Grimaldi was associated with Riccioli in making astronomical observations, and ho gave particular descriptions of the spots on the innon's disc. It was asserted by Alontucla that Grimaldi gave to thee. spots the designations by which they are now distinguished asneng astronomers ; thus superseding the names of the mountains and seas of the earth which had been given to them by Ilevelius; but this is apparently a mistake.
That which has given celebrity to Grimaldi is hie work entitled 'Phyelco-mathesie de Lumine, Culoribus, of Irides aliisque anneals,' which was published at Bologna, in 40, in 1665. The greater part of the work consists of a tedious discusaiun concerning the nature of light, the conclusion of which is that light is not a substantial but an accidental quality; the rest however pommies the highest interest, 44nce it contains acoonuta of nurnerons experiinents rela1ing to the interferences of the raye of light. A description of the work is given in the Philoeophioal Transactions' for that year.
Grimaldi, having admitted the sun's light into a dark room, through a small aperture, remarked that the breadths of the shadows of Blender objects, as needles and hairs, ou a screen, were much greater than they would have been if the rays of light bad passed by them in straight lines. Ho observed also that the circle of light formed on a screen by the rays passing through a very small perforation in a plate of lead was greater than it would be if its magnitude depended solely on the divergency of the rays ; and he arrived at the conclusion that the rays of light suffer a change of direction in passing near the edges of objects : this effect he designated 'diffraction: By Newton it was subsequently called inflexion.' He found that the shadow of a
small body was surrounded by three coloured streaks or bands which became narrower as they receded from the centre of the shadow ; end, when the light was strong, he perceived similar coloured bands within the shadow : there appeared to be two or more of these, the number increasing in proportion as the shadow was farther from the bod ving admitted tho sun's rays into a room through two small circular apertures, Grimaldi received the cones of light on a screen beyond the place where they overlapped each other; and he observed, as might be expected, that, within the space on which the raya from both apertures fell, the screen was more strongly enlightened than it would have been by one cone of light; but lie was surprised to find that the boundaries of the penumbral portions which overlaid one another were darker than the corresponding portions in which there was no overlaying. Thie phenomenon of interference was, at the time, enunciated as a proposition :—"That a body actually enlightened may become obscure by adding new light to that which it bas already received." Grimaldi also observed the elongation of the image, when a pencil of light from the sun is made to pass through a glass prism ; but he ascribed the dispersion of the light to irregularities in the material of which the prism was formed ; and he was far from suspecting the different refraugibilitiea of the rays. The discovery of this fact, which has led to so many important consequences in physical optics, was reserved for Newton.