94 MR. J. SMITH ON THE ORIGIN OF COLOUR 



Newton explained the play of colours on soap bubbles 

 by tbeir ever varying thickness. He protected thenij and 

 watched them with care, and the result of his reasoning, 

 and the uses he put it to, are generally known. I do not 

 mean to follow him in his observations, or in his argu- 

 ments, for our principles of reasoning have nothing in 

 common. I wish, however, to say that twenty years ago, 

 I examined most carefully the same phenomena, and 

 although I have not repeated my observations since then, 

 they made such an indelible impression on my mind that 

 nothing can remove it. When examining the play of 

 colours on the bubbles T remarked, what every one who 

 has attended to the subject must have done, that there 

 was a motion caused by the action of gravity, and also 

 a motion attributable to that of the air within the 

 bubble. The air, within and without, being of different 

 temperatures, there must necessarily be motion, either 

 until the bubble bursts or until an equilibrium is estab- 

 lished; or should there be sufficient tenacity in the 

 infusion, these waves or circles remain stationary for a 

 time. These two motions, or even one of them at a time, 

 are quite sufficient on our principles of reasoning to 

 explain the difficulties attending this hitherto intricate 

 subject; for it becomes only another case of the same 

 kind as that of thin plates. The motion within makes the 

 little waves or facets move in a different direction to those 

 on the outer surface of the bubble. We have, therefore, 

 two transparent planes or surfaces reflecting light, the one 

 from the eye, the other to the eye; the one causing a 

 shadow, and reducing the number of vibrations of the 

 incident light, the other illuminating that shadow. 



From these two notable phenomena of thin plates and 

 soap bubbles is to be derived the law which regulates the 

 production of colour in many other natural objects. 



145. The doctrine of complementary colours, as taught 



