On th AffiSlions and properties ef Light, -Ijl 



tng body's action reaches farther when exerted on fome rays than when exerted on others : 

 but then, the direction of the rays not paffing through the body, thofe which are fartheft off 

 and at too great a diftance to be bent, never coming nearer, are not bent at all ; and con- 

 fequently as the ieaft flexible rays are in this predicament at the fmalleft diilance, and the , 

 tnoft flexible not till the diftance is greater, the images formed out of the former muft be 

 lefs than thofe formed out of the latter. This difference in the way in which the pheno- 

 menon appears, does not argue the fmalleft difference in the caufe : it oa'y follows from the 

 different pofition of the rays, with refpe(^ to the a£ting body, in the two cafes. I infer then 

 from the whole, that different forts of rays come within the fpheres of flexion, reflexion, 

 and refracStion, at different diftances, and that the aftions of bodies extend fartheft v/hen 

 exerted on the moft flexible. It may perhaps be confiftent with accuracy and convenience 

 to give a name to this property of light ; we may therefore fay that the rays of light diff.r in 

 degree of refrangity, reflcxity, and flexity, comprehending inflexity and dcflexity. From 

 thefe terms (uncouth as, like all new words, they at firft appear) no confufion can arife, 

 if we always remember that they allude to the degree of diftance to which the rays are fub- 

 jciS to the aflion of bodies. I {hall only add an illuftration of this property, which may 

 tend to convey a clearer idea of its nature. Suppofe a magnet to be placed fo that it may 

 attract from their courfe a ftream of iron particles, and let this ftream pafs at fuch a dif- 

 tance that part of it may not be affected at all : thofe particles which are attracted may be 

 conceived to ftrilce on a white body placed beyond the magnet, and to make a mark there of 

 a fize proportional to their number. Let now another equal ftream confiderably adul- 

 terated by carbonaceous matter, oxygen, &c. pafs by at the fame diftance, and in the fame 

 diredlion. Part of this will alfo be attracied, but not fo far from its courfe, nor will an 

 equal number be affected at all ; fo that the mark made on the white body will be nearer 

 the dire£tion of the ftream, and of lefs fize than that made by the pure iron. It matters 

 not whether all this would a6tually happen, even allowing we could place the fubjeiSts in 

 the fituation defcribed : the thing may eafily be conceived, and affords a good enough illuf- 

 tration of what happens in the cafe of light. 



Purfuant to the plan I before followed, I now tried to meafure the different degrees of re- 

 flexity, &c. of the different rays ; but though the meafurements which I took agreed in 

 this, that the red images were much larger than the reft, and the green appeared by them of 

 a middle fize, yet they did not agree well enough (from the roughriefs of the images, and 

 feveral other caufes of error) to authorize us to conclude with any certainty " that the 

 action of bodies on the rays is in proportion to the relative fizes of thefe rays." This, how- 

 ever, will moft probably be afterwards found to be the cafe : in the mean time there is little 

 doubt that the fizes are the caufe of the fact. 



II. 



SEVERAL phenomena are eafily explicable on the principles juft now laid down. 



I. If a pin, hair, thread, he. be held in the rays of the fun refraded through a prifm, ex- 

 tending through all the feven colours, a very fingular deception takes place: the body ap- 

 pears of different fizes, being largeft in the red, and decreafing gradually towards the violet. 

 This appearance feemed fo extraordinary, that fome friends who happened to l(:e it as well as 



o myfclfj 



