840 On Quetelet's Rings and other Allied Phenomena. 



8. Summary. 



The following are the principal results of the investigation 

 described in the paper : — 



(a) Two simple methods o£ experimenting are described 

 in which a highly homogeneous scattering film of ammonium 

 chloride on the surface of a plate enables Quetelet's rings to 

 be satisfactorily observed at all incidences up to 90°. 



(b) An important difference is observed in the configur- 

 ation of the rings in the cases in which the refractive index 

 jof the material of the plate is equal to or greater than unity. 

 In the former case the width of the rings diminished con- 

 tinually from normal to grazing incidence, whereas in the 

 latter the rings first contract, reach a minimum width and 

 widen out again. 



(c) A new class of Quetelet's rings has been observed in 

 which scattered light which has passed twice through the 

 plate interferes with light which has traversed the plate four 

 or six times, undergoing multiple reflexion and scattering. 



(d) Observations of Quetelet's rings with a sheet of mica 

 of which one surface is dimmed by a film of ammonium 

 chloride show interesting effects, due to the doubly refracting 

 properties of the plate. The rings seen by white light show 

 dislocations along curved arcs of minimum visibility ana- 

 logous but not quite similar to those observed in Haidinger's 

 rings by monochromatic light. 



(e) The interferences observed in Quetelet's rings and 

 other allied phenomena do not always have maximum 

 visibility. The contrast between the maxima and minima 

 varies with the magnitude of the phase-differences intro- 

 duced by passage of the light through the scattering 

 film. 



(f) A new treatment of the theory of Quetelet's rings is 

 developed, applicable to the case of a continuous scattering 

 film, which enables the features mentioned in (<?), as also the 

 effect of obliquity on the visibilty of the rings, to be satis- 

 factorily explained. 



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Putney, London, S.W. 15. 



31st May, 1921. 



