440 



NOTE. 



Note on the use of the Mercury Vapour Lamp in observing the 



Rings and Brushes in Crystals. 



By E. B. Stringer. 



Whilst observing the rings and brushes in crystals with the 

 monochromatic sodium flame, it occurred to me that the light of 

 the mercury vapour lamp might be specially suitable for the 

 purpose, since it yields monochromatic light of several refrangi- 

 bilities at once. (Those who have seen its spectrum will under- 

 stand my meaning.) This actually proved to be the case. The 

 brilliance, and especially the definition with which they are shown, 

 is very remarkable. I know little of penological work, but much 

 of it, I believe, consists in the examination of these rings and 

 brushes, and the lamp might prove to be of great value for the 

 purpose. 



Postscript. 



Since writing the above, it occurred to me that other interesting 

 experiments in interference might be made with the same light. 



I find that whilst it is impossible to see the Newton's rings in 

 my 3 in. telescope objective by ordinary illumination, yet with the 

 mercury light (diffused by the interposition of a piece of ground 

 glass) they appear with the greatest distinctness, and of wonderful 

 fineness and delicacy, extending to the extreme edge of the lens.* 

 This should prove of value in adjusting pairs of lenses. 



The interference colours in a vertical soap-film are also very 

 beautiful when seen in this way, and so delicate as to require 

 examination with a magnifying glass. 



* To sec the rings by transmitted light it is best to look down upon a piece of 

 white paper, illuminated by the lamp. 



