210 Newton's Letters^ Hypothesis and Experiments 



so on ; yea the colours of every ring must spread themselves 

 something more both ways than is here represented, because 

 the quadrantal arcs here described represent not the verges, 

 but the middle of the rings made in a dark room by the ex- 

 treme violet and red; the violet falling on both sides the pricked 

 arches, and red on both sides the black line arches. And 

 hence it is that these rings or circuits of colours succeed one 

 another continually without any intervening black, and that 

 the colours are pure only in the three or four first rings, and 

 then interfering and mixing more and more, dilute one another 

 so much, that after eight or nine rings they are no more to be 

 distinguished, but seem to constitute an even whiteness; 

 whereas when they were made in a dark room, by one of the 

 prismatic colours alone, I have, as I said, seen above twenty 

 of them, and without doubt could have seen them to a greater 

 number, had I taken the pains to make the prismatic colour 

 more uncompounded. For by unfolding these rings from one 

 another by certain refractions expressed in the other papers* 

 I send you, I have even in daylight discovered them to above 

 a hundred, and perhaps the}' would have appeared innumera- 

 ble, had the light or colour illustrating the glasses been ab- 

 solutely uncompounded, and the pupil of my eye but a ma- 

 thematical point, so that all the rays which came from the 

 same point of the glass might have gone into my eye at the 

 same obliquity to the glass. What has been hitherto said of 

 these rings is to be understood of their appearance to an un- 

 moved eye; but if you vary the position of the eye, the more 

 obliquely you look upon the glass the larger the rings appear. 

 And of this the reason may be, partly that an oblique ray is 

 longer in passing through the first superficies, and so there is 

 more time between the waving forward and backward of that 

 superficies, and consequently a larger wave generated ; and 

 partly that the wave in creeping along between the two su- 

 perficies, may be impeded and retarded by the rigidness of 

 those superficies bounding it at either end, and so not overtake 

 the ray so soon as a wave that moves perpendicularly across. 

 The bigness of the circles made by every colour and at all 

 obliquities of the eye to the glasses, and the thickness of the 

 air or intervals of the glasses, where each circle is made, you 

 will find expressed in the other papers I send you, where also 

 I have more at large described how much those rings inter- 

 fere or spread into one another, what colours appear in every 

 ring, where they are most lively, where and how much diluted 

 by mixing with the colours of other rings, and how the con- 

 trary colours appear on the back side of the glasses by the 



* Obs. 24. 



