GKRI^N FIGMENT OF INTESTINAL WALL OF CH7ET0PTERUS. 459 



The acid Bonellin also exhibits four, and only four absorption 

 bands, but these do not coincide with any of the four bands of 

 the neutral solution. The alkaline solution presents a six- 

 banded spectrum, and of these six it is remarkable that the 

 strongest, viz. the first and the sixth, coincide in position with 

 those of neutral Bonellin ; whilst the remaining four are 

 similar to those of acid Bonellin, but all shifted alittle towards 

 the red end of the spectrum. 



I am not prepared to discuss here either Sorby^s slight 

 divergences from Engelmann^s record or Krukenberg's theory 

 of Bonellin and Bonellidin. My principal object is to show 

 how widely Bonellin differs from Chsetopterin (though resem- 

 bling it in general characters), and to present an accurate 

 record of the absorption phenomena of neutral, acid, and 

 alkaline alcoholic solutions of the pigment as obtained from 

 fresh specimens of Bonellia. 



It now only remains for me to give some explanation of the 

 method of observation and record of absorption spectra — intro- 

 duced by Professor Engelmann, — without which the reader 

 will not properly understand the value of Plates 36 and 37. 



IV. Measurements of the Absorption Spectra op Ch^- 



TOPTERIN AND BoNELLlN BY PrOFESSOR EnGELMANN. 



Professor Engelmann kindly offered last year (1896), when I 

 was on a visit to Utrecht, to apply his beautiful instrument 

 for the measurement of the absorption of the luminous spec- 

 trum by coloured media to Chsetopterin and Bonellin. I was 

 very glad to avail myself of his kind offer, in order to procure 

 a more accurate record of the position and intensity of the 

 absorption bands given by those pigments than is possible with 

 the ordinary micro-spectroscope. The charts forwarded to me 

 by Professor Engelmann as a result of his examination of the 

 solutions which I sent to him are reproduced in Plates 36 and 37. 



The instrument used by Professor Engelmann is described 

 by him in the ' Archives de Microscopic.' It is applied to the 

 body of an ordinary microscope, and consists in an arrange- 



