GEEEN PIGMENT OK INTESTINAL WALL OF CHiETOPTEttUS. 467 



milliontlis of a inillinietie on the left (red end) — a division line being ruled at 

 every point corresponding to the ])osition of a difference in wave length of 

 10 milliontlis of a niillinietre. In this drawing the dispersion of the spectrum 

 as actually seen is represented, the intervals corresponding to 10 millionths 

 of a millimetre of wave-length becoming increasingly larger as we pass from 

 the red (wave lengths of 750 — G50 milliontlis) to the blue and violet (wave 

 lengths of 500 to 400 millionths). But in the charts given in the next two 

 plates, for which I am indebted to Professor Engelmann, the intervals 

 occupied by wave lengths differing by ten millionths of a millimetre are laid 

 down without reference to dispersion at equal distances from one 

 another. The charts in fact correspond to a pure spectrum, whilst the 

 drawing, fig. 5, represents the appearance given by a prism of small dis- 

 persion. 



PLATE 35. 



Figs. 6 — 8. — Representation of the colour of the neutral, acid, and alkaline 

 alcoholic solutions of Chsetopterin, as seen by transmitted light. 



Figs. 9 — 11. — llepresentation of the colour of the neutral, aeid, and 

 alkaline alcoholic solutions of Bouellin, as seen by transmitted light. 



PLATE 36. 



Charts prepared by Professor Engelmann showing the intensity of absorp- 

 tion in different parts of the spectrum of acidulated, alkaline, and neutral 

 alcoholic solutions of Chsetopterin. The horizontal lines in the charts — 

 numbered in ten groups of ten — correspond to 100 units of light intensity. 

 The round dots indicate the successive parts of the spectrum observed and 

 measured. The position of the dot in vertical displacement records tlie per- 

 centage of light transmitted. Thus the highest horizontal = 100 j)er cent., 

 the lowest per cent, or complete absorption. The successive "dot points " 

 of observation are joined by oblique lines, giving thus a continuous but 

 irregular curve of absorption. The vertical lines as shown by lettering on 

 the chart correspond to millionths of a millimetre of wave length. The 

 position of the chief solar lines is also indicated by strong vertical lines on 

 the charts. The upper chart has the record of four distinct solutions. The 

 two records in dotted lines are those relating to experiments with an 

 acidulated solution — in the one case the light was passed through a thickness 

 of the solution amounting to 5 millimetres, in the other case only 2 millimetres 

 were used (of the same solution). [It is not possible in our present know- 

 edge of Chaetopterin to say what percentage of pure Chaetopterin was present 

 in the alcoholic solution.] The unbroken black lines are the records of 

 similar experiments with an alkaline alcoholic solution of Chsetopterin. In 

 VOL. 40, PART 3. NEW SER. K K 



