A photographic method of microspectrography applicable to living cells is described. Absorption spectra and the total amounts of absorbing substance in living chick fibroblast nuclei are measured. It is concluded that the nucleic acid of the living cell nucleus is in a condition in which it can absorb, and that the absorption spectra are similar in form to those obtained by other workers on fixed material. The increase in absorption of chick fibroblast nuclei after irradiation with ultra-violet light at 265 mp is a true increase in absorption and is accounted for by the decrease in projected nuclear area. The total amount of absorbing substance in the nucleus is actually found to decrease during irradiation. After certain assumptions have been made preliminary measurements of the total amount of desoxyribo-nucleic acid of individual fibroblast nuclei are obtained by the integration methods outlined. They are jn approximate agreement with the results of other workers obtained by bjochemical extraction.

The application of the techniques of microspectrography to living cells makes it possible to obtain data which will indicate the nature of some cellular components in their natural state. Furthermore, microspectrography may be linked with methods of obtaining a measure of the tctal amount of the absorbing substances in these components. Photographic methods of obtaining this information offer certain advantages: