130 COLOURS OF THIN PLATES. 



convex lens of glass upon a plane surface of the same mate- 

 rial. The thickness of the inclosed plate of air increases as 

 the square of the distance from the point of contact, and is, 

 therefore, the same at all equal distances from that point ; 

 and, as the reflected colour depends on the thickness, the 

 bands of the same colour will be arranged in concentric 

 circles, of which that point is the centre. The same suc- 

 cession of colours is produced when any other transparent 

 fluid is inclosed between the glasses. The colours, how- 

 ever, are more vivid, the more the refractive power of the 

 thin plate differs from that of the substances within which it 

 is inclosed. 



When we look attentively at these rings, the light being 

 reflected always at the same angle, we observe that the cen- 

 tral one is not a mere annulus, but a complete circle of nearly 

 uniform colour. If then we diminish the thickness of the 

 plate of air, by pressing the two glasses more closely together, 

 this central circle is observed to dilate, and a new circle of a 

 different colour to spring up in its centre. This will dilate 

 in turn, driving the former before it, and another circle ap- 

 pear within it ; until at length a black spot shows itself in 

 the centre of the system, after which no further diminution 

 of thickness will alter the succession. "When the black spot 

 makes its appearance, we have obtained a plate of air so 

 thin as no longer to reflect any colours, and the phenomenon 

 is complete. Newton traced seven coloured rings round 

 this spot, the colours of which are said to be of the first, se- 

 cond, third, &c., order, according to the order of the ring to 

 which they belong. Thus, the red of the third order is the 

 red in the third ring from the central black, &c. The whole 

 succession of colours is called Newton' 's scale. 



(143) The principal laws of these phenomena are included 

 in the following propositions : 



