Gratings are of the utmost importance in spectroscopy (q.v.), as they furnish a means of measuring the wave-lengths of ether-waves. They were invented and first used by Joseph von Frannhofe•, of Munich. in 1821. To be of value. gratings must he ruled with the scratches at e.racay equal intervals. This is scoured by plac ing the surface to be ruled on a platform. which is carried forward by a long screw, whose thread is perfectly uniform. and by having a diamond point so arranged as to draw lines at right angles to the direction of the screw. By means of a toothed wheel fastened to the screw, it is turned through a definite proportion of one com plete revolution. thus carrying the platform for ward a definite distance: for example. if the threads of the screw are at a distance 1-15th of an inch apart. and if the toothed wheel has 1000 teeth, then if this wheel is pushed round by an amount equal to one tooth. the screw will turn through 1-I0tiOth of a complete revolution. and twill early the platform forward 1-15,000111 of an ineh. Then the diamond is drawn the surface of the grating. making a groove; it is raised. pushed back to the other side of the
grating. dropped down and again drawn across the surface, the platform in the interval of time. while the diamond is pushed leek, having been carried forward another step. The process is continued until as many lines are made as are desired. Such a machine is called a 'dividing or a 'ruling engine.' (See DiviniNG ExcfNe..) The most perfect gratings now in u-e have been made at the physical laboratory of Johns Hop kins where Professor P,o•land super. wised their preparation. His grating: have. as a rule. 15.000 lines or scratches to the inch: an! the surfaces vary from two inches to six in width.
To use gratings ruled on plane snrfores for speetroseopie purposes, it is necessary to bine flout with tele-copes and collimators; but Professor Rowland has shown that reflecting gratings ruled on spherically concave surfaces can be used uithi,ut the addition of Tin".0 •concave.gratings" Jaye Other marked advantages °NCI' ours for almost every spectroscopic purpose. A good description of their properties is gk on in Preston's Thtory of 1.10i (London,