170 H. C. SORBY. 
particular solution of bonelleine is seven, but in the com- 
plete theoretical spectrum sixteen. On the whole, it seems to 
me the best to indicate the position of the absent bands by 
small figures and that of those present by larger, and to 
express the intensity of the absorption by symbols printed 
under the numbers of millionths of millimétres, as already 
explained. For the sake of simplicity I give only the theo- 
retical positions, and merely say that in no case does obser- 
vation differ from this theory by more than a millionth of a 
millimétre. In accordance with this system the spectra of 
the different solutions are as follows : 
In carbon bisulphide— 
643 630 618 606 594 582 571 560 549 538 527 517 507 497 
In alcohol, neutral— 
662 649 636 624 611 599 587 576 564 553 542 531 520 509 500 490 
In alcohol, slightly acid— 
636 623 611 599 588 577 565 554 543 534 522 511 502 492 
In alcohol, strongly acid— 
617 603 590 577 565 552 540 529 517 
It will thus be seen that when dissolved in alcohol the 
bands are raised towards the blue end by an interval of about 
seven millionths of a millimetre of wave-length, and that, 
whilst the faint band at 549 disappears, that at 527 is made 
darker when in alcohol. There is also a new faint band 
developed at 662. On adding a very little acid to the alco- 
holic solution the band at 662 vanishes, that at 611 is made 
darker, that at 587 fainter, two new bands are developed at 
565 and 543, whilst that at 520 is made much more faint. 
On adding more strong acid the bands at 636, 543, and 492 
disappear, those at 611 and 522 are made darker, and both 
moved in opposite directions more towards the centre of the 
spectrum, and a fresh band is developed at 551. 
The character of the different spectra and the relation of 
the various absorption-bands will be better understood by 
means of the following figure : 
