Invention of achromatic lens: a battle of intellectuals
1750s was an exciting time in the technology scene of England. Marine navigation of the empire demanded better time keeping and ever more precise celestial observation. Educated public was keenly interested in astronomy, partly from beliefs (still strong in our time) that celestial bodies determine our fate, partly from genuine desire to understand the Universe. Fortunes were made in manufacturing watches, navigation instruments, and telescopes.
Although refractive (lens-based) telescopes were already well known at that time, they suffered from chromatic aberrations, because of light dispersion in glass: light with different wavelength (color) propagates with different speed and thus gets focused at different distances from the lens, which creates colored fringes in the image. It was widely believed, due to Sir Isaac Newton’s works, that chromatic aberration cannot be corrected with lenses. However, some scientists kept pursuing this lead, including Leonhard Euler, who proved theoretically that such lens can exist, but could not build it.
An unlikely hero in this story, Chester Moore Hall was a barrister (a lawyer, don’t confuse with barista), an educated men of independent means, and amateur optician. He surmised that since we don’t see color fringes when using our bare eye, it must correct chromatic aberrations by some unknown mechanism. Since eye consists of refractive humours of different kinds, which are essentially lenses, a combination of glass lenses may also have the properties of color distortion correction. Around 1729 he succeeded in finding a lens pair which would do this job, by combining a positive and a negative lens of two dispersion types (crown, low dispersion, and flint, high dispersion). He wanted to keep his invention secret and ordered the two lenses from two different opticians, so any of them couldn’t reverse-engineer his design. He didn’t have time or need to make a profit, and it was designed solely for his own home-built telescope.
From there the story becomes an inventor’s blockbuster. The two opticians whom Hall ordered his lenses sub-contracted their orders to the same lens maker, George Bass. He realized that the two orders are from the same customer, so there must be some secret which the customer wanted to keep. Bass found the secret — the two lenses placed next to each other cancelled the chromatic aberration. Bass didn’t keep the secret and started producing achromats and sharing the idea with others. The cat was out of the box.
One of the people with whom Bass shared it was John Dollond, a son of silk-weaver and self-educated polymath, who recently started a business in optical instruments with his son Peter. John Dollond immediately realized the importance of the idea. He conducted optical experiments to study the problem and published a comprehensive paper in 1758, which immediately made him famous. He became a member of the Royal Society and received the Copley Medal, the most prestigious scientific award in the United Kingdom, an honor that can be possibly compared to winning a Nobel in our time.
John Dollond also patented the achromat design, despite the fact it was already known and practiced by others, and him being not the original inventor. However, the Hall’s original invention was never published, while Dollond took the idea, performed scientific experiments, re-designed the lens and published it in a way accessible to others. Unfortunately John Dollond suddenly died in 1761 at the age of 54, at the peak of his fame.
After John Dollond’s death his son Peter launched commercial production of achromatic telescopes and also took action against several London opticians who “infringed” his father’s patent. He soon was counter-sued by thirty-five opticians for exploiting the patent on invention which was known and practiced before. This led to a long and bitter court fight, which Peter Dollond eventually won. The court ruled that although Hall is the original inventor of achromat, it was Dollond who had the merit of bringing it before the public as a useful invention.
Peter Dollond later became famous for inventing apochromatic lens, which corrected 3 colors instead of two as the achromat. He continued the optical business he started with his father. Dollond telescopes for terrestrial use were amongst the most popular in Great Britain and abroad for over a century. Their company, Dollond and Aichison, existed for more than 250 years, famous for the quality of their instruments. Among famous owners of the D&A telescopes were Admiral Lord Nelson and Captain James Cook.
During the recent market meltdown of 2009, Dollond and Aichison merged with Boots Opticians due to financial difficulties, and their famous brand name became history.
Achromatic lenses became the pillar of almost every modern optical device: telescopes, binoculars, camera lenses, microscopes, anything that requires precise focusing of multi-colored light. When an optical engineer needs a lens, achromat often becomes option #1. There are numerous improvements of the original Hall design, suggested by bright minds in 19 and 20-th centuries, and the process of improvement continues even today. All those designs flourish on a root that grew from a hobby project of a professional lawyer and amateur optician Chester Moor Hall.
References
Chester Moor Hall, Britannica.com.
John Dollond, Encyclopedia.com.
Peter Dollond, Wikipedia.
Peter Dollond answers Jesse Ramsden, archive of the Museum of Science.
Dollond and Aitchison, Wikipedia.
Image courtesy.
