Eddie Yeadon will talk about “John Dolland and the achromatic lens”.
Isaac Newton investigated the dispersion of light into a colour spectrum by refraction and concluded that making a colour-free lens was impossible, so he designed and made a telescope using a mirror instead. But some theorists argued that, since the eye does not show colour fringing, it must be possible to correct a lens in a similar way. It was left to John Dolland to re-investigate the problem experimentally and devise rules for correcting doublet lenses. A better understanding of the nature of light and colour by Fraunhofer and Abbe in Germany led to complete colour correction and the supremacy of German optical technology at the end of the 19th Century.