On Anomalous Dispersion in Incandescent Sodium Vapour. 53 



Calculated Values of g for Tokio. 



From the formula, section 222, p. 167,") ( cent, per 



Thomson and Tait's 'Natural Phi- V 979*74 < sec. per 

 losophy,' published 1867 ... J ( sec. 



From the formula given on p. 21 of 1 



Dr. Everett's Units & Phys. Const., V 979*803 „ „ 

 published 1879 J 



From the formula given by Major") qvq.qii 



Herschel, published 1880 . . J J ' JMi » » 



In our paper we merely, as a rough approximation, took 

 the first of these calculated values to the first place of de- 

 cimals. 



Might we again draw attention to an important point ad- 

 vocated in our paper, but which runs the risk of being missed 

 altogether in this discussion. It was our simple proposal of 

 an easy way of measuring the relative value of g over the 

 earth. Let rigid pendulums be sold, each accompanied by a 

 table giving the number of swings per hour at many different 

 temperatures in London. An observer in Siberia, say, finds 

 the number of swings per hour at an observed temperature, 

 and from his table at once sees the value of g for the place 

 relatively to that of London. 



In conclusion might we venture to suggest that, whatever 

 may be Major HerschePs opinion of our ability as experi- 

 menters, whatever may be the estimate he forms of the value 

 of our results, the style and tone of his letter is hardly what is 

 usually employed by scientific men when discussing an inves- 

 tigation patiently made and faithfully recorded. 



We are, Gentlemen, 



Faithfully yours, 



W. E. Ayrton, 



June 16th, 1880. John Perry. 



VII. On Anomalous Dispersion in Incandescent Sodium 

 Vapour. By A. Kundt*. 



THE first observation of an anomaly in the dispersion of 

 light was, as is well known, made by Le Houx upon 

 iodine vapour. The vapour of iodine, which chiefly transmits 

 only the extreme blue and red rays, shows a stronger refrac- 

 tion for the red than for the blue rays. As then anomalous 

 dispersion was discovered in fuchsine by Christiansen, and I 



* Translated from Wiedemann s Annalen. 1880, No. G, vol. x. pp. 321- 

 325. 



