Taking the colour out of light.

The early history of telescopic astronomy was plagued by the imperfections of the lenses used to construct the telescopes. These imperfections were of two types, those resulting from the physical difficulties of glass production, lens grinding etc. and those inherent to the optics of the system. Of the second type it was known from the beginning that spherical lenses suffer from spherical aberration, i.e. when the surface of the lens is a segment of a sphere then the light rays refracted by the lens don’t focus at a single point but over a small spread causing the resulting image to be fuzzy. The problem and its solution was known long before the discovery of the telescope but it required grinding the lenses as a segment of a hyperboloid instead of a sphere and that was not technically  possible in the 17^th century. Trail and error showed that the longer the focal length of the objective the less the distortion caused by spherical aberration leading to the development of telescopes of more than 60 metres in lenght!

In the 1670s the whole situation became even more complicated as Isaac Newton discovered chromatic aberration. Newton showed that white light was not homogeneous but  a heterogeneous mixture of coloured light each of which has a different index of refraction leading to the fact that lenses separate the colours producing different focal points for each colour very slightly separated meaning that the image is not sharpe and has coloured fringes. In fact most of the aberration that had been previously considered to be spherical aberration was in fact chromatic aberration. Newton now made one of the major scientific mistakes in his life and claimed that it was impossible to avoid chromatic aberration and developed his reflecting telescope. However one of his disciples David Gregory thought that it should be possible to avoid chromatic aberration if one constructed double lenses out of two different materials with indices of refraction that effectively cancelled each other. His own attempts to construct such a lens were a failure but he had found the right theoretical answer.

The man who is credited with creating the first achromatic lenses using the theory outlined by Gregory was John Dollond who was born on the 10^th of June in 1706.

John Dollond By Benjamin Wilson Source: Wikimedia Commons

Dollond patented his discovery and became very rich and the optics company of Dollond and Aitchison still exists today. However John Dollond did not produce the first achromatic lenses this honour should go to Chester Moore Hall born 9 December 1703.

Chester Moore Hall

Hall himself was not a lens grinder so to protect his discovery from piracy he gave the commissions for the two halves of his double lens to two separate opticians, Edward Scarlett and James Mann but as fate would have it they both farmed their commissions out to the same lens grinder George Bass. Hall’s double lens worked and being a somewhat unusual individual, he was a highly successful lawyer, for whom optics was only a hobby; he did not patent his invention but gave it to the instrument maker John Bird. Bird, who was himself highly successful, had no time for this and in turn he gave the invention to another instrument maker, James Ayscough who unfortunately went bankrupt and never utilized Hall’s discovery.

Enter John Dollond, like Hall a passionate amateur optician, he was a weaver by trade, he also tried to solve the problem of how to construct an achromatic lens. His initial attempts failed until one day fate brought him to the workshop of George Bass who told him of the two lenses that he had ground for Hall; and so Dollond now possessed the secret of how to construct achromatic lenses, which he as opposed to Hall patented.

With time the opticians’ grapevine spread the information that Hall and not Dollond was the true inventor of the achromatic lens, which in the hands of Dollond had developed into a real money spinner and various competitors brought court cases against the granting of the patent to Dollond claiming that by rights the patent belonged to Hall and as he had chosen not to register it then the achromatic lens was so to speak free ware. The issue was settled when a judge ruled in favour of Dollond arguing that it is not he who hides an invention in his desk drawer who has a right to a patent but he who makes it public.