On the IiifHicion, RrJJex'ion, and Colours of Lighl. 555- 



tends to a diHance TMch greater than that of apparent contaa, at vl'hich an attmaion again 

 begins ftill ata diftance, though fefs thair fiiat at which before there \va5 a repul/ion ; as 

 will appear by the following demon (Iration, which occurs to me, and which is general with 

 -repea to the theory of Bofcdvich*. In Fig. 4, let the body A have for P an attraflion 

 which at the diftarice of A P is proportional to P M ; then let P move towards A, fo as 

 to come to the fituationP', and let the attraaion here be'T'M'; as it is continual during 

 ■the motion of P to P', M M' is a curve line. Now in the cafe of the attraaion of bodies 

 •for light, and for one another,'? M is lefs than P' M' ; and confequently MM'does nat 

 ever return into itfelf, and therefore it muft go ad hiJinUum, having its arc between A B 

 and AC, to which it approaches as afymptotes, the abfcifla always reprefenting thc"^- 

 tance, and the ordinate the attraaion at that dillance. Let P' now continue its motion to 

 P", and M'will move M"; and if P" meets A, or the bodies come into perfea contaa, 

 V M" will be infinite ; fo that the attraaion being changed into cohefion will be infinite, 

 - and the bodies infeparable, contrary to uriiverfal experience ; fo that P can never come 

 nearer to A than a given difiance. In the cafe of gravity P M is inverfcly as the fquare of 

 A P, fo that the curve >J M M"' is the cubic hyperbola ; but the demonftrarion holds, what- 

 ever be the proportion of the force to the diftaace. It appears then, that fleilon, refraaion, 

 and reflexion, are performed by a force adling at a definite diftance ; and it is rei,fonable 

 to think even a priori, that, as this fame force in other circumllances is exerted to a different 

 degree on the different parts of light in refraaing, infleaing, and defleaing tiiem, it 

 Ihould alfo be exercifed with the like variations in refleaing them. Let us attend to the 

 proof which enables us to change conjeaure into conviaion. 



Obj. I. The fun fhining into ray darkened chamber through a fmall hole ^gth of an inch 

 in diameter, I plated a pin of ^th of an inch diameter in the cone of light (one half inch 

 from the hole) inclined to the rays at an angle of about 45° j and its fliadow was received 

 on a chart parallel to it, at the diftance of two feet. The fliadow was furrounded by the 

 three fringes on each fide, difcovcred by Grimaldo ; beyond thefe there were two ftreaks of 

 white light diverging from the fii.ido'.v, arid mottled with' bright colours, very irregularly 

 fcattered up and down; but pnufing. another pifl, whofe furface was well poliflied,' an'd 

 placing it nearer the"hole|than before, the coJaurs \\\ the iheaks became much ■ brigjiter 

 (and the ftreaks themfelves narrower), being extended from ene fide to theother, fo thai 

 except in a yery few points, here and <here, no white w,as now to be fcen.; and, on moving 

 the pin, the colours moved alfo< But tliey difappeared if the pin was. deprived of its no- 

 lidi by' being held in the flame of a candle, or if a roll of paper was ufed inflead of the 

 j)i^ii,alfo they were much brighter in diiea thjm in n^lliaed light, and 'ill thclio-ht of the 

 .furiji^ the focifs of a lens, cluin in his direa unrefraacd light. Placing a pitjce of paper 

 round the hole in the windoyi-fliut, I obfcrved the colours concinuqd there 1 aivdinriiriinK 

 •tlie chart to tlte point where they left off, Ifaw them continued 0n it ■ aiid then pixiceed 

 as before to the fhadow. If the piu was Iicld hovizontally, or nearly fo, they were feen of 

 a great fizc on the floor the walls and roof of the room, forming a large circle ; and if tire 

 ^hart; was laid horizontally, and ihe pin held between the hole and it, in a vertical pofition, 

 thcicirclc vvas fccn ou the chart, and became -.u> oval by Jiiftlining tiic piri a little to the ho- 



,Ni/va T.iioria Philnfophia; Nuturalis. 



A B Z ' Obf. 



