Colour and Chemical Constitution. 
5^ 
spectrum, and A; and the wave-lengths of the lowest band in the two 
spectra respectively. Thus in uranyl metaphosphate the lowest fluorescent 
band is at \ 495 — Now suppose that the absorption spectrum shows a 
band at X 460 = A„ and a last band at \ 446 = A;. The value of -r-^' is 
F 
1*031, and this, raised to the 4/3 power, gives the value of viz. r042. 
Hence F« calculated is 5154: the observed value was 515. Expressed in 
words, the equation means that the cube of the fluorescent wave-lengths 
varies as the fourth power of the absorption wave-lengths, or that the cube 
of the ratio of two fluorescent frequencies is equal to the fourth power of the 
ratio of the corresponding absorption frequencies. If we employ a constant 
to eliminate the necessity for an observation of the lowest wave-length the 
formula becomes : 
\f I three figures only in\ 
n' Wave-length reading'. 
Thus if there is an absorption band at X 499, A„ = 499, and F,^ is 0-145 x 
499 X 7*93 = 574. This gives the central wave-length of the fluorescence. 
What the physical basis of this may be in terms of light-waves and periodic 
time of electrons I must leave to physicists to elucidate. Meanwhile I may 
point out that the ratio of fluorescent emission to absorbed wave-length is 
much the same amongst organic compounds, viz. about 1*15 for both 
fluorescein and rhodamine, so that the law appears to hold for many 
substances other than uranium compounds. 
F = '-'^ (A 
