DISPERSION. 173 



a plano-convex lens, which is consequently the best 

 form for a simple lens. 



Dispersion, or Chromatic Aberration. The rays of 

 light have so far been considered as simple. They 

 are, however, in reality compound, consisting of a 

 number of primary- coloured rays, of which seven kinds 

 are easily distinguishable, viz. red, orange, yellow, 

 green, blue, indigo, and violet. The coloured rays of 

 the sun are, as is well known, often seen separated 

 by the action of the triangular glass bars or prisms 

 forming the lustres of a chandelier ; the separation 

 arising from the different refrangibility of the coloured 

 rays, by which each is refracted to a different degree 

 from that of the others. This is shown in PL XII. 

 fig. 19, representing a ray of white light entering a 

 triangular prism, at the surface of which the paths of 

 the rays become different according to the degree of 

 their refrangibility, whence they emerge separately, 

 forming a spectrum at v r ; the most refrangible violet 

 rays (v) being most refracted, the less refrangible red 

 (r) least so, the intermediate rays being refracted to 

 intermediate degrees according to their respective re- 

 frangibilities. This separation of the coloured rays 

 is called dispersion; and as different substances or 

 media disperse the coloured rays over a larger or 

 smaller space, so as to produce spectra of different 

 lengths, they are said to possess different dispersive 

 powers. Thus the dispersive power of flint glass and 

 balsam are about equal, while that of crown glass is 

 considerably less. 



The extent to which dispersion is produced by the 

 same medium also depends upon the angle of the 

 prism, being greater as the angle is larger ; increased 

 obliquity of the incident light also increases the dis- 

 persion, so that the spectrum produced by a small 

 prism may be equal to that produced by a larger one 

 upon which the light is less obliquely incident. 



In consequence of the dispersion of light, rays pass*. 



