Ronchi's Method of Optical Testing

i. Ronchi's mei/zod.-Light from a slit is brought to a focus by the lens or mirror to be tested. A short distance in front of (or behind) the focal plane is placed a line- screen having a spacing of 40-200 lines per centimeter, so oriented that the lines are parallel to the slit image. The shadow lines seen by the eye placed at the focus indicate clearly the characteristics of the wave surface as it leaves the lens or mirror. This meth- od, which is described in Italian publications, is probably not so familiar to American readers as it deserves to be. 2. Modification of Ronchi's nzelhod.-Instead of a single slit, a line-screen ifiumi- nated by a frosted lamp or even by sky light may be used as the source, with the ad- vantage that the ifiumination is so greatly increased that all observations can be made in a fully illuminated room. Also, for testing concave spherical mirrors, or lenses and paraboloids with parallel light, only one screen is required, one-half of which serves as the source, the other half as the observing screen. In this case the lines of the image are automatically parallel to those of the observing screen. ~. A simple mounting for the screen and light-source is described. ~. Results of the examination of a number of lenses and of a slightly defective con- cave mirror are given; these show that the method is fully as sensitive as the Foucau~t knife-edge test