168 H. C, SORBY. 
Very faint absorption : ; : . 
Quite decided on ; : : ee 
Dark and strong ,, ‘ ; ‘ —— 
Black and very strong absorption : — 
Now, by carefully discussing the wave-lengths of the cen- 
tres of the absorption-bands in the spectra of substances 
which have a number of well-defined character, I find that 
they appear to be related to one another in the following 
manner. If the wave-lengths of a series of such bands, 
passing from the red end to the blue, be a, J, c, d, &c., the 
ratio between any consecutive pair is very closely, if not 
at bink 
absolutely, the same; that is to say, p=7~=G &e. If, 
however, the complete spectrum be a, 6, c, d, it does not by 
any means follow that all the bands are always present. In 
the case of some substances the spectra of the acid and 
alkaline solutions are related thus: 
Alkaline solution : : ere Noy ve 
: bane 
Acid solution d 
It also frequently happens that under certain conditions 
some of the bands, which in accordance with this law ought 
to be present, are absent, so that the spectrum may be, for 
example, a, c, d, and in other conditions of the substance 
(for instance, dissolved in some other solvent) the missing 
band may make its appearance at the true interval. It must 
also be borne in mind that without any considerable change 
in the general character of the spectrum the wave-lengths of 
the bands may vary very considerably, according to the 
nature of the liquid in which the substance is dissolved, or 
according as it is ina free state or in solution; so that we 
have to consider both the alterations in the position of any 
particular band, as well as the development of new bands. 
Very much remains to be learned in connection with this 
subject, and I do not yet feel quite certain that the above- 
described relations are the true law, or only a better approxi- 
mation than any hitherto made. 
Having now given such a short account of my general 
conclusions as seemed necessary for the subject before us, I 
proceed to describe the spectra of the colouring matter of 
Bonellia, which for convenience may be named Bonelleine. 
Being green and fluorescent, and the spectrum showing a 
very well-marked absorption-band in the red—being, more- 
over, insoluble in water, but soluble in alcohol and in carbon 
bisulphide—it might at first be mistaken for chlorophyll, or 
