He concluded that there could be no refraction without dispersion, and hence that achromatism was impossible of attainment.
Leonhard Euler in 1747 had suggested that achromatism might be obtained by the combination of glass and water lenses.
The whole history of his researches proves how fully he was aware of the conditions necessary for the attainment of achromatism in refracting telescopes, and he may be well excused if he so long placed implicit reliance on the accuracy of experiments made by so illustrious a philosopher as Newton.
The absence of this error is termed achromatism, and an optical system so corrected is termed achromatic.
If all three constants of reproduction be achromatized, then the Gaussian image for all distances of objects is the same for the two colours, and the system is said to be in " stable achromatism."