87 



AN EXPERIMENTAL METHOD OF PRESENTING THE 



PRINCIPLES DETERMINING THE GENERAL 



PROPERTIES OF OPTICAL GRATINGS. 



By a. L. McAulay, B.Sc, B.A., Ph. D. 

 (Read 6th August, 1923.) 



In a laboratoi'y which is not well equipped with modern 

 optical instruments, the actual appearance and meaning of 

 many of the phenomena whose theory is studied is not in the 

 least appreciated. This seems especially the case in con- 

 nection with the elements of the more advanced theory of 

 optical gratings, and with the theory of interference spectro- 

 scopes. 



The following presentation is an attempt to provide a 

 set of experiments, made with fairly cheap and simple 

 apparatus, which shall illustrate the various effects that 

 produce the phenomena exhibited by an optical grating, and 

 includes such simple theoretical discussion as is necessary to 

 understand the experiments. 



In the course of the experiments the effects that 

 determine such things as the resolving power of a grating, 

 the intensity of the spectrum of a given order, and so on, are 

 directly observed. At the end of the paper the results 

 b^jtained in the discussion are applied to the cases of the 

 ordinary plane grating and the Michleson Echelon. 



SECTION 1. APPARATUS. 



The arrangement of apparatus is shown diagrammatically 

 in Figure I. G is an ordinary metallic filament electric lamp 

 of about 60 candle power, enclosed in a box which is fairly 

 light tight; a kerosene tin lying on its side, with a black cloth 

 over one end, and a window cut in the other serves the pur- 

 pose very well. A is a vertical slit placed against the 

 window. It should have good straight jaws, and be ad- 

 justable. Considerable use may be made of this adjustment, 

 for some purposes it is convenient to have the slit very much 

 wider than for others. It is best to have a large diameter 

 short focus lens in the box to diffuse the light falling on the 



