174 NEWTON'S SCALE OF COLOUBS. SECT. XX. 



The phenomenon of the coloured rings takes place in vacuo as 

 well as in air, which proves that it is the distance between the 

 lenses alone, and not the air, which produces the colours. How- 

 ever, if water or oil be put between them, the rings contract, but 

 no other change ensues ; and Newton found that the thickness 

 of different media at which a given tint is seen is in the inverse 

 ratio of their refractive indices, so that the thickness of laminge 

 which could not otherwise be measured may be known by their 

 colour ; and, as the position of the colours in the rings is inva- 

 riable, they form a fixed standard of comparison, well known as 

 Newton's scale of colours ; each tint being estimated according 

 to the ring to which it belongs from the central spot inclusively. 

 Not only the periodical colours which have been described, but 

 the colours seen in thick plates of transparent substances, the 

 variable hues of feathers, of insects' wings, mother-of-pearl, and 

 of striated substances, all depend upon the same principle. To 

 these may be added the coloured fringes surrounding the shadows 

 of all bodies held in an extremely small beam of light, and the 

 coloured rings surrounding the small beam itself when received 

 on a screen. 



When a very Blender sunbeam, passing through a small pin- 

 hole into a dark room, is received on a white screen, or plate of 

 ground-glass, at the distance of a little more than six feet, the 

 spot of light on the screen is larger than the pin-hole : and, 

 instead of being bounded by shadow, it is surrounded by a series 

 of coloured rings separated by obscure intervals. The rings are 

 more distinct in proportion to the smallness of the beam (N. 201). 

 When the light is white there are seven rings, which dilate or 

 contract with the distance of the screen from the hole. As the 

 distance of the screen diminishes, the white central spot contracts 

 to a point and vanishes ; and, on approaching still nearer, the 

 rings gradually close in upon it, so that the centre assumes suc- 

 cessively the most intense and vivid hues. When the light is 

 homogeneous red, for example the rings are alternately red 

 and black, and more numerous ; and their breadth varies with 

 the colour, being broadest in red light and narrowest in violet. 

 The tints of the coloured fringes from white light, and their obli- 

 teration after the seventh ring, arise from the superposition of 

 the different sets of fringes of all the coloured rays. The shadows 

 of objects are also bordered by coloured fringes when held in this 



