0// the Lifie^-hn, R,J!,xh», and Clours of Light. ;6l 



iTJt 7'V: "" ":1°" ■' °'" °' '"^'^ ' '^"^"'-' '°S<^"^" -^'' ''^ ^''t--e from 

 d Leer of " "". 1 y°'" '™" *^ '""''' *^ ''^"''''' °^ '"^^ "-'^°-. -^ "'^ 

 c^c n ""r I T[V? ""^ ^-^ '°''°^^'-'- f" ««• 7' C - "- centre, and B . . 



he crcumfcrence of .he pm, G M the chart, and G D a line in it, being the axis of all the 

 .ages, a r,glu angles to C D, the diftance of C from D the centre of "the fnadow. and al- 

 fotothefl,acowufeir; G E is the parallel fide of the image, G being red, E violet, and F 



throngh B, the pomt where O B .s incident at the angle OB A, to which (by what was be- 

 fore (hown) AB F ,s equal. By meafurement G E is |th of an inch, CB -L,h CD 4" • 



to°II,f d-^ ?""°"''?"'' ^' ' P'"""'"' "^" "'^'"' ^° '^^''f 'J^^ fl.adow,^;d' their fum 

 to the d.Tlance between the penumbra and the violet, gave E D A/.^d.s of an in^h. From 

 whence ,t,seafy to calculate, that the angle of incidence being 77= 2oVthe angle of the 

 reds reflea,on A BG.S 75° 50' and that of the violets 78=5.'. Now the natural fines 

 ot 77J 20, 75° 5C, and 78- 5,', are as 9756, 9695, and 981 1 ; or as 250, 248, and 251. 

 which are very nearly as 77', 77, and 78; and making an allowance for the omiffion. 

 made m the reduftions, the errors in the operations and meafurements, they may be ac 

 counted as accurately in the above proportion. Now their extremes 77 and 78 are the 

 very proportions of the red's refrangibility to the violet's*. So that the reflexibility of the 

 red IS to that of the violets as the refrangibilities inverfely. But it is obvious that the fine 

 of modence is not the fame in the two cafes; for in the one it is equal to that of the mean 

 rays refieaion ; while in the other none of the rays are refrafled at an angle equal to' that 

 of incidence, otherwife they would not be refraded at all. This, however, being a confe- 

 quence of the elTential diftinaion in the circumftances, does not impair the beautiful ana- 

 logy which we have feen is preferved in the two operations, and which proves them to b- 

 different exertions of the fame power. Now we may find, from the data obtained, the fines 

 of all the rays in the fpeftrum by adding to 77, the lengths of the fpaces into which it is 

 divided, and ^vhlch are, without any fenfible error, as the differences of thofe fines The 

 fines of the red will be from 77 to 77' ; the orange from 77.^ to 77! -, the yellow from 77 ' 

 to 77 ■ ; the green from 77 ■ to 77^ j the blue from 77 Ho 77 J ; the indigo from 77i;: to 

 77-.- -. the violet from 77^ to 78. So that, the fine of incidence being given, that of the re- 

 flexion of all the different rays may be found; and the angle of incidence being 50° 48' the 

 angles of reflexion are as foUows : Of the extreme red 50- 21 '; of the orange 50= 27'- of 

 the yellow 50" 32'; of the green 50' 39'; of the blue 50= 48'; of the indigo 50" c;''; of 

 the violet 51" 3'; and of the extreme violet 51° 15'. 



I fliall conclude this part of the fubjeft with a kw remarks on the phyfical caufe of re- 

 flexibility. As light is rclleded by a power extending to fome dillunce from the lefleaing 

 furface, the different reflexibility of its parts arifes from a conftitutional difpofilion of thcfe 

 to be aflcd upon differently by the power. And as tliefc parts are of different fizes, thofe 

 which are largell will be afled upon mod ftrongly. I fliall not liefitatc to go a (lep farther. 

 In fig. 8, let EC be the refleding furface, DH the perpendicular, and Ai3 a ray incident 

 at B.-and produced to F, and refleaed into GB; draw Gil parallel to FB, and GF tq LIB • 

 then HB : (HG :) : BF : : fin. HGB : fin. HBG or : : fin. GBF : fin. IIBG. But GPF il 



• Optics, Book i. Part I. rro].. -. 



Vol. I.-M..RCH 1798. 4C t[.« 



