88 Dr Searle, Experiments with a plane diffraction grating 



Experiments with a plane diffraction grating. By G. F. C. Searle, 

 Sc.D., F.R.S., University Lecturer in Experimental Physics. 



[Read 3 May 1920.] 



Part I. Parallel Light. 



§ L Introduction^ . When a plane grating is employed in 

 accurate measurements of wave length, the ruhngs are set per- 

 pendicular to the direction of the incident beam of parallel hght. 

 When these two directions are not at right angles, the diffracted 

 beam is no longer parallel to a plane containing the directions of 

 {a) the incident beam and (6) a hne intersecting the ruhngs at 

 right angles. The formulae apphcable to this general case are 

 obtained in §§ 4, 5, 7 ; they are tested by the experiment of §§ 8, 9 

 for the restricted case in which the directions {a) and (6) are at 

 right angles. 



§ 2. The grating axes. It is necessary to specify the three axes 

 of a plane grating and the origin from which they start. 



For a transmission grating, the origin is a point on the centre 

 hne of one of the openings. In a reflecting grating, would he 

 on the centre line of one of the reflecting portions. 



The axes are 



(1) The normal ON to the plane of the grating. 



(2) The transverse axis OT, a line through cutting the 

 ruhngs at right angles. 



(3) The longitudinal axis OL, a line parallel to the ruhngs. 

 The grating interval, i.e. the common interval measured along 



OT from centre to centre of the openings, will be denoted by d. 



§ 3. Diffracted wave front and ray. At a distance of thousands 

 of wave lengths from the grating, the wavelets due to the separate 

 openings will merge into practically a single wave. For the mathe- 

 matical purposes of this paper we shall speak of this wave as the 

 diffracted wave front and of a normal to it as the diffracted ray. 

 We may speak of the diffracted wave front passing through the 

 origin 0, if we understand it to be a surface through cutting at 

 right angles the normals to the distant wave fronts. The normal 

 through may be called the diffracted ray through 0. 



In the case of reflexion or refraction at a pohshed surface, the 

 time of passage from an incident wave front to a reflected or 



' * 1 have to thank Dr J. A. Wilcken of Christ's College, and Mr C. L. Wiseman, 

 M.A. of Peterhouse. Dr Wilcken took the observations of § 12, Part T, and assisted 

 in other ways. Mr Wiseman gave valuable help and criticism in the mathematicaV 

 parts of the paper. 



