1411 
In particular substances with strong absorption-bands or lines 
deserve attention, such as didymiwm compounds, which can be 
obtained as solid solutions in glass, and the vapour of sodium. 
When these substances are chosen to work with, horizontal 
interference lines must be thrown on the slit of the spectroscope. 
Horizontal lines are observed in the telescope, which diminish in 
distance from red to violet. Through the Fizrav-effect the lines 
would move up and down in case of rapid motion to and fro of 
selectively absorbing substances, and at those places in the spectrum 
d, 
where = assumes large values the amplitude of the movement might 
become considerable. 
From preliminary experiments on the dispersion of didymium glass 
at the ordinary temperature. and at that of liquid air it appeared 
d 
that the value of En assumes nowhere great values in the visible 
spectrum. Though rods of didymium glass of excellent quality are 
to be had, I am yet of opinion that it would not be worth the 
trouble to make experiments with them on the Fizrav-effect. 
Nor can results on the Fizeav-effect be expected with sodium 
i d 
vapour. Close to the absorption D-lines = can, indeed, become very 
great, but at the most interesting place near the D-lines the absorption 
too becomes very great. On continuation of the experiments it might 
perhaps have deserved recommendation to work with a stationary 
tube with sodium vapour and moving prism (see $ 1 of this appendix). 
It appeared however, clearly enough from some experiments that 
the observation of the Fizrav-effect with sodium vapour was out of 
the question. 
Though these experiments did not yield the result for which they 
were undertaken, they gave occasion to the observation of an 
interesting interference phenomenon in sodium vapour, about which 
a separate communication will shortly follow *). 
Py oe 
1) This communication has been published already. These Proceedings Vol. 24, 
p. 206, 1922. 
