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ESSENTIALS OF CHEMICAL PHYSIOLOGY 



source of light parallel : they fall on the prism P, and then the 

 spectrum so formed is focussed by the telescope T. 



The third tube, D, seen in the next figure (fig. 33), carries a small 

 transparent scale of wave lengths, as in accurate observations the 

 position of any point in the spectrum is given in the terms of the 

 corresponding v^ave-lengths. 



If we now interpose between the source of light and the slit S 

 a piece of coloured glass (H in fig. 32), or a solution of a coloured 

 substance contained in a vessel with parallel sides (the hsematoscope 

 of Hermann, F in fig. 33), the spectrum is found to be no longer 

 continuous, but is interrupted by a number of dark shadows, or 

 absorption hands, corresponding to the light absorbed by the coloured 

 medium. Thus a solution of oxyhaemoglobin of a certain strength 

 gives two bands between the D and E lines ; haemoglobin gives only 

 one ; and other red solutions, though to the naked eye similar ta 

 oxyhsemoglobin, will give characteristic bands in other positions. 



Fig. 34. — Arraugement of prisms in direct-vision spectroscope. 



A convenient form of small spectroscope is the direct-vision 

 spectroscope, in which, by an arrangement of alternating prisms 

 of crown and flint glass, placed as in fig. 34, the spectrum is 

 observed by the eye in the same line as the tube furnished with 

 the slit — indeed slit and prisms are both contained in the same 

 tube. 



Such small spectroscopes may be used for class purposes, and 

 may for convenience be mounted on a stand provided with a gas- 

 burner and a receptacle for the test-tube (see fig. 35). 



In the examination of the spectrum of small coloured objects, a 

 combination of the microscope and direct-vision spectroscope, called 

 the micro-spectroscope, is used. 



Fig. 36 illustrates a method of representing absorption spectra 

 diagrammatically. The solution was examined in a layer 1 cen- 

 timetre thick. The base line has on it at the proper distances 

 the chief Fraunhofer lines, and along the right-hand edges are 



