210 HEMOGLOBIN. 



ately the position of any line or the boundaries of any absorption-band 

 observed in the spectrum, it being usual to express the position in terms of 

 the wave length of the light corresponding to it. With this object the 

 spectrum of sunlight is observed, and the position of the principal lines of 

 Frauenhofer is determined in reference to the divisions of the photographic 

 scale, or, in the case of the finer spectroscopes and spectrometers, in reference 

 to the divisions of the graduated circle of the instrument. From the results 

 of these observations a curve is readily plotted, enabling the experimenter at 

 any time to convert the readings of the arbitrary scale of his instrument into 

 wave lengths. 1 



For all exact spectroscopic work the eyepiece of the spectroscope should 

 be provided with cross-threads ; and, when employed in the investigation of 

 absorption spectra, if possible with the arrangement employed in spectro- 

 photometry, which enables the observer* to limit, by a variable slit in the 

 eyepiece, any particular spectral region and to shut out of the field of view 

 the remainder of the spectrum. 



As a source of light, for some investigations the light of the sun reflected 

 from the mirror of a heliostat driven by clock-work is desirable ; for general 

 purposes the light of the sun, reflected from a white surface, may be employed. 

 Artificial sources of illumination possess the great advantage of being available 

 at all times, and susceptible of considerable constancy. A gas lamp, furnished 

 with the Auer incandescent burner, is the best of all lamps for the examina- 

 tion of absorption spectra. 



In examining the absorption-spectra of liquids, it is convenient to employ 

 cells or troughs with perfectly parallel glass or quartz sides, which are a 

 definite width apart. Such vessels are made according to the model of 

 Hoppe-Seyler, and sold under the name of hcematinometers (Fig. 23), the 

 internal surface of the parallel glass plates being exactly 1 cm. apart, and the 

 little trough being so arranged as to be readily taken to pieces for cleaning. 

 The small troughs employed in spectrophotometry, and which are usually 

 constructed with great care, are well adapted to the general purposes of the 

 spectroscopist. 



Instead of a vessel of which the sides are at a constant and known dis- 

 tance apart, it is convenient for many purposes to employ the so-called 

 hcematoscope, or licemoscope, of Hermann, 2 as shown in the accompanying 

 woodcut (see Fig. 24). F is a glass plate, forming the anterior wall of the 

 tube D, which is supported on the stand A. C is a metallic tube, sliding in 

 and out of the tube D, and closed anteriorly by a glass plate parallel to F. E 

 is a funnel communicating with the interior of D F B. By sliding the piston 

 C in and out of the tube I), the capacity of the vessel I) F B and the 

 depth of a stratum of liquid contained between the two glass plates, may be 

 modified at will within wide limits. 



The depth of the stratum is read off by the aid of a millimetre scale, 

 engraved on the sliding tube C. 



As the absorption of light passing through a coloured liquid depends 

 upon the number of absorbing molecules in its path, by doubling the thick- 

 ness of the stratum of a coloured liquid examined, we obtain the same result 

 as by examining a solution of double concentration. With such a contrivance 

 as the hsematoscope, we are, within certain limits, able therefore to obtain the 

 same result with a solution of constant concentration as with a large number 

 of solutions of which the concentration varies in known proportions. 



1 In a work intended for the advanced student of physiology, it appears superfluous to 

 enter into such details concerning the construction of the spectroscope, or the method of 

 working with it, as can be learned in all courses of practical physics, or may be found in 

 any elementary treatise devoted to this branch of science. 



2 " Notizen fur Vorlesungs und andere Versuche," Arch. f. d. ges. Physiol., Bonn, 

 Bd. iv. S. 209. 



