METHODS OF SPECTROPHOTOMETR Y. 
217 
their firm. Whatever bhe precise form of the slit iii the eyepiece, it must 
permit of the isolation of a perfectly defined spectral region, and of the precise 
determination of the limits of that region, those hem-' 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 oxyhemoglobin, 
llufner 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 hand (A 542'5-A 531-5). 
There remains to he described an absolutely essential accessory to the 
spectroscope, without which it would he 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 
Fir;. 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- 
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 Schulz'sche Glaskorper) 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 1 1 mm. thick. 
easily and gradually 
(I 
\ 
) 
Fig. 29. — Section of glass 
trough with the Schulz- 
'sche Glaskorper, 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 
