^86 On the Irfixhn, Rcf^xit'i, and Co'ours of Light. 



poTtant V I haftcn now to tlic natural phenomena, tlic explanation of wliich depend* on 

 the property whofe e iftcncc and nature we have jull now been inveltijjaiiiig } and. that \vc. 

 may treat this part of the fu'jj-'ft v/l;h condfenefs and order, we fliall rank the plienomena 

 under a divlfion llinilar to that under which we laid down the principles, beginning with 

 thofe appearances wl-.ich are explicable on the principles of flexion. 



1. It is obfervablc, that when a body is expo.'ed in the fun's light fo as to cafl a fliadow,. 

 and another body is approached to it, either between the fun aiid it, or the fliadow and it, 

 or in the fame line with it, the fhadow of the one bodycomes out a eonfidorablc way, and 

 meets that of the other. Now it is evident, that when the bodies are held at a fufTicient dif^ 

 tancc from one another, a penumbra is formed round the fhadow of each, nuking it Icfs 

 than it fliould be were there no inflexion; but when the bodies are brought fo clofe to one 

 another th.it the edge of the one is within the fphereof the other's inflexion, the light being 

 already bent by this laft, the former can have none to bend, and confequcntly no penum- 

 bra in the part of the fliadow correfponding to that part of the body which i's within the 

 other's fphcre of infleiiion ; and the reft of the fhadow having a penumbra, this part that 

 has none will be larger than it, and increafe as the bodies approach, till at laft it meets 

 the other fliadow : the like appearance happening when the fliadows are thrown on the 

 eye. Mr. Mctvill has endeavoured to ftiew that it belongs fimply to a cafe of vifron*. 

 However, we have now fecn that it has no reference to the (truclure or pofition of the eye,: 

 but only to the common nature of all fliadows+. 



2. If w'C fliut out all the light coming into a room from external objects, except 

 ■what may pafs through a fmall hole of -i- or J th of an inch in diameter, the images of the 

 external objects, as clouds, houfes, trees, will be painted on the oppofite v/all by tiie rays 

 of light eroding at the hole ; but if a piece of rough glafs or of very fine paper be held fo 

 as to cover it all over, the light docs not pafs through; then if the paper be wetted with 

 oil, or the gliifswith water, fo as to give either a fmall degree of tranfparency, the firft rays 

 that come through are thofe from red and orange obje£ts, and laft &ora blue and violet. 

 Now it is evident that tranfparency in general, and this particular fa£l, are explicable by 

 what was before laid down. It was found by Newton, that a body tranfraits the light inci- 

 dent on it more or lefs according to the continuity of its particles; and that a ftrong re- 

 flexion takes place on the confines of a vacuum J: How does this happen? The initial* 

 velocity of light is fufTicient to carry it through the firft furfiice or fet of particles ; but it is 

 fo much diminiflied that it is refletled by the repullive power of the back fide of thefe parti- 

 cles, unlets there be others behind at a certain diftance, namely, that at which inflexion or 

 attraiflion afts, that is, apparent contact : this attraction renews the impetus of light, and 

 tranfmits it to another fct, and fo on. Now this aclion being ftrongeft on the largeft and 

 red particles, and weakeft on the blue and violet, if the continuity be dlminifhcd, the former 

 will be tranfmitted, and not the latter; which is conformable to the experiment jufl now men- 

 tioned. 



3. The dodlrine of flexibility furnifhes an eafy and fatisfaflory explanation of the 

 difl^erent colours which are aflumed by flame. Whether we fuppofe the light to come frora 

 the burning body or the oxygenous gas, the largell or red particles have the ftrongeft attrac- 

 . -' Edinburgh Literary ElTays, Vol. 11. -1 See, liov. ever, Philof. journ. I. 431. 



; OiJiics, Book ii. Part lU. Pruf. .:. 



5 ti«>n 



