Let us begin with a celebration of Progress in Optics, edited since it was launched in 1961 by E. Wolf, which has reached volume 50 [1]. To mark the occasion, Wolf has collected eight essays on the history of optics, beginning with "From millisecond to attosecond laser pulses" by N. Bloembergen. Not only is this of intrinsic interest but it concludes with an arresting comment: "The understanding of the attosecond processes of soft X-ray emission and its inverse of photo-ionization can be formulated in terms of electron interferometry." Next, a fascinating account of "Conical diffraction: Hamilton's diabolical point at the heart of crystal optics" by M.V. Berry and M.R. Jeffrey. Berry has accustomed us to amazing findings in his work on optical singularities over the last few decades but this may be his finest yet. His list of 10 "reasons why conical refraction is worth revisiting" is too long to reproduce here in full, so I must extract the essence. The effect was predicted by W.R. Hamilton in 1832 (published 1837) and "was an early (perhaps the first) example of a qualitatively new phenomenon predicted by mathematical reasoning .... Prediction of qualitatively new effects by mathematics may be commonplace today but, in the 1830s, it was startling." Berry and Jeffrey then remind us that "critical refraction was the first non-trivial application of phase space" and "Its observation [by H. Lloyd, in 1837] provided powerful evidence that light is a transverse wave." "It was the first physical example of a conical intersection (diabolical point) ... involving a degeneracy." It "displays a subtle interplay of ray and wave physics" and "Analysis of the theory [by Berry] led to an identification of an unexpected universal phenomenon in mathematical asymptotics." Moreover, "There are extensions ... of the case studied by Hamilton and their theoretical understanding still presents challenges .... Conical diffraction is a continuing stimulus for experiments. Although the fine details of Hamilton's original phenomenon have now been observed [by Berry, Jeffrey and Lunney], predictions of new structures that appear in the presence of chirality, absorption and nonlinearity remain untested." Finally, the comment I most enjoyed "The story of conical diffraction, unfolding over 175 years, provides an edifying contrast to the current emphasis on short-term science."
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