2l6 
HAEMOGLOBIN. 
It is usual to determine the value of the constant A of any coloured 
body under examination for, at least, two spectral regions. The reasons 
for this practice will appear in the sequel. 
(b) The actual methods of spectrophotometry.— The elementary theo- 
retical discussion of the theory of spectrophotometry which has preceded has 
shown that, as developed by Yierordt, it resolves itself into the determination 
of the extinction-coefficient and of the absorption relation of coloured bodies, 
and that the optical investigation is concerned with, and confined to, the 
determination of the value of e. We have now to consider the two principal 
methods by which this determination can be effected. 
Vierordfs method. — For the determination of the extinction-coefficient 
according to the original method of Yierordt, any good spectroscope of the 
type introduced by Bunsen for laboratory purposes may be employed, provided 
certain modifications and additions are made. The most essential of these 
modifications consists in replacing the usual single slit of the collimator by a 
double slit, i.e. by a slit composed of two independent halves — an upper one 
and a lower one — each of which is controlled by a micrometer screw provided 
with a divided circle or drum, so that the Avidth of each half of the slit may 
be ascertained by direct reading (see Fig. 26). In so-called symmetrical slits, 
Fig. 26. — Double slit employed in Vierordfs method of spectrophoto- 
metry, as adjusted to their spectrophotometers by the Brothers 
Kriiss of Hamburg. 
both edges of the slit move symmetrically. When the two halves of such a 
slit are of the same width, if the illumination he uniform, the observer, on 
looking through the telescope of the spectroscope, observes two superposed 
spectra of equal brightness. If one slit be narrower than the other, the 
illumination of the corresponding spectrum will be diminished in proportion. 
The second modification which has to be made in the ordinary spectro- 
scope consists in substituting for the usual eyepiece, one which is provided 
with a slit for isolating any desired region of the spectrum, the remainder of the 
spectrum being concealed from view. In Yierordt's original instrument this slit 
was formed by two lateral shutters, moving in the focal plane of the eyepiece, 
which could be approximated to any desired extent. This simple contrivance 
has been perfected by Hiifner, and adapted to his beautiful spectrophotometer. 
A very ingeniously contrived and readily adjusted slit has been devised by the 
Brothers Kriiss of Hamburg, and adapted to the spectrophotometers made by 
