There are difficulties about this combination of apparently incompatible aspects. The holographer who tries to reconstruct the real image from a Fresnel hologram with too wide a beam, overlays on the screen different views of the subject, and generates diffuseness; whereas the painter can separate them spatially, and combine them into a harmonious composition. Scott Fitzgerald, the young American writer of the 1920s, wrote that it was a mark of genius "to be able to hold in the mind simultaneously two contrary views, ... and continue to function."
But quantum physics requires us to do just this, and - like the Cubists? - has devised formal procedures for doing it - they comprise what's called "the transformation theory".
When I was asked to review the developments in some branch of physics during John's working life, it seemed to me that I could link John's interest in the superficially paradoxical view of nature which quantum theory challenges us with to my abiding interest in optics which has been, in this period, one of the most exciting and most spectacularly developing branches of physics.
Of course, this has been an exciting period of progress in many fields. In society we have seen
the decline of the safety pin and the rise of the Velcro fastener
the decline of the picture palace and the rise of the VTR
the decline of spelling and the rise of the word processor
the decline of the Scottish educational system ... ? and the rise of 'trivial pursuits'.
In optics we have seen - I hope and trust - the decline of the photon and a deeper understanding of the wave-picture: there may be room for photons in high-energy physics, but they've been a confounded nuisance, and at times a source of bad temper and destructive cross-purposes, in optics.
We've seen the development of coherence theory, and out of it, part way along its progress, an elegant, and powerful, and profoundly satisfying discipline of quantum optics.
We've seen the development of the laser, of non-linear optics, of the optical image processor, of phase-conjugate optical devices, any minute now the optical computer; on the High Street, the laser light shows at Christmas, and shops overflowing with binoculars, cameras, zoom lenses and what-not at prices which, in real terms, are trivial compared with those of 1946.
But what I want to share my thoughts with you about is not optical hardware, but how the way we think about light has developed over the past 40 years. Much of what I say - in particular the last part, will relate to work by an old friend E. Wolf, whom John I know, remembers as one of his teachers in Edinburgh.