Microscopy Pioneers
Aberrations, a Way of Life Peter Hawkes
CEMES-CNRS, B.P. 94347, F-31055 Toulouse cedex, France
peter.hawkes@cemes.fr
Editor’s note: Peter Hawkes received the 2015 Distinguished Scientist Award (Physical Sciences) from the Microscopy Society of America. Dr. Hawkes was unable to attend the awards ceremony or his invited presentation; he sent this text to be read at the time he was to speak. T ere is a certain ambiguity about “aberrations.” In the days of routine requests for off prints of one’s publica- tions, I occasionally received requests from Departments of Psychology, perhaps in the hope of revelations about kinky physicists. Before I start looking at the past, let me fi rst express my pleasure at joining this elite club. A glance at the membership list reveals that I have been on friendly terms with more than 20 of the physicists and 15 of the biologists, and it is a reminder of those who would have been strong candidates if they had not died before 1975. Walter Glaser of course, Bodo von Borries, and Ernst Ruska’s brother Helmut come to mind. In 1959, when I joined Mr Cosslett’s Electron Microscopy
Group † in the Cavendish Laboratory, I knew very little about lens aberrations. I had met the chromatic aberration of glass lenses in school physics, but I doubt if I knew much—if anything—about spherical aberration and the other Seidel aberrations (nor did I know any kinky physicists). Peter Sturrock, a pure mathematician, had studied for his Ph.D. in Cosslett’s group in the 1950s, and I was told to read his Static and Dynamic Electron Optics and his papers in the Proceedings and Philosophical Transactions of the Royal Society. Aſt er mastering these, my project was to apply Sturrock’s methods to the aberrations of quadrupole and related multipole lenses ( Figure 1 ). At that time, only one publication contained expressions for the aberration coeffi cients of such lenses: Alexander Melkich’s dissertation published in 1947 and based not on Hamiltonian optics and the eikonal method, perfected by Sturrock though introduced much earlier by Walter Glaser, but on the so-called trajectory method, favored by Glaser’s only serious rival, Otto Scherzer. In the course of my investigations, I looked into the relation between system symmetry and permitted aberrations; from this, it emerged that sextupoles suff er from an aberration of exactly the same nature as the spherical aberration of round lenses and should hence be capable of correcting that aberration. Sextupoles have the big advantage over quadrupoles that they have no linear focusing eff ect and the equally big disadvantage that
†Cosslett was still “Mr” Cosslett, as his Bristol Ph.D. did not entitle him to be called Dr Cosslett in Cambridge – only when he acquired an ScD (Cantab.) did we start calling him Dr Cosslett.
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they exhibit second-order eff ects. T e presence of the latter was so discouraging that I did not pursue the potential role of sextupoles as correctors—wrongly, for some 15 years later Beck and later Crewe, Rose, Chen and Mu, and Shao showed that sextupole doublets could provide correction, and a later design of Rose is at the heart of all the CEOS sextupole correctors.
I remained in Cosslett’s electron microscopy group for
16 years in all, funded at diff erent times by my college (I was a research fellow of Peterhouse), the Department of Scientifi c and Industrial Research, the Royal Society, ICI, and Churchill College (where I held a Senior Research Fellowship). T e Peterhouse years were very happy ones. I came to know (or just to meet) such fi gures as the mathematicians Charles Burkill and Hallard Croſt ; the historians Denis Brogan, Denis Mack Smith, and Maurice Cowling; the archaeologist Grahame Clark; the Hans Christian Andersen scholar Elias Bredsdorff ; the molecular biologists Max Perutz, Aaron Klug, and John Kendrew; and many others, as well as fi gures from the past such as E.M. Forster and Shane Leslie. But not all was “scholarship”; the college book club never overlooked a weak spot in a Fellow’s armor: thus Ash by Bredsdorff ’s son was treated mercilessly, as was a (very readable) “menopausal” novel by another Fellow’s wife, Menna Gallie. My Churchill years too were enjoyable partly because I was invited to join the wine committee whose task was to taste and buy large quantities of wine to lay down for future generations in the magnifi cent temperature- and
Figure 1 : Peter Hawkes at Cambridge, about 1970. doi: 10.1017/S1551929515001194
www.microscopy-today.com • 2016 January
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