CHAP. II.] THE BLOOD. 97 



The first to apply a spectroscope to the microscope was Mr Sorby 1 , and 

 very numerous modifications of his original micro-spectroscopes have been 

 made. In all cases the micro-spectroscope consists of a modified microscopic 

 eye-piece which has superadded to it a direct- vision prism, an arrangement 

 of slits for allowing definite quantities of light to reach the prism, usually 

 arrangements for comparing two different spectra, and finally some micro- 

 metric arrangement. In consequence of the admirable nature of the micro- 

 metric arrangement we give the preference to the instrument made by Zeiss 

 and of which a vertical and horizontal section are given in Figs. 20 and 21. 



Being provided, then, with one or other of the spectroscopes 

 piously described, or with a similar instrument, let the observer 



previous 



interpose between it and some source of light a solution of blood, say 

 made by diluting defibrinated blood with ten times its volume of 

 distilled water contained in a haematinometer (Fig. 16, p. 92) 1 cen- 

 timetre wide. It will then be found that the whole of the more 

 refrangible portion of the spectrum has been cut off but that the red 

 end of the spectrum remains visible, or rather, those rays having a wave- 

 length greater than about 600 millionths of a millimetre. 



If now a stream of hydrogen or nitrogen be passed for a consider- 

 able time through the diluted blood it will be observed that the absorp- 

 tion is least between Frauenhofer's line a (W. L. 718) and Frauenhofer's 

 line B (W.L. 6867), but that the rest of the spectrum is less bright than 

 before the gas was passed. The effect of the N or H has been to drive 

 more or less of the respiratory oxygen from the haemoglobin, and in 

 consequence there is more light absorbed ; this difference in the spec- 

 trum corresponds to the change which the blood undergoes from a 

 bright vermilion colour to a brown-red when it passes from the arterial 

 to the venous condition, in other words from a condition in which its 

 haemoglobin is nearly saturated with its respiratory oxygen, to one in 

 which a portion of that oxygen has been given up. 



If now the blood solution be rendered much more dilute so as to 

 contain - 8 p.c. of haemoglobin, on examining a stratum 1 centi- 

 metre wide the spectrum becomes distinct up to Frauenhofer's line D 

 (W.L. 589), i.e. the red, orange and yellow are seen, and in addition 

 also a portion of the green between b and F. Immediately beyond D 

 and between it and b, however (between W. L. 595 and 518), the ab- 

 sorption is intense. (See Fig. 22, 4.) On diluting still further, what 

 appeared one wide black band between D and E is seen to resolve 

 itself into two beautifully distinct absorption bands separated by a 

 green interspace (Fig. 22, 3). Of these absorption bands, the 

 one next to D is narrower than its fellow; it has more sharply 

 denned edges and is undoubtedly blacker; its centre corresponds 

 with wave-length 579, and it may conveniently be distinguished as 

 the absorption band a in the spectrum of oxy-haemoglobin. The 

 second of these absorption bands, i.e. the one next to E, which we 

 shall designate /3, is broader, has less sharply defined edges, and is 

 not so dark as a. Its centre corresponds approximately to W. L. 553'8. 

 1 Sorby, Quarterly Journal of Science, 1865, xi. p. 198. 



