RAYS. 



I 



the eye, are painted in the eye, behind the cryllaUine, very 

 Imall, but very dilUndL 



And lalUy, from other experiments, that each ray 

 produces an image of the radiating point : and that the 

 feveral rays emitted from the fame point are again united in 

 one point, by the cryllaOine, and other humours of the eye, 

 and thus thrown on the retina. 



It is the fpifTitude or clofenefs of the rays emitted from a 

 luminous body, that conllitutos the intenfenefs of the hght. 

 Yet the direction in whicli the rays ilrike the eye, has acon- 

 fidcrable influence. In cifea, a perpendicular ray, ilriking 

 with more force than an obhque one, in the ratio of the 

 whole fine to the fine of the angle of obliquity (as follows 

 from the laws of pcrcujfion, which fee,) a perpendicular 

 ray will affeft the eve more vividly than an oblique one, in 

 that ratio. 



If then the fpiffitude of the rays be equal, the intenfity 

 will be as the direftion : if the dircftion be the fame, the 

 intenfenefs will be as the fpifiitude : if both differ, the in- 

 tenfenefs will be in a ratio compounded of the direction and 

 the fpiffitude. 



Hence, firft, if light be propagated in parallel rays 

 through an uiirefilling medium, its intenfity will not be varied 

 by diftance. 



Secondly, if light be propagated in diverging rays through 

 an unrefifling medium, its intenfity will decreafc in a du- 

 plicate ratio of the dillances from the radiant point, re- 

 ciprocally. 



Thirdly, if light be propagated in converging rays through 

 an unrefiiling medium ; its intenfity will increafe in a dupli- 

 cate ratio of the dillances from the point of concourfe, re- 

 ciprocally. 



Fourthly, if the breadth of an illumined plane be to the 

 diftance of the radiant point as i to 2,000,000, it is the fame 

 thing as if the ray ftruck upon it parallel : and hence, fince 

 the diameter of the pupil of the eye, when largeft, fcarcely 

 exceeds -^th or ^ th of an inch ; the rays will fall upon it pa- 

 rallel as tofenfe, at the diftance of 3860 Englifhfeet, which 

 is nearly fix furlongs. See Light. 



The effeft of concave lenfes, and convex mirrors, is to 

 make parallel rays diverge j converging rays become parallel ; 

 and diverging rays to become more divergent. 



The enedt of convex lenfes, and concave mirrors, is to 

 make diverging rays become parallel; parallel rays become 

 convergent ; and converging rays to converge the more. 

 See Lens. 



The rays of light are not homogeneous, or fimilar, but 

 differ in all the properties with which we are acquainted ; 

 liiz. refrangibility, reflexibility, and colour. 



It is probable that from the different refrangibility the 

 other differences have their rife ; at leaft it appears, that thofe 

 rays which agree or differ in this, do fo in all the reft. 

 Thus, from the different fenfations the differently dif- 

 pofed rays excite in us, we call them red rays, yellow 

 rays, &c. 



The effeft of the prifm is to feparate and fort the differ- 

 ent kinds of rays, which come blended promifcuoufly from 

 the fun ; and to throw each kind by itfelf, according to 

 its degree of refrangibility and colour, red to red, blue to 

 blue, &c. 



Befides refrangibility, and the other properties of the 

 rays of light already afcertained by obfervation and expe- 

 riment, fir Ifaac Newton fufpefts they may have many 

 more ; particularly, a power of being infledled, or bent, 

 by the aftion of diftant bodies ; and thofe rays which differ 

 in refrangibility, he conceives likewife to differ in this 

 flexibility. 



In paffmg by the edges and fides of bodies, he conceives 

 that the rays may be bent feveral times backwards and for- 

 wards, with a motion like tliat of an eel ; and that thofi 

 rays which appear to fall on bodies are reflcfted or refrafttd 

 before they arrive at tlie bodies : and adds, that they 

 may be refrafted, reflefted, and inflcfted, all by the fame 

 pnnciple afting in different circumftances. See Inblec - 

 'iiON and Light. See alfo Reflection and Rkfrac- 



TION. 



Again, do not the rays, falling on the bottom of the 

 eye, excite vibrations in the retina ; which, being propa- 

 gated along the fibres of the optic nerve into the brain, 

 caufe vifion ? and do not feveral forts of rays make vibrations 

 of feveral bigneffes, which excite fenfations of feveral co- 

 lours, much after the manner as the vibrations of the air, 

 according to their feveral bigneffes, excite fenfations of fe- 

 veral founds ? V 



Particularly, do not the moft refrangible rays excite the 

 fhorteft vibrations, to make a fenfation of a deep violet ; 

 and the leaft refrangible the largeft, to make a fenfation of 

 a deep red ? and the feveral intermediate kinds of rays, vi- 

 brations of intermediate bigneffes, to make fenfations of the 

 intermediate colours ? 



And may not the harmony and difcord ef colours arife 

 from the proportion of thefc vibrations ; as thofe of found 

 depend on the vibrations of the air ; for fome colours, if 

 viewed together, are agreeable, as gold and indigo ; others 

 difagreeable. See Colol'k. 



Again, iiave not the rays of light feveral fides endued 

 with feveral original properties ? It is certain we find, that 

 every ray of hght has two oppofite fides, originally endued 

 with a property, on which the unufual refrattion of ifland 

 cryftal depends, and other two oppofite fides endued with 

 that property. 



Laftly, are not the rays of light very fmall bodies emitted 

 from fhining fubftances ? 



Such bodies may have all the conditions of light : and 

 there is th*t aC\ion and re-aftion between tranfparent bodies 

 and light, which very much refembles the attraftive force 

 between other bodies. Nothing more is required for the 

 produftion of all the various colours, and all the degrees of 

 refrangibility, but that the rays of light be bodies of dif- 

 ferent fizes ; the leaft of which may make violet the weakeft 

 and darkeft of the colours, and be the moft eafily diverted 

 by refracting furfaces from its reftilinear courfe ; and the 

 reft as they are bigger and bigger, may make the ftronger 

 and more h cid colours, blue, green, yellow, and red. (See 

 Colour and Light.) Nor is any tlung more requifite 

 for the putting of the rays into fits of eafy refleftion, and 

 eafy tranfmiffion, than that they may be fmall bodies, 

 which, by attrsftion, or fome other force, excite vibrations 

 in the bodies they act upon ; which vibrations, being fwifter 

 than the rays, overtake them fucceffively, and agitate them 

 fo as by degrees to increafe and diminifti their velocity, and 

 thereby put them into thofe fits. Laftly, the unufual re- 

 fraction of ifland cryftal appears very much as if it were 

 performed by fome attractive virtue lodged in certain fides 

 both of the rays, and of the cryftal. 



Rays of Heat denote feparate portions of that emana- 

 tion, which proceeds fr«m an heated body, when placed 

 in a colder temperature, and which expands itfelf in every 

 direftion, provided it be not prevented by the interpofition 

 of particular fubftances. This appellation is adopted and 

 thus applied, not becaufe that emanation is certainly known to 

 confilt of feparate ftreams, but merely for the convenience of 

 explanation. The rays of heat are not the fame with the 

 ray& of light. If this were the cafe, a certain quantity 



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