164 DISPERSION OF LIGHT. SECT. XIX. 



of gases and flames. In a highly-magnified spectrum from light 

 passed through nitrous acid gas, Sir David Brewster counted 

 2000 dark bands. In the spectrum of a lamp, and generally of 

 all white flames, none of the defective lines are found ; so all 

 such flames contain rays which do not exist in the light of the 

 sun or stars. Brilliant red lines appear in the spectrum pro- 

 duced by the combustion of nitre upon charcoal ; and in all 

 artificial flames dark and bright bands exist, sometimes corre- 

 sponding in position with those in the solar spectrum, and 

 sometimes not. 



A sunbeam received on a screen, after passing through a small 

 round hole in a window-shutter, appears like a round white spot ; 

 but when a prism is interposed, the beam no longer occupies the 

 same space. It is separated into the prismatic colours, and 

 spread over a line of considerable length, while its breadth re- 

 mains the same with that of the white spot. The act of spread- 

 ing or separation is called the dispersion of the coloured rays. 

 Dispersion always takes place in the plane of refraction, and is 

 greater as the angle of incidence is greater. It varies inversely 

 as the length of a wave of light, and directly as its velocity : 

 hence towards the blue end of the spectrum, where the undula- 

 tions of the rays are least, the dispersion is greatest. Substances 

 have very different dispersive powers ; that is to say, the spectra 

 formed by two equal prisms of different substances, under pre- 

 cisely the same circumstances, are of different lengths. Thus, 

 if a prism of flint-glass and one of crown-glass of equal refracting 

 angles be presented to two rays *of white light at equal angles, 

 it will be found that the space over which the coloured rays are 

 dispersed by the flint-glass is much greater than the space occu- 

 pied by that produced by the crown-glass : and as the quantity 

 of dispersion depends upon the refracting angle of the prism, the 

 angles of the two prisms may be made such that, when the 

 prisms are placed close together with their edges turned opposite 

 ways, they will exactly oppose each other's action, and will 

 refract the coloured rays equally, but in contrary directions, so 

 that an exact compensation will be effected, and the light will 

 be refracted without colour (N. 195). The achromatic telescope 

 is constructed on this principle. It consists of a tube with an 

 object-glass or lens at one end to bring the rays to a focus, and 

 form an image of the distant object, and a magnifying-glass at 



