touching his Theory of Light and Colours. 207 



ought to be transmitted at E, I, n and K; and this because 

 the ray B C arrives at the superficies A C, when it is con- 

 densed by the first wave that overtakes it; D E when rarified 

 by the interval of the first and second ; F G when condensed 

 by the second wave ; H I when rarefied by the interval of the 

 second and third, and so on for an indeterminate number of 

 successions; and at A, the centre, or contact of the glasses, 

 the light must be transmitted, because there the aetherial me- 

 diums in both glasses are continued as if but one uniform 

 medium. Whence if the glasses in this posture be looked 

 upon, there ought to appear at A, the contact of the glasses, 

 a black spot, and about that many concentric circles of light 

 and darkness, the squares of whose semi-diameters are to sense 

 in arithmetical progression. Yet all the rays without excep- 

 tion ought not to be thus reflected or transmitted : for some- 

 times a ray may be overtaken at the second superficies by the 

 vibrations raised by another collateral, or immediately succeed- 

 ing ray; which vibration being as strong, or stronger than its 

 own, may cause it to be reflected or transmitted when its own 

 vibration alone would do the contrary. And hence some 

 little light will be reflected from the black rings, which makes 

 them rather black than totally dark ; and some transmitted at 

 the lucid rings, which makes the black rings appearing on 

 the other side of the glasses not so black as they would other- 

 wise be. And so at the central black spot, where the glasses 

 do not absolutely touch, a little light will be reflected, which 

 makes the spot darkest in the middle, and only black at the 

 verges. For thus I have observed it to be, by tying very hard 

 together two glass prisms which were accidentally (one of 

 them at least) a very little convex, and viewing by divers 

 lights this black spot at their contact. If a white paper was 

 placed at a little distance behind a candle, and the candle and 

 paper viewed alternately by reflexion from the spot, the verges 

 of the spot which looked by the light of the paper as black as 

 the middle part, appeared by the stronger light of the candle 

 lucid enough, so as to make the spot seem less than before ; 

 but the middle part continued as absolutely black in one case 

 as in the other, some specks and streaks in it only excepted, 

 where I suppose the glasses through some unevenness in the 

 polish did not fully touch. The same I have observed by 

 viewing the spot by the like reflexion of the sun and clouds 

 alternately. 



But to return to the lucid and black rings ; those rings 

 ought always to appear after the manner described, were light 

 uniform. And after that manner, when the two contiguous 

 glasses AQ and AR have been illustrated in a dark room 



