Interference-Bands. 201 



inverted by the refraction in such manner that if I first held 

 the prism very near the object-glasses, and then gradually 

 removed it further off towards my eye, the colours of the 2nd, 

 3rd, 4th, and following rings shrunk towards the white that 

 emerged between them, until they wholly vanished into it at 

 the middle of the arc, and afterwards emerged again in a con- 

 trary order. But at the ends of the arcs they retained their 

 order unchanged." 



" I have sometimes so laid one object-glass upon the other, 

 that to the naked eye they have all over seemed uniformly 

 white, without the least appearance of any of the coloured 

 rings ; and yet, by viewing them through a prism, great 

 multitudes of these rings have discovered themselves. And 

 in like manner, plates of Muscovy glass, and bubbles of glass 

 blown at a lamp-furnace, which were not so thin as to exhibit 

 any colours to the naked eye, have through the prism exhibited 

 a great variety of them ranged irregularly up and down in the 

 form of waves. And so bubbles of water, before they began 

 to exhibit their colours to the naked eye of a bystander, have 

 appeared through a prism, girded about with many parallel 

 and horizontal rings ; to produce which effect it was necessary 

 to hold the prism parallel, or very nearly parallel, to the 

 horizon, and to dispose it so that the rays might be refracted 

 upwards." 



Newton was evidently much struck with these " so odd 

 circumstances," and he explains the occurrence of the rings 

 at unusual thicknesses as due to the dispersing power of the 

 prism. The blue system being more refracted than the red, 

 it is possible, under certain conditions, that the nth blue ring 

 may be so much displaced relatively to the corresponding red 

 ring as at one part of the circumference to compensate for the 

 different diameters. White and black stripes may thus be 

 formed in a situation where, without the prism, the mixture of 

 colours would be complete, so far as could be judged by the eye. 



The simplest case that can be considered is when the " thin 

 plate " is bounded by plane surfaces inclined to one another 

 at a small angle. Without the prism, the various systems 

 coincide at the bar of zero order. The width of the bands is 

 constant for each system, and in passing from one system to 

 another is proportional to X. Regarded through a prism of 

 small angle whose refracting edge is parallel to the intersection 

 of the bounding surfaces of the plate, the various systems no 

 longer coincide for zero order ; but by drawing back the 

 prism, it will always be possible so to adjust the effective dis- 

 persing power as to bring the nth bars to coincidence for any 

 two assigned colours, and therefore approximately for the 



