RAY 



many more by his correfpordents, throw much light on his 

 charafter and purfuits. We cannot help remarking that his 

 hand-writing was peculiarly fair and elegant. A fpccimen 

 of it exifts among fir Henry Spehnan's vatt and curious col- 

 Icftion of mannfcripts, now in the poileflion of John Patter- 

 fon, efq. late M. P. tor Norwich. 



The portraits of Ray arc not numerous. One in oil, taken 

 at an advanced period of his life, remains in the Britifh 

 Mufeum, and Dr. John Sims is iiolleflid of a miniature, of 

 an earlier date, of whicli its owner has given an engraving, 

 by way of frontifpiece to the firft volume of the " Annals of 

 Botany," publifhed in 1805. The latter is rather deficient 

 in that ftrength of charafter wl.xh appears in the more com- 

 mon prints, after a pifture by laithorne, often prefixed to' 

 the third edition of the Synofjii, and fometimes to the hi/- 

 tor'ta Plantarum. One of theie prints was engravtd by 

 Elder; the other by Vcrtue. Neither of them is llnftly 

 appropriated to any particular book. That in the German 

 edition of his " Wifdom of God in the Creation," publifhed 

 at Leipfic in 1732, appears to be copied from one of thefe, 

 and yet is fo uniike them in expreffir r, that if it were 

 poflible, one w ould fuppofe it taken from lome other portrait 

 of the fam.e perfon. Ray's Works. His Life lay Dr. 

 Derham. Haller's Bibl. Bot. Pulteney's Sketches. Ai- 

 kin's General Biography. S. 



Ray, in Geography, a town of France, in the department 

 of the Upper Saone ; 12 miles. E. of Champlitte. 



Ray, in Geometry. See Radils. 



Ray, Radius, hi Optics, a beam, or line of light, propa- 

 gated from a radiant point, through any n.i ditm. 



Sir Ifaac Newton definet rays to be tlif Icalt parts of light, 

 whether fucceflive in the fame line, or contemporary in ie- 

 veral lines. 



For, that light confilts of parts of both kinds, appears 

 hence, that one may Hop what comes this n oment m any 

 point, and let pafs that which comes the next ; and again, one 

 may flop what comes in this point, and let pafs that in the 

 next. Now, the leaft light, or part of light, which may 

 be thus flopped alone, he calls a ray of light. 



A ray of light is, therefore, confidertd as an infinitely 

 narrow portion, or an evanefcent element ot a ftream of 

 light ; and a pencil, as a fmall detached itreani, compofed of 

 a coUedlion of fuch rays accompanying each other. As we 

 cannot exhibit to the lenfes a fingle mathematical line, ex- 

 cept as the boundary of two furtaces ; in the fame manner, 

 we cannot exhibit a fingle ray of liglit, except as the confine 

 between light and darknefs, or as the lateral limit of a 

 pencil of light. 



If the parts of a ray of light do all lie ftraight between the 

 radiant and the eye, which is the cafe when it moves 

 through the fame uniform fubllance or a vacuum, called a 

 medium, the ray is faid to be dlreS : the laws and properties 

 of which make the fubjeft of optics. If any of them be 

 turned out of that direftion, or bent in their paflage, the ray 

 is faid to be refraileti : and that branch of optics which 

 treats of thefe refratted rays, is called dioptrics ; which fee. 

 See alfo Refraction. 



If it ftrikeon the furface, or medium attached to the fur- 

 face, of any body, and be driven back, it is faid to be 

 refleSed •• and that branch of optics which treats of thefe 

 refrafted rays, is called catoptrics ; which fee. See alfo 

 Reflection. 



In each cafe, the ray, as it falls either direftly on the eye, 

 or on the point of refleftioii, or of refraflion, is faid to be 

 incident : and the angle which the incident ray makes with 

 the perpendicular to the reflefting furface at the point i'{ 

 incidence, is called the angle of incidence ; and the angle 



RAY 



which the rcfi( fled ray makes with the fame ptrprndi- 

 tular, i.s called ihe angle of refl(ftion ; and alfo, the angle 

 which the refraiited ray makes with a perpendicular to the 

 refradting furface produced, is calhd the angle of re- 

 fraftion. 



Again, if feveral rays be propagated from the radiant 

 cquidiftantly fnm one another, they are called /i/raZ/c/ rays. 

 It they come inclining towards each other, they are called 

 converging rays. And if thi y go tontiniiiJly receding from 

 each other, they are called diverging rays. 



It is evident that the rays ot light, which come from a 

 luminous point, muft fall divergingly upon any given fur- 

 face ; yet when the objtft is very diftant, c(>mpared with 

 the interval that feparates thefe rays, the divergency of the 

 rays becomes infenfible ; hence the rays of the fun, of the 

 moon, of the ftars, &c. arc deem.ed parallel rays. When 

 the luminous point is near, then the rays are fcnfibly di- 

 verging. 



It is from the various circumftances of rays, that the fe- 

 veral kinds ot bodies are diftinguiflied in optics. A body, 

 e.gr. that difiiiles its own light, or emits rays of its own, is 

 called a /ucid, or luminous body. If it only refleft rays 

 which it receives from another, it is called an illuminated 

 body. 



This diftin£l;ion, however, is not always obferved ; for a 

 lun-.inous body, in common language, means any vifibleob- 

 je<ft, whether it be vifible by thecminion of original light, 

 like the I'un, a candle, &c. or by reflefted light, like the 

 moon, or any other celeltial or terreftrial objeft that has no 

 native light, or that docs not emit its own light-. 



If an objeft only tranfmit rays, it is called a tranfparenl 

 body. So that tranfparent bodies, fuch as water, glafs, 

 &c. are thole through which light will pafs, or through 

 which our eyes can perceive objefts fituated on the other fide, 

 and all fuch bodies, as alio a vacuum, are denominated me- 

 diums in optics. 



If it intercept the rays, or refufe them paffage, fo that 

 nothing can be feen through it, it is called an opaquehoAj . 



Hence no body radiates, /'. e. emits rays, unlefs it be either 

 luminous, or illumined. 



It is by means of rays reflefted from the feveral points of 

 illumined objefts to the eye, that they become vifible, 

 and that vifion is performed ; whence fuch rays are called 

 vifual, rays. 



When an eye views any objeft direftly, fome of the 

 rays, which proceed from every perceivable point of the 

 objtft, enter the eye, and the whole number of rays, or 

 quantity of light, which thus enters the eye, is circum- 

 fcribed by the rays which proceed from the extreme points 

 of the objed : and the angle which thefe extreme rays 

 form at the eye is called the vifual angle, from the mag- 

 nitude of which we principaUy judge of the diftance of a 

 known objeft. Thus, if the fame objeft is reprefented at 

 different diftances, it is evident that the farther the objeft is 

 from the eye, the fmaller will the vifual angle be. Suppof- 

 iiig alfo the diftance between the eye and the objeft to re- 

 main the fame, if by any means the rays of light are bent fo 

 as to enlarge the vifual angle, then the objeft will appear 

 larger, or it is faid to be magnified ; and on the contrary, if 

 the vifual angle be diminifhed, then the objedl will appear 

 fmaller, in which cafe it is faid to be diminifhed. 



In effetl, we find that any point of an objeft is feen in 

 all placet to which a right line may be drawn from that point : 

 but it is allowed, nothing can be feen without light ; there- 

 fore every point of an objeft diffufes innumerable rays evcrv 

 way. Again, from other experiments it appears, that the 

 images of all objefts, whence right lines may be drawn to 



the 



