equally refracted, but fome more and others lefs; and the colour which they are difpofed to exhibit, is connected invariably with the degree of their refrangibility. The red-coloured rays are the leait, and the violet the most refrangible; and the reft are more or lefs fo, in proportion to their nearness to thefe, which are the extremes, in the following order; violet, indigo, blue, green, yellow, orange, red.

Thefe colours, when they are feparated as much as poffible, are ftill contiguous, and all the fhades of each colour have likewife their feparate and invariable degrees of refrangibility. When they are feparated by refraction, the extremes are removed from one another to fuch a diftance, that they divide

the

the whole fpace between them exactly as a musical chord is divided, in order to found the feveral notes and half notes of an octave. The mixture of all thefe differently-coloured rays, in the proportion in which they cover the fpace, fo divided, makes a white, and the ab. fence of all light is blackness.

The degree in which thefe differently-coloured rays are feparated from one another, is not in proportion to the mean refractive power of the medium, but depends upon the peculiar conftitution of the fubftance by which they are refracted. The difperfing power of glafs, into the compofition of which lead enters, is great in proportion to the mean refraction; and it is little in proportion to it in that glafs in the composition of which there is much alkaline falt.

Not only have the different rays of light thefe different properties with refpect to bodies, fo as to be more or lefs refracted, or difperfed by them; but the different fides of the fame rays have different properties; for they are differently affected according to the fides with which they are prefented to Ifland chryftal. With the fame degree of incidence, they are refracted in different angles.

Rays of light are not reflected or refracted by impinging on the folid parts of bodies, but by virtue of a power which extends to fome diftance from the furface. They are refracted by a power of attraction, and reflected by a power of repulfion.

At the first furface of any body, rays of all kinds are promifcuoufly reflected or tranfmitted; but if the next furface be very near to it, fo that their powers of attraction and

repulfion interfere, the rays are affected in fuch a manner, that, in fome particular places, thofe of one colour only are reflected, and those of another colour, chiefly, are tranfmitted; and thofe places occur alternately for rays of each of the colours, in paffing from the thinneft to the thickeft parts of the medium; so that several series, or orders of colours, will be visible on the furface of the fame thin tranfparent body.

When rays of light pafs near to any body, fo as to come within the fphere of its attraction or repulfion, an inflection, that is a partial refraction or reflection, of all the rays takes place; all the kinds being bent either towards or from the body; and these powers affecting fome rays more than others, within the fame diftance, they are, by this means, alfo, feparated from one another; fo that coloured ftreaks appear both within the fhadow, and on the outfide of it. The red is inflected at the greatest distance from all bodies. There are several diftances at which the different rays are differently affected bythepowers that are lodged at the furfaces of the bodies, to which they make a near approach, so that different orders of colours are made by rays which come within different diftances from the bodies. Three of thefe orders have been observed.

Part of the light which falls upon bodies is retained within them, and proceeds no farther, This is more efpecially the cafe in refpect to light falling with certain degrees of obliquity on the furfaces of bodies. Part of this light is retained fo loofely by fome kinds of bodies, that a very small degree of heat makes them emit it again;

but