The Achromatic Refractor

In 1733, the achromatic lens (Fig. 14) was invented by Chester Moore Hall, an English barrister. This was accomplished by combining a convex crown and a concave flint lens in such a way that their focal lengths were inversely proportional to their dispersions.

Although a number of telescopes were made according to Hall's instructions, the benefits of the achromatic lens do not appear to have been made available to the public until John Dollond invented it independently in 1758, and patented it. Dollond's efforts led to a demand for clearer glasses of more varied densities and of less equal dispersions, needed to improve achromatism, and chemists pursued experiments in learning

Fig. 14. The achromatic lens. The upper half of the diagram illustrates the color correction. The lower half shows how spherical aberration is corrected by the same compound lens.

how to control the refractive indices of melts, and in the pouring of large disks of limpid, homogeneous glass. Altogether, excellent progress began to be made, and by 1800 achromatic objectives 6" in diameter were being turned out. Some of the best glass had been manufactured by Guinand, a Swiss who worked with Fraunhofer from 1805 to 1814. Fraunhofer produced a number of splendid achromats up to 9" in diameter.

Achromatic Refractor continued