2 1 o H. EMO GL OB IN. 
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 liner 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 held 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 hcemoscope, of Hermann,- 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 D, the capacity of the vessel J) 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- 
iii ss 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 hamiatoscope, 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 "Xotizen fiir Vorlesungs und andere Yersuche," Arch. f. d. c/es. Physiol., Bonn, 
Bd. iv. S. 209. 
