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The variation in refractive index for different colours 

 results in the breaking up of a beam of white light into its 

 constituent colours when it suffers refraction at a glass 

 surface. The phenomenon is spoken of as the " dispersion " 

 of the light; and the extent to which a glass of given index 

 will produce this dispersion is alluded to as the " dispersive 

 power " of the glass. 



A single lens, therefore, under the most favourable con- 

 ditions can only bring light of one particular colour to a 

 focus at a particular point with the result that, when an 

 object is examined in ordinary light with such a lens, the 

 image is ill-defined and heavily fringed with colour. To 

 correct this fundamental defect in all refracting media, it 

 is necessary to make use of the device of combining convex 

 and concave lenses of different glasses. The dispersion and 

 deviation produced by the two lenses are thus opposed. 

 With crown and flint glasses it may be arranged that the 

 dispersion of the concave flint lens balances and neutralises 

 that of the crown lens, whilst the deviation due to the flint 

 only partially balances that of the convex crown lens. The 

 two lenses together, therefore, form a system which converges 

 rays of all colours to the same focus without dispersion, 

 giving an achromatic image practically free from all colour 

 fringes. 



It will, therefore, be seen from this rough indication of 

 the use of optical glass that, besides the " mean " index of 

 refraction, it is essential in the design of lens systems that 

 the computer should be acquainted accurately with the 

 dispersion for different rays of light which is produced by 

 the glass under consideration. 



It is usual for the difference between refractive indices for 

 the red and green rays, C and F, to be stated ; n v n c 

 being spoken of as the " mean," " middle," or " medium " 

 dispersion of the glass. 



From what has been said above, it will be seen that it 

 is of importance to know the relation between the deviation 

 and dispersion produced in optical glass, and it is, therefore, 

 frequently convenient to have stated the value of the number 

 which results by dividing the mean index minus I by the 

 mean dispersion; this quantity, which is the reciprocal of 

 the dispersive power of the glass, is spoken of as u, and has 

 sometimes been called the " reciprocal dispersive power." 

 We, therefore, have the relation 



