194 LENGTH OF THE UNDULATIONS. [SECT. xx. 



which consists of all the colours, is consequently a mixture 

 of waves of all lengths between the limits of the extreme 

 red and violet. The determination of these minute portions 

 of time and space, both of which have a real existence, being 

 the actual results of measurement, do as much honour to 

 the genius of Newton as that of the law of gravitation. 



The phenomenon of the coloured rings takes place in 

 vacua as well as in air ; which proves that it is the distance 

 between the lenses alone, and not the air, which produces 

 the colours. However, 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 laminae which could not otherwise be 

 measured, may be known by their colour ; and, as the position 

 of the colours in the rings is invariable, 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 prin- 

 ciple. To these may be added the coloured fringes, sur- 

 rounding 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 slender 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. 196). When the light is white, 

 there are seven rings, which dilate or contract with the dis- 



