The Initial Approximations for Achromatic Doublets of the XVIII Century

Analysis of the both type (flint-forward and crown-forward) achromatic doublets was carried out. The investigation revealed possible initial approximations which could be used by opticians at producing of the achromatic doublets in last half of XVIII century. The comparative analysis of approximate versions of achromatic doublets has provided additional explanation to some historical events. One more confirmation that the earliest achromatic doublets were really flint-forward type was found.