890 Profs. Trowbridge and Wood on Groove-Form and 



Neglecting the probable disturbances in phase continuity 

 resulting from the breaking up of the wave into narrow 

 strips, we should expect all of the energy in the first order 

 spectrum. If, however, we work with waves twice as long, 



the path-difference will be ^ instead of \ and we should 



find the energy about equally divided between the central 

 image and the first order spectrum, which in this case will 

 lie well to the left of the direction in which the reflected 

 waves start. Grating No. 8, which will be described pre- 

 sently, comes the nearest to fulfilling these conditions of any 

 thus far examined. With waves 4*3 /x in length the first 

 order spectrum lies nearly in the direction of the reflected 

 waves, and contains 70 per cent, of the energy. With the 

 " Reststrahlen"" from quartz (\ = S'6) we have 34 per cent, in 

 the first order spectrum and 66 per cent, in the central image. 

 The preponderance in the central image is due to the fact 

 that the " oblique image " (direction of reflexion) lies nearer 

 to the central image than the first order spectrum for 8*(> yu. 



A large number of gratings have been examined and the 

 work is not yet completed. For a complete solution of each 

 case, it is necessary to know whether any of the original flat 

 surface has been left between the grooves. This is often the 

 case with the coarser rulings, and results in the formation of 

 strong central images when the gratings are examined with 

 visible light. Each grating element may thus consist of 

 three strips, the two edges of the groove and the flat portion 

 between. Thus far, but a single type of groove has been 

 tried, viz. the one ruled by the 120° carborundum crystal. 

 The angle at which the crystal was mounted with respect to 

 the surface has, however, been varied over a wide range, as 

 well as the depth of the groove, &c. Other types of grooves 

 will be investigated with a view of finding the one best suited 

 for work in the infra-red. It seems probable that a 90° 

 groove will be the best, as with a groove of this type one 

 edge can be made almost inoperative, and a larger proportion 

 of the surface brought into play. A symmetrically placed 

 90° groove with the light incident normally will be an 

 interesting type to investigate, for in fhis case we have a 

 two-fold reflexion in the groove, each element of the plane 

 wave being broken into two, which are turned end to end 

 and reunited, as can be seen by constructing the reflected 

 rays for a surface of this nature. A 90° double mirror has 

 the property of returning a twice-reflected ray back to its 

 source, regardless of its direction, provided it cross the groove 

 in a direction perpendicular to the groove. 



