jSS Oil the Inflexion, Re/lfxion, and Colours of LigSt, 



•colours; and in like manner by the dilTerent deflexibilityof the parts whereof S'D'conrifl»> 

 an image without the fliadow, as V'G'R', will be formed, fimilar to VGR, R' being red». 

 and V violet, all which is both theory and experience ; and the fame explanation may be 

 extended to the other cafes. Now in all thefe, the bending power ftretching to a very fmall 

 •defmite dillance, and being of different degrees of ftrength at different diftances from the 

 body, feveral pencils or fmall beams, palTing through ditferenc parts of the fphcres, will be 

 acted upon by the power in its different ftates of ftrength, and each beam will be difp-fcd 

 into an image in the way before defcrlbed. Of thefe images [ have fometimes obrer\'ed four, 

 and even, by ufing great care, the faint lineaments of a fifth. In forming them, the power 

 ads ftrongeft at the fmalleft diftances, and of confequence bends the mean flexible rays 

 that pafs near, farther inwards or outwards than thofe that pafs farther off; fo that the 

 extreme rays will in the former cafe be more feparated from the mean than in the latter; and 

 tl»e nearer image will always be the largeft and moft highly coloured ; which is confiftent 

 »itl» facl. This e.\plains fully the celebrated experiment of Sir Ifaac Newton with the 

 knives, and the explanation is confirmed by the experiments which I related above on 

 flexibility, where the bending force a£led moft ftrongly on thofe images formed out of 

 red light, and leaft ftrongly on thofe formed out of violet and blue light. A number of 

 other phenomena are explicable on the fame principles, being only particular cafes, as it 

 were, of the coloured fringes or images: I (hall here mention a few of the moft remarkable. 

 6. When making fome of the experiments which I have related in the courfe of this 

 paper, I obfiirved, that when the fun was furrounded, but not covered, by clear white 

 clouds, the white image on the chart (the hole being i [■ inch in diameter) was furrounded 

 by two rainbows pretty broad and bright ; in the colours were red on the outfide, and violet 

 next the white of the image. Thefe bows muft not be confounded with one which fome- 

 times appears wholly of a dull red and yellow, when the fun or moon ftiines through a 

 cloud, and which is owing to the direcl tranfmiffion of the red rays, and reflexion of the 

 others; for not only are the colours different in fpecies, in brightnefs, and in nuniber, in 

 the phenomena under difcuffion, but lilccwife they are formed by the hole in the window, 

 as I knew by altering its ftiape into an oblong; and the colours now were not difpofcd in 

 circles, but in broad lines of the fame breadth as the bows had been, running along the 

 fhadow of the hole's fides, and in the fame pofition of colours as before. It is evident that 

 thtir caufe is the inflexion of the light which comes from the clouds by the fides of the 

 hole (for if the flcy have no clouds the colours do not appear) which feparatc the white light 

 into the parts of which it is compofed. 



7. It is cbfervable, that when we look at any luminous body at a diftance greater than 

 one or two feet, its flame appears furrounded by two bows of faint colours, the inner- 

 noft of them terminating in a white which continues to the flame ; and the colours arc red 

 outcrmoft, and green and blue innermoft : the appearaice is moft remarkable if we look 

 at a fmall hole in ihe window-lhur, the room being otherwlfe dark; and if the eye be prcffcd 

 upon, ano then opened, the colours are more lively than before, as Dcfcartes obfcrved * ; 

 from which bjth he and Newton concluded that the appearance was owing entirely to 

 wrinkles formed on the furface of tlie eye by the preffure t- But this could neitiier form. 



• Dc Mttcorilus. t Lefl. Optica. Scfl. III. ad fiuem. 



the 



