ON CERTAIN GREEN PIGMENTS IN INVERTEBRATES. 397 



of acid do or do not indicate the existence of acid derivatives 

 may seem at first sight one which can be readily determined. 

 When, however, it is recollected that we are dealing not with 

 a well-defined chemical substance capable of being invariably 

 recognised by definite reactions, but with an unstable, unknown 

 substance, which, apart from the presence of impurities, may 

 be a mechanical mixture of several pigments ; when, further, 

 it is found that virtually the one available test is that of the 

 spectrum, whose validity as a test is the point to be proved, it 

 is then possible to obtain some notion of the difficulties. It is 

 curious to note that in point of fact, in spite of the frequent 

 occurrence of pigments whose spectra change on the addition 

 of acids, Schunck's beautiful work on chlorophyll (10) seems 

 to be the only case where the reasons for the changes have 

 been fully investigated. The difficulties mentioned above 

 perhaps afford a ready explanation of the blanks in our know- 

 ledge of such pigments. 



The most obvious characteristics of the acidified solutions 

 are, of course, their spectra. The spectrum of the blue acid 

 solution has been completely mapped by Professor Engelmann, 

 whose method shows its peculiar characteristics in a very striking 

 manner. My own observations were made both with a Sorby's 

 microspectroscope and the large double prism spectroscope of 

 the Cambridge Instrument Company. The results are recorded 

 here (see fig. 2) only because they are essential to the course 

 of the argument; they agree very closely with those of 

 Engelmann, but on account of the indefiniteness of the margins 

 of the bands are less accurate, — that is, the point given as the 

 centre of the band does not always coincide with the point of 

 maximum absorption. 



The general characters of the spectrum of the blue acid 

 solution have been already described. In regard to detail, the 

 most important point is the character of the dominant band in 

 the red. In freshly extracted solutions of chsetopterin this 

 band, as measured by a table spectroscope, has the following 

 position : 

 A 679 - A 643, centre = A 661 (see the first band of fig. 1). 



