ON LIGHT. 253 



of separating the differently-coloured rays and spreading 

 them over an angular space greater or less in proportion 

 to the total deviation of some one ray, taken as a stand- 

 ard, from its former course, is called in optics the " dis- 

 persive power" of the medium. It differs very widely in 

 different media, and in consequence, the lengths of the 

 spectra which they produce, corresponding to one and 

 the same mean or average refraction, differ accordingly. 

 Thus, for example, the total lengths of the spectra pro- 

 duced by prisms of fluorspar, water, diamond, flint glass, 

 and oil of cassia (the mean refractions being the same), 

 are to each other in the proportions of the numbers 22, 

 35, 38, 48, and 139. 



(36.) This quality of dispersion stands in very dis- 

 tinct relation to the chemical constitution of the refract- 

 ing medium. Thus it is found that all the compounds 

 of lead, whether in liquid solution, natural or artificial 

 crystals, or glasses into which that metal enters largely, 

 possess very high dispersive powers; while those into 

 which strontia enters exhibit remarkably low ones. It 

 is on this property of lead that the formation of highly 

 dispersive glasses, to imitate the brilliant colours of gems, 

 and to give the vivid prismatic colours of the pendants 

 of chandeliers by candle-light, depends, as well as that far 

 more important application which, by the combination 

 of two glasses of different dispersive powers, the one con- 

 taining lead, the other none, enables the optician to 

 effect refraction without producing colour, and so to con- 

 struct that admirable instrument, the achromatic telescope. 



(37.) Not only are the total lengths of the spectra pro- 



