Diffraction Gratings in Immersion

AT the London Optical Conference in 1950, it was suggested1 that the resolving power R of diffraction gratings might be increased by immersing the grating in a medium of high refracting index μi. Thus: where W is the ruled width of the grating and where α applies for the case of autocollimation of incidence and diffraction. This idea, which is a strict parallel to that of the immersion microscope, was further developed2 in accordance with Figs. la and b.