Diffraction gratings

The first descriptions of diffractive effects induced by periodic modulation of the refractive index date back to the 17th century, with an experiment described by Scottish astronomer, mathematician and physicist James Gregory. In a letter to publisher John Collins dated May 13, 1673, he describes a diffraction experiment based on the use of a bird's feather: "Let in the sun's light by a small hole to a darkened house, and at the hole place a feather, (the more delicate and white the better for this purpose, ) and it shall directto a white wall or paper opposite to it a number of small circles and ovals, (if I mistake them not,) whereof one is somewhat white, (to wit, the middle, which is opposite to the sun,) and all the rest severally coloured. I would gladly hear his thoughts of it." We recognize in this experiment the creation...