Quetelet's Rings and other Allied Phenomena. 835 



rings. The further examination, both theoretical and expe- 

 rimental, of this subject must be reserved for a separate 

 paper. 



6. Influence of the Structure of the Scattering Film. 



It has already been remarked that the luminous field in 

 which Quetelet's rings appear shows a fibrous or granular 

 structure depending on the irregular distribution on the 

 surface of the plate of the sources of diffracted light, and 

 that this structure may be controlled by altering the aperture 

 of the observing instrument. Apart from this, there is 

 another important respect in which the nature and distri- 

 bution of the diffracting particles influences the observed 

 phenomena. It seems generally to be supposed that in 

 Newton's Diffusion Rings or in Quetelet's rings as observed 

 in focus in monochromatic light the interference minima are 

 perfectly black; in other words, that the visibility of the 

 rings is the maximum possible. This is, however, very far 

 from being generally true. Observation shows that the 

 visibility is different under different conditions. The most 

 interesting results under this head are obtained when the 

 surface of the mirror is dimmed by a more or less continuous 

 film. The effects are then seen to depend upon the structure 

 of the film. 



Probably the easiest way of exhibiting the effect mentioned 

 above is to observe the diffusion rings at the focus of a 

 concave mirror in the usual way, the surface of the mirror 

 being dimmed by breathing on it. The structure of the 

 deposit may be controlled by breathing either less or more 

 heavily and for different intervals. It will be noticed that 

 the lightly-breathed upon mirror shows rings which are 

 rather evanescent no doubt, but which are very vivid and 

 beautifully coloured so long as they are seen. The heavier 

 deposit gives rings which are much duller in appearance. 

 The distribution of the scattered light around the focus is 

 also different in the two cases. When white light (with or 

 without a red glass to approximately monochromatize the 

 light) is used, the effect may be seen with the rings focussed 

 on a screen in the usual way. The effects are, however, 

 far more striking when the monochromatic light of the 

 quartz-mercury vapour lamp with a green filter is used. 

 Instead of receiving the rings on a screen, it is preferable 

 to view them directly by the eye, a fairly large totally- 

 reflecting prism being placed near the focus to make any 



