METHODS OF SPECTROPHOTOMETR Y. 



217 



their firm. Whatever the precise form of the slit in the eyepiece, it must 

 permit of the isolation of a perfectly denned spectral region, and of the precise 

 determination of the limits of that region, these being expressed in wave 

 lengths. 1 For all coloured solutions there are regions in which the absorption 

 of light is peculiarly distinctive, and which are specially favourable to the 

 determination of the coefficient of extinction. In the case of oxyhaemoglobin, 

 Hiifner has in his most recent researches selected a part of the region 

 between the two absorption-bands (A. 550-A. 540) and a part of the region 

 lying within the second band (A 542'5-A 531 -5). 



There remains to be described an absolutely essential accessory to the 

 spectroscope, without which it would be impossible to determine the spectro- 

 photometric constants. This is a specially contrived glass trough, for holding 

 the solutions to be investigated, the anterior and posterior walls of which 

 are formed by two perfectly parallel glass plates. Two forms of this trough 

 are shown in Fig. 27, whilst Fig. 28 exhibits a trough mounted on its 



FIG. 27. Glass troughs for containing the liquids 

 to be examined by the methods of spectro- 

 photometry. After Kriiss. 



FIG. 28. Trough mounted on stand, 

 as used in spectrophotometry. 

 After Kriiss. 



stand, the stand permitting of the trough being easily and gradually 



lowered or raised, and of its being accurately levelled. 



The inner surfaces of the parallel glass plates of the 



little trough are exactly 11 mm. apart. A glass 



cube (called after the person who suggested its use, 



der Schuh'sche Glaskurper) exactly 10 mm. broad, 



and half the height of the interior of the trough, 



rests on the floor of the latter, so that the anterior 



and posterior surface of the cube shall be parallel 



with the glass plates of the trough (Fig. 29). When 



the coefficient of extinction of a coloured liquid is 



to be determined, such a trough is filled with it. 



When light passes through the lower half of the 



trough, it must traverse a stratum of coloured liquid 



1 mm. in thickness, whilst light passing through 



the upper half traverses a stratum 11 mm. thick. 



FIG. 29. Section of glass 

 trough with the Schuh- 

 'sche Glaskorpcr, a, in situ 

 (schematic). After Kriiss. 



In the latter case, the light is subjected to the absorbing action of a layer of 



1 The reader who wishes to understand the details which are necessary for practical 

 work in spectrophotometry is advised to read in the first instance a useful, indeed almost 



