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The refracting telescope and John Dollond
A brief look at the case of John Dollond illustrates a different way of dealing with the problem of relating individual skill to universal principle. Here we find a reversal in the distribution of intellectual and mechanical aspirations among telescope makers, with mechanics passionately seeking the status of theorists and engaging in squabbles over their theoretical credentials. When Dollond presents himself in his portrait, there is not a telescope in sight, even though making and selling telescopes became his trade. Here he clearly reads Newton’s Opticks, the book on the table, and practices experimental natural philosophy: the two instruments in the composition are experimental demonstrations of the principle of achromatisation, separated from its application in the telescope.
© Science Museum/Science & Society Picture Library
Portrait of John Dollond, English optician, c 1750s
Dollond began as a silk-weaver in a Huguenot family in the unfashionable district of Spitalfields in London but with optics becoming a field of general interest, open to all comers – the first ‘popular’ science as presented by Smith – Dollond encouraged his son Peter to take up this more profitable trade and also followed his own investigations (Clifton, 2013; Gee, 2014; Sorrenson, 2013; Willach, 1996). Naturally he began with complete confidence in Newton’s conclusions but he came to doubt the reliability of the assumption that the dispersion of light for a given degree of refraction was the same for all types of glass. His subsequent experiments showed that this was not the case and that high-lead ‘flint’ glass had a significantly greater dispersive power than the more common optical ‘crown’ glass. This opened the possibility for an ‘achromatic’ combination of two lenses that could produce overall deviation without dispersion. Newton had been wrong.
Dollond explained his technique in a paper to the Royal Society in 1758 (the paper was communicated to the Society by James Short) and he was awarded the Copley Medal. In the same year he also took out a patent on his new lenses. He was elected a fellow of the Royal Society in May 1761, with Short’s support, but died suddenly in November and his share in the patent passed to his son Peter.
In subsequent squabbles between Peter and the many London opticians who challenged his patent, Peter’s consistent position was that, whatever precedents there may have been to his achromatic lenses, his father had independently reached his practical technique on the basis of his theoretical command of Newtonian optics. It was central to Peter’s cause that his father had not simply been a jobbing optician, but a natural philosopher. We can note, for example, the stance he adopted in response to a challenge from Jesse Ramsden to the originality of his father’s work:
In the beginning of the year 1757 Mr Dollond having tried the…8th Expt. of the 2d part of the first book of Newton's Opticks and by that means having discovered the new principle which was that the dissipation of the different coloured rays was not in the same proportion to the mean refraction, in water as in glass... (Bennett, 1998)
Peter acknowledged that there had been an early conversation between his father and one of the earlier makers, George Bass, but held that Bass himself had not understood the significance of what he had been saying: 'He did not know that such a difference in the quality of two kinds of glass could be applied to any advantage. Mr Bass was a practical optician; not a theorist.'
With reflectors we found mathematicians with mechanical aspirations; with refractors we find mechanics anxious to be mathematicians. Optical mirrors entered the eighteenth century with scarcely any trade history but with an account of their manufacture in Newton’s Opticks, soon to be elaborated in Smith’s Opticks. Lenses, on the other hand, had a long history of obscure workshop practice and trade protection.
Component DOI: http://dx.doi.org/10.15180/140209/005