In a prophetic letter to Albert Einstein on 22 April 1928, Irish scientist Edward Hutchinson Synge proposed a microscopic imaging method in which an optical field scattered from a tiny gold particle could be used as a radically new light source. The gold particle, Synge argued, would act as a local probe of some sample of interest, with the scattered light transmitted through the sample and into a detector. The sample’s image could then, at least in principle, be obtained by raster scanning the particle over the sample surface while continuously recording the detected light’s intensity.

Along with a sketch of the proposal, shown in figure 1, Synge wrote, “By means of the method the present theoretical limitation of the resolving power in microscopy seems to be completely removed and everything comes to depend on technical perfection.” The resolution was limited not by the wavelength of light, he realized,...

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