REFLECTION. 



refleftiiig body, V)Ut by fome power of the wliole body, 

 evenly difTufcd all ovlt its furface, and by which it afts on 

 a ray wlihoiit immediate contadt ; for that the parts ot bo- 

 dies do ad upon light at a diftance, is already fliewn under 

 ' the articles Ini'LECTIon and I^ight. 



2. If the colours, feparated by a prifm placed at the 

 entrance of a beam of light into a darkened room, be 

 fuccefllvely caft on a fecond prifm placed at a greater dillance 

 from the former, in fuch manner as that they all fall alike, 

 or with an equal obliquity, upon it ; the fecond prifm 

 may be fo inclined to the incident rays, that thofe which 

 are of a blue colour fliall be all reflcded by it ; and yet 

 thofe of a red colour pretty copioufly tranfmitted. Now, 

 if the rcfleftion were caufed by the parts of the air or 

 glafs, we wotdd alk, why, at the fame obliquity of inci- 

 dence, the blue (hould wholly impinge on thofe parts lo as 

 to be all relieved ; and yet, the red find pores enough 

 to be, in a great meafure, tranfmitted. 



3. Where two glades touch one another, there is no fen- 

 fible reflcftion ; and yet we fee no reafon why the rays 

 fliould not impinge on the parts of the glafs, as much when 

 contiguous to other glafs, as when contiguous to air. 



4. When the top of a water bubble, by the continual 

 fubfiding and exhaling of the water, grows very thin, there 

 is fuch a httle, and almoft. infenfible quantity of light re- 

 fledled from it, that it appears intenfely black ; whereas 

 round about that black fpot, where the water is thicker, 

 the reflexion is fo flrong, as to make the water feem very 

 wliite. Nor is it only at the lead thicknefs of thin plates or 

 bubbles, that there is no manifeil refleftion, but at many 

 other thicknefl'es, gradually greater and greater. For, in 

 one of our author's obfervations, the rays of the fame co- 

 lour were, by turns, tranfmitted at one thicknefs, and re- 

 fledled at another thicknefs, for an intermediate number of 

 fucceffions : and yet, in the fuperficies of the thinned body, 

 where it is of one thicknefs, there are as many other parts 

 for rays to impinge on, as where it is of any other thicknefs. 



5. If the red and blue rays, feparated by a priim, fall 

 fuccefllvely on a thin plate of any pellucid matter, whofe 

 thicknefs increafes in contmual proportion, (fuch as a plate 

 of air between two glafl'es, the one plane, and the other a 

 little convex,) the fame plate will, in the fame part, refleft 

 all the rays of one colour, and tranfmit all thofe of the 

 other ; but, in different parts, will refleft the rays of one 

 and the fame colour at one thicknefs, and tranfmit them at 

 another ; and thus alternately, and In infinitum. Now, it 

 can never be imagined that at one place the rays, which, for 

 inllance, exhibit a blue colour, (hould happen to ftrike on 

 the folid parts, and thofe which exhibit a red, to hit on the 

 void parts of the body ; and at another place, where the 

 body is either a little thicker, or a little thinner, that, on 

 the contrary, the blue iliould hit on the pores, and the red 

 upon the foUd parts. 



6. In the pailage of light out of glafs into air, there is 3 

 reflcftion as llrong as in its padage out of air into glafs, or 

 rather a little llronger, and by many degrees ftronger than 

 in its palTage out of glafs into water. Now, it feems im- 

 probable, that air flunild have more reflefting pjrts than 

 water or glafs : but if that (liould be fuppofed, yet it will 

 avail nothing ; for the refleftion is as ftrong, or llronger, 

 when the air is drawn from the glafs by the air-pump, as 

 when it is adjacent to it. If any fhould here objttt-, on 

 Defcartes's hypothefis, that, though the air be drawn away, 

 there is a fubtle matter remaining to fupply its place, which, 

 being of a denfer kind, is better fitted for the refleftion of 

 light than any other body ; befide? that we have elfewhere 

 Oiewn fuch fubtle matter to be fiftitious, and that, fuppof- 



ing its exiitence, and its reflefting power, no li^ht could 

 ever have been propagated, but nmfl. have been all rcflefted 

 back to the lucid body, immediately after it was firIL 

 emitted, the following experiment does evidently convift it 

 of falfity. 



7. If light, in its paffage out of glafs into air, (trikc more 

 obliquely than at an angle of forty or forty-one degrees, it 

 is then wholly reflefted : if lefs obliquely, it is in great mea- 

 fure tranfmitted. Now, it is not to be imagined, that light 

 at one degree of obliquity fhould meet with pores enough in 

 the air to tranfmit the greater part of it, and at another dc. 

 gree fhould meet with notliiiig but parts to refleft it wholly ; 

 efpecially confiderlng, that, in its pa'lage out of air into 

 glafs, how oblique foever be its incidence, it finds pores 

 enough in the glafs to tranfmit a great part of it. If any 

 fuppofe, that it is not reflefted by the air, but by the ut- 

 moll fupcrficial parts of the glafs, there is ilill the fame dif- 

 ficulty : befides, that fuch a fuppofition is unintelligible, 

 and will alfo appear to be falfe, by applying water behind 

 fome part of the glafs, inllead of air ; for fo in a convenient 

 obliquity of the rays, fuppofe of forty-five or forty- fix de- 

 grees, at which they are all reflefted, where the air is adja- 

 cent to the glafs, they fliall be in great meafure tranfmitted 

 where the water is adjacent to it ; which argues, that their 

 refleftion or tranfmiffion depends on the conllitution of the 

 air and water behind the glafs, and not on the ftriking of 

 the rays upon the parts of the glafs, the rays not being re- 

 flefted until they have reached the lafl part of the furface, 

 and have begun to go out. For if, in going out, they fall 

 upon a furface of oil and water, they proceed, the attrac- 

 tion of the glafs being balanced by an equal force the con- 

 trary way, and prevented from having its cffeft by the at- 

 traftion of the liquor adhering to it : but if the rays, in 

 pafTmg out of this lafl furface, fall into a vacuum, which 

 has no attraftion, or into air, which has but little, not 

 enough to counterbalance the efteft of the glafs in this cafe, 

 the attraftion of the glafs draws them back, and rePefts 

 them. 



This will appear ftill more evident by laying two glafs 

 prifms, or the objeft-glafles of two telefcopes, the one plane, 

 and the other a little convex, upon each other, fo as they 

 may neither touch, nor yet be too far apart ; for that light 

 which falls on the hinder furface of the firfl glafs, where the 

 glafles are not above -rsTroTro-ijtli part of an inch apart, will 

 be tranfmitted through the furface, and through the air or 

 vacuum between the glaffes, and will pafs into the fecond 

 glafs ; but if the fecond glafs he taken away, then the light 

 palling out of the fecond furface of the firfl glafs into the air 

 or vacuum, will not proceed, but will return into the firft 

 glafs, and be reflefted. 



Whence it follows, that the r.iys are drawn back agaia 

 by fome force in the firfl glafs, there being nothing elfe to 

 occafion their return. And hence too it follows, that the 

 refleftion is not effefted by means of any fubtle matter con- 

 tiguous to the hinder furface, accordmg to the principles 

 of Defcartes ; fince that matter ought to refleft them when 

 the glaffes were nearly contiguous, as well as when the fe- 

 cond glafs is quite removed. 



Laflly, if it be afked, how fome of the rays come to be 

 reflefted, and others tranfmitted ; and why they are not all 

 alike reflefted, fuppofing the refleftion owing to the aftion 

 of the whole furface ? the fame great author ihews, that 

 there are, both in the rays of light, and in the bodies them- 

 felves, certain vibrations (or fome fuch property) impreffed 

 on the rays, by the aftion either of the luminary that emits 

 them, or of the bodies that refleft them ; by means of which 

 it happens that thofe rays, in that part of their vibration 

 8 which 



