836 Prof. C. V. "Raman and Mr. G. L. Datta on 



desired part of the field visible to the observer who takes his 

 stand on one side. The aperture through which the light issues 

 should be as small as possible, the light of the mercury arc 

 being concentrated upon it with a condenser. When the 

 mirror is lightly breathed upon, a very large number of beauti- 

 fully sharp and clear rings may then be seen. With a heavy 

 deposit the rings are very poorly visible, and may even 

 entirely disappear in the higher orders. Some improvement, 

 however, occurs as the deposit evaporates and is about to 

 vanish completely. 



With a tarnish of milk or grease on the mirror similar 

 effects may be noticed, the appearance of the rings deterior- 

 ating with the heaviness of the deposit. Figs. 4 and 5 in 

 PI. XXV. exhibit the difference in the appearances observed 

 with a very thin and a heavier coat of dried milk-and-water 

 respectively. It will be noticed that though the heavier 

 deposit does not distort the rings in any way, it diminishes 

 the observable contrast between the maxima and minima. 

 Perhaps the most interesting results are those observed when 

 a film of ammonium chloride is deposited by volatilization on 

 the surface of the mirror. The deposit as initially formed is 

 highly fine-grained in texture and scatters light through 

 very large angles. By breathing lightly on it. the structure 

 of the film may be altered permanently, the ammonium 

 chloride being drawn by the moisture before it evaporates 

 into small microscopic globular masses. Still heavier 

 breathing further makes the structure coarser, semi- 

 crystalline aggregates being formed. These changes are 

 accompanied by corresponding alterations in the distri- 

 bution of light in the field and in the visibility of the 

 rings. 



That these changes of visibility of the rings depend on 

 the magnitude of the phase-changes which occur in the 

 passage of the light through the more or less irregular 

 structure of the film, is made clear by the following experi- 

 ment. A front-silvered glass plate and another plate of 

 glass one of whose surfaces is smoothly ground by fine 

 emery, are placed face to face with little slips of paper at the 

 corners to keep them at a fixed distance apart. With this 

 combination, Quetelet's rings cannot be observed at all in the 

 field of light diffused by the glass. When, however, a little 

 water is allowed to enter the space between the plates, the 

 rings can be faintly seen, only the first few rings, however, 

 being visible even in monochromatic light, and the contrast 

 between the maxima and minima is very small. When, 



