Section III, 1922 [313] Trans. R.S.C. 



A Note on Missing Spectra 



By A. S. Eve, F.R.S. 

 (Read May Meeting, 1922) 



Mining engineers use well-constructed sieves made of phosphor 

 bronze wire with various sizes of mesh, ranging up to three hundred 

 wires to the inch. 



When a narrow beam of parallel light falls on the sieve diffraction 

 bands and patterns are produced which may be observed directly, or 

 after projection. 



By a calculation involving simple proportion only the wave- 

 length of any light can be determined. 



A sieve with 300 wires to the inch was found to have the intervals 

 between the wire^ precisely equal to their diameters. It necessarily 

 follows that the even order spectra are entirely absent, while the odd 

 order spectra are enhanced in brilliancy. 



On inclining the sieve to the ray the gaps are effectively narrower 

 while the circular wires are not, hence at about 30° the missing spectra 

 suddenly appear. 



A photograph of the 300 sieve was taken with a white screen 

 behind it. This photograph was used as a grating and all the spectra 

 were found present. Possibly the light flooded the plate when the 

 photograph was taken, so that the gaps were enlarged relatively to 

 the image of the wires. There is no new principle involved in the 

 above, but the method is a valuable and effective method of demon- 

 strating missing spectra. 



