92 



METHODS OF OBSERVING ABSORPTION SPECTRA. [BOOK I. 



vessels are made, after the plan of Hoppe-Seyler, for the purpose of 

 the physiological chemist, and sold under the name of Haematinometers 1 ; 

 the glass plates are exactly one centimetre apart, so that when the 

 apparatus is filled with liquid, the observer knows that he is examining 

 a stratum 1 cm. broad. Instead of such a vessel the Haematoscope or 

 Haemoscope of Hermann 2 , shewn in the accompanying woodcut, may 

 be employed. F is a plate of glass, and the piston G is a metallic tube 

 closed at its inner end by a plate of glass. Ity sliding the piston C in and 

 out of the tube D the capacity of the vessel DFB and the depth of a 

 stratum of fluid contained between the two glass plates may be modified at 

 will within wide limits. The depth of the stratum is read off by means of 

 a millimetre scale engraved 011 the sliding tube (7. 



FIG. 16. THE HAEMATINOMETER. 



FIG. 17. HAEMATOSCOPE. 



Whichever the exact form of vessel containing the blood to be 

 examined, it is interposed between a source of light and a suitable 

 spectroscope. 



Various forms of spectroscope may be employed in these researches. 

 Any ordinary spectroscope adapted to the requirements of the chemiht 

 will answer ; it is advisable, however, that the instrument shall be 

 provided with an arrangement for observing simultaneously two spectra, 

 and with a scale. 



1 These are sold by Schmidt and Haensch, Berlin. 



2 Hermann, "Notizen fur Vorlesungs- und andere Versuche." Pfliiger's Archiv, 

 Vol. iv. (1871) p. 209. 



