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

The Initial Approximations for Achromatic Doublets of the XVIII Century