CHAP. IV.] 



THE TELESCOPE. 



253 



construction of optical instruments by -'an English optician, of French 

 origin, named Dollond. 1 We refer to the achromatism of the lenses, 

 of which we have spoken when describing the microscope. 



When a ray of white light is refracted by a lens, the coloured 

 rays of which it is composed not having the same degree of refran- 

 gibility are dispersed, and give to the images formed fringes of 

 prismatic colours, which constitute a serious defect in the production 

 of sharp and true definition. This dispersion is caused because each 

 of the coloured rays has a distinct focus at a different distance from 

 the lens. This defect is called chromatic aberration, arid Dollond 



FIG 190.- Achromatic lenses : A, Gauss' object glass ; B, C, Herscliel's object glass. 



discovered a method of counteracting this by making the object-glasses 

 and eye-pieces of two or more different lenses, either convergent or 

 divergent, and varying the nature of the glass of which these lenses 

 are formed. 



By forming the converging lens of ordinary crown-glass and the 

 diverging lens (bi-concave or plano-concave) of flint-glass ; and by 

 giving certain curves to each surface of the combination furnished 

 by calculation or experiment, Dollond made systems of achromatic 

 lenses, so that the rays of white light, on being refracted in the 

 desired direction, retained their parallelism on leaving the lens, 



1 Dollond was descended from a French Protestant family, which the Revocation 

 of the Edict of Nantes obliged to take refuge in England. 



