INTRINSIC ABSORPTION SPECTRA 97 



7. Antigens. It is probable that individual proteins can conveniently 

 be studied by their reaction with coloured antibodies. 



The number of methods available for study of the nucleic acids 

 and the proteins is considerable, although as yet far from suffi- 

 cient for obtaining all the information which is required about 

 their cytochemistry. The methods include: 



1. Observation of the intrinsic absorption spectrum. 



2. Observations made after the use of chromogenic reagents which 

 react with particular groups. 



3. Determination of the localization of enzyme activity. 



4. Observation of the reactions of tissues with colored antibodies. 



5. Staining with dyes. 



6. Study of the action of enzymes such as proteolytic enzymes and 

 nucleases, on tissue sections and squashes, etc. 



In this chapter we shall consider only the methods involved 

 in 1, 2, and 4. The methods concerned in 3 have already been 

 considered in Chapter 3. The methods involved in 5 and 6 are, 

 in my view, not sufficiently reliable except for preliminary studies. 

 The reasons for this have been given elsewhere (Danielli, 1946), 

 and there seems to be no reason for revising the opinions given 

 in this earlier publication. 



The Analysis of Intrinsic Absorption Spectra 



As has been shown by the accomplished work of Caspersson 

 and his school, a remarkable amount of information about pro- 

 teins and nucleic acids can be obtained even when one is limited 

 to studying the ultraviolet absorption spectra. When reflecting 

 microscopes become more readily available it should be possible 

 to use, with more or less equal facility, absorption bands ranging 

 from the infrared to the far ultraviolet. At present most studies 

 have been limited to the regions between 2500 A.U. and 3000 

 A.U., which is the region in which the pyrimidine ring and some 

 of the groups of proteins have strong absorption bands. It is un- 

 likely that the extreme ultraviolet beyond 2400 will be a very 

 profitable region for study, since so many substances have strong 

 absorption bands in this region. There is also a considerable lim- 

 itation on the usefulness of moving into the infrared regions, due 

 to the decline in resolving power as one moves into the longer 

 wavelength regions of the spectrum. This limitation is more 

 severe than is generally realized. Thus, although the limit of 



