THE USE OF ABSORPTION SPECTRA 13 



rather rapidly diminishes the amount of nucleic acid which can 

 be demonstrated in a section. On the other hand, if a heavy 

 metal such as mercury is present in the fixative, the hydrolysis 

 may profitably be more prolonged, with an increase in the 

 intensity of reaction and no evidence of loss of material even 

 after 30 minutes' hydrolysis. It seems clear from these obser- 

 vations of Bauer that the Feulgen hydrolysis, in some way at 

 least, must be making it possible for nucleic acid to diffuse, and 

 that it may be more firmly anchored in the section by conver- 

 sion into the salt of a heavy metal such as mercury before the 

 hydrolysis is carried out. If the hydrolysis makes it possible 

 for nucleic acid to diffuse out of the section, then it may also be 

 possible for it to diffuse into the section. In consequence, there 

 must remain some slight doubt as to whether the details of the 

 picture revealed by the Feulgen technique are not to some de- 

 gree invalidated by diffusion artefacts. It is to be regretted 

 that no attempt has been made to study carefully the extent 

 to which diffusion does, in fact, take place during the Feulgen 

 procedure. 



It would be of considerable value if a film could be made, by 

 means of ultraviolet light, of changes in the distribution of 

 nucleic acid in cells which are undergoing chemical fixation, 

 and the results so obtained compared with the results obtained 

 by freeze-drying, following which a study should be made of 

 the course of distribution of ultraviolet-absorbing material in 

 the same specimen, preferably by filming the cells during hy- 

 drolysis. It seems likely that the difficulties in such a problem 

 caused by prolonged exposure to ultraviolet light can be mini- 

 mized by using a newly developed instrument, the television mi- 

 croscope, which requires much less intensity of light than does 

 a direct photograph with a photographic film. 



The Use of Absorption Spectra 



Practically all cytochemical methods involve at one stage or 

 another the observation of the absorption spectrum of a speci- 

 men. It is, therefore, necessary to bear in mind certain prob- 

 lems which constitute limitations in the interpretation of such 

 studies even when the instrumental problems have been solved. 



