98 



•KOI'EUTIKS or Ol'TICAI OUATr.SCS 



SECTION 9. COMPARISON OF THE EXPERIMENTAL 



RESULTS OF SECTION 7 WITH THOSE FOUND 

 THEORETICALLY IN SECTION 8. 



Using methods similar to those of section 4, and dia- 

 grams of the nature of Figure IV., find the number of second- 

 ary maxima that should appear between spectra given by 

 gratings of 3, 4, and 5 lines. Roughly estimate their posi- 

 tions with respect to the specti-a, and compare your results 

 with those obtained experimentally in section 7. 



SECTION 10. APPLICATION OF THE FOREGOING TO 



THE CASE OF THE ORDINARY TRANSMISSION AND 



REFLECTION GRATINGS. 



The plane transmission grating consists of a transparent 

 surface closely ruled with lines; 15,000 lines to the inch is an 

 average spacing. In such a case, a transparent aperture will 

 have a width of roughly lO^-* cms. The light from these 

 apertures passes through a lens, and is focussed, all the in- 

 dividual visibility curves being superimposed. The angle 

 through which the central maximum of the visibility curve 



extends on one side of the centre is given by sin e = ~- (see 



sections 3 and 4). Here therefore hiH •; • lO-«)/(l x 10-<), 

 i.e., it extends over more than 30 degrees on either side of 

 the direction of the incident light. The form of the curve 



