216 



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. 



Vierordt's 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 width 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 Vierordt's 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 saine width, if the illumination be 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 



