may be confidered in two refpects, viz. when the reflection is total, and when it is partial.

furface abounds with fuch, the reflected rays will rather be fcattered like a parcel of pebbles thrown upon a rough pavement, than reflected with that regularity with which light is obferved to form a well polished one, which are far from being fo; for to polish is no other than to grind off the larger eminences and protuberances of the metal with the rough and harp particles of fand, emery, or putty, which muft of neceffity leave behind them an infinity of rafures and fcratches, which, though inconfiderable with regard to the former roughneffes, and too minute to be difcerned by us, muft, nevertheless, bear a large propor tion to, if not vaftly exceed, the magnitude of the particles of light. Secondly, that it is not reflected by impinging upon the folid particles which constitute this fecond furface, is fufficiently clear from the foregoing argument; the fecond furfaces of bodies being as incapable of a perfect polish as the fift; and it is farther confirmed from hence, viz. that the quantity of light reflected, differs according to the different density of the medium behind the body: and that it is not reflected by impinging upon the particles which conítitute the furface of the medium behind it, is evident, because the strongest reflection of all at the fecond furface of the body, is when there is a vacuum behind it.

2. It has been thought by fome, that it is reflected at the firft furface of a body, by a repulfive force equally diffused over it, and at the fecond by an attractive force.

If there be a repulfive force diffused over the furface of bodies, that repels rays of light at all times, then, fince by increafing the obliquity of a ray, we diminish its perpendicular force (which is that only whereby it must make its way thro' this repulfive force) however weakly that force may be fuppofed to act, rays of light may be made to fall with fo great a degree of obliquity on the reflecting fur face, that there shall be a total reflection of them there, and not one particle of light be able to make its way through, which is contrary to obfervation; the reflection of light at the first furface of a transparent body being never total in any obliquity whatever. The hypothefis, therefore, in this particular, must be falle.

As to the refl-&tion at the second furface by the attractive force of the body, this

And first, in cafes where the reflection is total, the cause of it, undoubtedly, is that fame attractive force by which light would be refracted in paffing out of the fame body: this is manifelt from that analogy which is obfervable between the reflection of light at this fecond surface and its refraction there. For, otherwife, what can be the reafon that the total reflection fhould begin just when the obliquity of the incident ray, at its arrival at a fecond furface, is fuch, that the refracted angle ought to be a right one; or when the ray, were it not to return in reflection, ought to país on parallel to the furface, without going from it? For, in this cafe, it is evident, that it ought to be returned by this very power, and in fuch manner, that the angle of reflection fhall be equal to the angle of incidence; just as a stone thrown obliquely from the earth, after it is fo far turned out of its courfe by the attraction of the earth, as to begin to move horizontally, or parallel to the furface of the earth, is then, by the fame power, made to return in a curve fimilar to that which is defcribed in its departure from the earth, and fo falls with the fame degree of obliquity that it was thrown with.

But, fecondly, as to the reflection at the fecond furface, when it is partial; an attractive force uniformly fpread over it, as the maintainers of this hypothefis conceive it to be, can never be the cause thereof, because it is inconceivable, that the fame force, acting in the fame circumstances in every respect, can sometimes reflect the violet-coloured rays and transmit the red, and at other times reflect the red and tranfmit the viclet.

This argument concludes equally against a repulfive force uniformly diffused over the fift furface of a body, and reflecting light there; because some bodies reflect the violet and tranfinit the red, others reflect the red and tranfmit the violet at their first furface; which cannot poffibly be upon this fuppofition, the rays of whichever of thefe colours we fuppofe ta be the strongest.

3. Some being apprehenfive of the infufficiency of a repulfive and attractive force diffufed over the surface of bodies, and acting uniformly, have fuppofed, that by the action of light upon the fur

faces

faces of bodies, the matter of these forces is put into an undulatory motion, and that where the furface of it is fubfiding, light is tranfmitted, and in those places where it is rifing, light is reflected. But this feems to advance us not one jot farther; for in thofe cafes, fuppofe where red is reflected and violet tranfinitted, how comes it to pass that the red impinges only on those parts when the waves are rifing, and the violet when they are fubfiding?

4.

The laft hypothefis is that of Sir Ifaac Newton; who is of opinion, that light, in its paffage from the luminous body, is difpofed to be alternately reflected by, and tranfmitted through, any refracting furface it may meet; and this difpofition he calls fits of easy reflection, and eafy tranfmiffion. Thus, if we take the diftances as the numbers 0, 1, 2, 3, 4, 5, 6, 7. 8, 9, 10, &c. then at the distances o, 2, 4, 6, 8, &c. the light will be tranfmitted; and, at the diftances 1, 3, 5, 7, 9, &c. it will be reflected in coloured rings.

These fits he thinks probably owing to fome fubtile and elastic fubftance, diffufed through the univerfe, in the following manner as bodies falling into water or paffing through the air, caufe undulations in each, fo the rays of light may excite vibrations in this elaftic fubftance: the quickness of which vibrations depending on the elafticity of the medium, the motion of the particles of it may be quicker than that of the rays; and therefore, when a ray, at the inftant it impinges upon any furface, is in that part of a vibration of this elastic fubftance which confpires with its motion, it may be easily tranfmitted; and when it is in that part of a vibration which is contrary to its motion, it may be reflected. Allo, when light falls on the first furface of a body, none is reflected there; but all that happens to it there is, that every ray that is not in a fit of eafy tranfmiffion is there put into one, fo that, when they come to the other fide, the rays of one colour fhall be in a fit of eafy tranfmiffion, and thofe of another in a fit of ea fy reflection, according to the thick. nefs of the body, the intervals of the fits being different in rays of a different kind. This doctrine of the easy reflection and eafy tranfmiffion of the rays of light, ought by no means to be looked on as a mere hypothefis, fince Sir Ifaac has vinced, by experiments, that this is the

cafe. The first experiment he mentions is the compreffion of two prifms hard together, whofe fides were a little convex, by which means they touched by a small part of their furfaces, and contained every where else a thin plate of air, as it may be properly called, whofe thickness did every where gradually increase from the touching parts. He obferved the place where they touched became abfo lutely transparent, as if they had there been one continued piece of glass. For when the light fell fo obliquely on the plate of air between the prifms as to be all reflected, it feemed in that place of contact to be wholly transmitted, infomuch that when looked upon, it appeared like a black or dark spot, by reafon that little or no fenfible light was refle&ted from thence, as from other places. When he looked through the prifms, this place of contact seemed, as it were, a hole in the plate of air; and through this hole objects that were beyond might be feen diftin&tly, which could not be feen through other parts of the glaffes where the air was interjacent. By harder compreffion the spot was dilated by the yielding inwards of the parts of the glaffes.

When the plate of air, by turning the prifins about their common axis, became fo little inclined to the incident rays that fome of them began to be tranfmitted, there arofe in it many leader coloured arches, which at first were shaped almost like the conchoid, as in ibid. n° 2, and by continuing the motion of the prisms, thefe arches increased and bended more and more about the said transparent spot, till they were completed into circles or rings encompaffing it; and afterwards. continually grew more and more contracted. These arches and rings became tinged with various colours, as the mction of the prifms was continued, being at first of a violet and blue; afterwards of a white, blue, violet; black, red, orange, yellow, white, blue, violet, &c. after this the coloured rings contrзded, and became only black and white. The prifms being farther moved about, the colours all began to emerge out of the whiteness, and in a contrary order to what they had before.

But to obferve more nicely the order of the colours which arofe out of the white circles, as the rays became less and less inclined to the plate of air, Sir Isaac Newton made ufe of the two object

gtaffes,

glaffes, one a plano-convex, and the other a double-convex, of the fame Sphericity on both fides, of fifty-one feet focal diftance; and upon this he laid the plane fide of the other, preffing them lowly together to make the colours fucceffively emerge in the middle of the circles, and then flowly lifted the upper glass from the lower to make them fucceffively vanish again in the fame place.

Upon compreffion of the glaffes, various colours would emerge and spread into concentric circles or rings of different breadths and tints encompaffing the central spot. Their form, when the glaffes were moft compreffed, is delineated, ibid. n° 3. where a is the central black fpot, and the circuits of colours from thence outwards as follows:

451, green,

r, red. u, greenish blue. x, pale red.

1, purple. m, blue.

n, green. o, yellow.

, red.

Ss, greenith blue. 2t, red.

{greenish blue. 7%, reddish white.

Thele rings were obferved to be leaft when the eye was held perpendicularly over the glaffes in the axis of the rings: whereas, viewed obliquely, they became bigger, continually fwelling as the eye was removed farther from the axis and the coloured rings made in air, became much more diftinct and visible, when viewed in a dark room by the reflection of the coloured light of the prifm. The rings made by reflection of red light were manifeftly bigger than those made by the blue and violet; and it was very pleasant to fee them gradually fwell and contract according as the colour of the light was changed. The motion was quickest in the red, and floweft in the violet; and, by an estimation made of the ciameters of the rings, the thickneffes of air in the places where the rings are made by the limits of the feven colours, red, orange, yellow, green, blue, indigo, violet, fucceffively in order, were to one another as the cube roots of the fquares of the eight lengths of a chord which found the notes of an octave, that is, of the numbers

3 5 3 2 3 9 1, 3, 8, 4, 3, 3, I Thele rings were not of various colours, as thofe made in the open air, but appeared all over of that prifmatic colour only with which it was illumined; and

by throwing the coloured light directly on the glaffes, that which fell on the dark spaces between the rings was tranfmitted through the glaffes without any variation of the colour. This appeared by placing a white paper behind, on which the rings were painted of the fame colour as those by reflected light, and of the bigness of their immediate spaces. Hence the origin of these rings is manifeft; namely, that the air between the glaffes, according to its various thicknefs, is difpofed in fome places to reflect, in others to tranfmit the light of any one colour; and in the fame place to reflect that of one colour, where it tranfmits that of another; in the manner as you fee reprefented ibid. n° 4. where A B, CD, are the glaffes, as before; and a, c, e, g, i, l, n, p, the parts of the beam tranfmitted; and b, d, f, b, k, m, o, the parts of the beam reflected, making the coloured rings. REFLECTION of the moon, the fame with her variation. See VARIATION. REFLECTION is alfo ufed, figuratively, for an operation of the mind; whereby it turns its view backwards as it were upon itself, and makes itself and its own operation the object of its difquifition; and by contemplating the manner, order, and laws which it obferves in perceiving ideas, comparing them together, reafoning, &c. it frames new ideas of the relations dilcovered therein. See the articles IDEA, KNOWLEDGE, REASONING, &c.

REFLEX, or REFLECT, in painting, is understood of thofe places in a picture which are fuppofed to be illuminated by a light reflected from fome other body, represented in the fame piece, REFLEX VISION, that performed by means of reflected rays, as from mirrours. See the articles VISION, REFLECTION, and MIRROUR.

REFLUX of the fea, the ebbing of the water, or its returning from the shore. See the article TIDES.

REFORM, a re-establishment, or revival of formerly neglected difcipline, or a correction of the reigning abuses therein. To REFORM, in a military fenfe, is to reduce a company, regiment, &c. either by dibanding the whole, or breaking a part, and retaining the reft. REFORMADO, or REFORMED OFFICER, one whofe troop or company is fuppreffed in a reform, and he continued