346 0. A. MACMUNN. 



Postscript. 



The Spectrophotometry of Spongioporphyrin. 



In this Journal (vol. 40), in his paper "On the Gi'een Pig- 

 ment of the Intestinal Wall of Chgetopterus," Professor 

 Lankester has given curves showing the percentages of light 

 transmitted by solutions of chsetopterin and bonellin. These 

 curves were drawn by Professor Engelmann, being worked 

 out by means of his microspectrophotoraeter. At Professor 

 Lankester's request I here give spectrophotometi-ic curves for 

 feebly alkaline and fully acid spongioporphyrin. I may 

 briefly describe the insti^ument by means of which these 

 curves were obtained. It is constructed on the lines first 

 proposed by Vierordt/ but with improvements since made 

 by the brothers Kriiss^ and others. It consists of a double 

 slit, the jaws of which open symmetrically to the optic axis ; 

 this was made by Mr. A. Hilger, and he has constructed the 

 screws so well that there is not the slightest " lash," or any 

 other defect. The eye-piece is provided with the usual 

 moveable shutters, and the instrument with a measuring 

 arrangement ; by means of these a slice of the spectrum can 

 be isolated and measured in wave-lengths. The glass cells 

 are provided with Schulze's "glass body," and are 11 mm. 

 from front to back, the "glass body" being 10 mm. in the 

 same direction ; thus one can compare a depth of fluid of 

 1 mm. with one of 11 mm., which is the same as compai'ing 

 10 mm. with a depth of nothing. The graduated drumhefids 

 attached to the screws, moving the jaws of the slit, allow 



' ' Journ. Physiol.,' vol. vii, No. 3 ; vol. viii, No. 6. 



^ Vierordt, ' Die Anwendung- des Spectralapparates zur Photometrie der 

 Absorptionsspectren,' etc., 1873 ; and ' Die Qiiantitativ Spectralanaljse in 

 ilirei- Anwendunjr,' etc., 1876. 



2 Kiiiss, G. and II., ' Koloriineirle nnd Quantitativ Spektral Analyse,' 

 etc., 1891. 



