26 



to the diverging rays of a concave lens. It will be 

 remembered however, that in order to have image 

 forming rays they must converge. 



As dispersion depends upon two conditions, the 

 angle of the prism or convexity of the lens and 

 dispersive power of the glass, it will be seen that 

 in two prisms of the same angle but made of glass 

 of different dispersive power, the separation of the 

 red and violet rays will be greatest in that having 

 the greatest dispersive power. Or, in two prisms of 

 the same density but one having one half the 

 angle of the other, the dispersion will be one half 

 as great in the prism of smaller angle. Again, in 

 the prism of twice the angle but one half the dis- 

 persive power of another the dispersion will be the 

 same. \Ve also know that in a prism having a 

 greater angle the refraction will be greater than 

 in one of lesser angle and as a refraction must be 

 maintained which will create convergence while at 

 the same time neutralizing the dispersion, a solu- 

 tion will offer itself in using in conjunction with a 

 prism of a certain angle and dispersion, another 

 of lesser angle and greater dispersion. If the 

 same principle be carried out in lenses, a convex 

 lens of low dispersion would be combined with a 

 concave lens of high dispersive power. Such a 

 lens is shown in Fig. 11, and is called an 

 achromatic or corrected lens. If the chromatic and 

 spherical aberrations are both corrected it is called 



