touching his Theory of Light and Colours. 197 



cles of various sizes springing from shining bodies at great 

 distances one after the other, but yet without any sensible in- 

 terval of time, and continually urged forward by a principle of 

 motion, which in the beginning accelerates them, till the re- 

 sistance of the ffitherial medium equal the force of that prin- 

 ciple, much after the manner that bodies let fall in water are 

 accelerated till the resistance of the water equals the force of 

 gravity. God, who gave animals motion beyond our under- 

 standing, is without doubt able to implant other principles 

 of motions in bodies which we may understand as little. Some 

 would readily grant this may be a spiritual one; yet a mecha- 

 nical one might be shown, did not I think it better to pass it 

 by. But they that like not this, may suppose light any other 

 corporeal emanation, or an impulse or motion of any other 

 medium or aetherial spirit diffused through the main body 

 of aether, or what else they imagine proper for this pur- 

 pose. To avoid dispute, and make this hypothesis general, 

 let every man here take his fancy; only whatever light be, I 

 would suppose it consists of successive rays differing from one 

 another in contingent circumstances, as bigness, force, or vi- 

 gour, like as the sands on the shore, the waves of the sea, the 

 faces of men, and all other natural things of the same kind 

 differ, it being almost impossible for any sort of things to be 

 found without some contingent variety. And further, I would 

 suppose it diverse from the vibrations of the aether, because 

 (besides that were it those vibrations, it ought always to verge 

 copiously in crooked lines into the dark or quiescent me- 

 dium, destroying all shadows, and to comply readily with any 

 crooked pores or passages as sounds do) I see not how any 

 superficies (as the side of a glass prism on which the rays 

 within are incident at an angle of about forty degrees) can be 

 totally opake. For the vibrations beating against the refract- 

 ing confine of the rarer and denser aether must needs make 

 that pliant superficies undulate, and those undulations will 

 stir up and propagate vibrations on the other side. And fur- 

 ther, how light, incident on very thin skins or plates of any 

 transparent body, should for many successive thicknesses of the 

 plate in arithmetical progression,- be alternately reflected and 

 transmitted, as I find it is, puzzles me as much. For though 

 the arithmetical progression of those thicknesses, which reflect 

 and transmit the rays alternately, argues that it depends upon 

 the number of vibrations between the two superficies of the 

 plate, whether the ray shall be reflected or transmitted, yet I 

 cannot see how the number should vary the case, be it greater 

 or less, whole or broken, unless light be supposed something 

 else than these vibrations. Something indeed I could fancy 



