10 II. METHODS OF INVESTIGATION 



employed. A number of works are available {H^, 991, 1213, 2200, 

 2529, 2588, 3026) which provide this information, and more extensive 

 bibliographies may be found there also. Rather is it necessary that 

 the reader shall be familiar with the general outline of the theory 

 and technique of the methods, and their particular application and 

 significance to the present subject. 



2. SPECTROSCOPY 



The property of characteristic light absorption has been utilized 

 as the most readily available means of distinguishing many of the 

 substances with which we are concerned. It must, however, be appre- 

 ciated that, although observation of absorption spectra is a versatile 

 and powerful weapon when properly used, it is open to serious errors. 

 While it is certainly true that under identical conditions the same 

 substance cannot have different absorption spectra, spectra which 

 are apparently identical are insufficient evidence for chemical identity. 

 As an example may be cited the variety of substances having an 

 absorption band at about 630 m/u {cf. Chapter X). It is always neces- 

 sary to demonstrate identical alterations in spectra when chemical 

 reactions are performed, before identity of two substances can be 

 considered in any degree certain. 



2.1. The Direct Vision Spectroscope 



The simple direct vision spectroscope is quite indispensable for 

 work on compounds in this field. The instrument should be of 

 medium dispersion, since high dispersion may obscure faint bands. 

 A wavelength scale is an advantage for approximate location of 

 bands, but the instrument should be regarded as primarily qualita- 

 tive, and accurate wavelength determinations made by other means. 

 The more elaborate small dispersion microspectroscope is of advantage 

 for investigation of complex spectra, e.g., those of the cytochromes, 

 in tissues and turbid media. 



2.2. The Hartridge Reversion Spectroscope 



This instrument is designed for accurate location of sharp absorp- 

 tion bands. It consists essentially of two relatively high dispersion 

 grating spectroscopes, arranged so that the two spectra produced 

 from one entering light beam are contiguous along their length, but 

 are reversed in direction, one having the short wavelengths (blue) 



