C. J. p. SPRUIT AND A. SPRUIT-VAN DER BURG 111 



colored substance with considerable absorption in the blue and violet 

 regions of the spectrum. From visual observation we feel reasonably 

 sure that the emission maximum of Cypridina is at shorter wavelengths 

 than that of Ph. phosphoreum (472m/A). Three spectra of the emission 

 of Photinus pyralis are known to us. They do not agree among each 

 other save that the maxima of emission are rather close together (563, 

 565, and 568 m/i ) . As this is far in the green, where there is not much 

 self-absorption, this maximum should be quite reliable. 



At any rate we may conclude that the emission spectra do prove 

 that the emitting molecules in these various organisms are definitely 

 different. As all these spectra are related to one band in the absorption 

 spectra of the compounds involved, it will be clear that little infor- 

 mation can be drawn from them without the assistance of further 

 independent observations. 



It has been suggested on several occasions that bioluminescence is 

 connected in some way or other with the presence in the cell of de- 

 rivatives of riboflavin. It is therefore advisable to point out that none 

 of the emission spectra obtained so far show much resemblance with 

 the fluorescence of riboflavin (Fig. 2), the maximum of which is sit- 

 uated at 534 m/x. 



If the luminescent system occurs in the cell in the form of an en- 

 zyme with a specific substrate, the concentration of this compound in 

 all probability will be so low that direct observation by absorption 

 spectroscopy will be impossible. Accidentally, during a study of car- 

 bon monoxide inhibition of luminescence, van Schouwenburg and van 

 der Burg came across an indirect method for the measurement of the 

 absorption spectrum of the luminescent system in Ph. phosphoreum. 

 The idea has been the basis of an investigation by van der Kerk and 

 van der Burg. Whereas irradiation with red light has no effect upon 

 luminous bacteria, irradiation with comparable intensities of blue 

 light tends to dim the light emission of the bacteria. A quantitative 

 study of this effect, which is fully described in the thesis of van der 

 Kerk (1942) (Kluyver et al, 1942) led to the establishment of an 

 action spectrum for the quenching of bacterial luminescence which 

 should be proportional to the absorption spectrum of the system which 

 emits the light. Later observations ( Spruit, 1946, 1949a ) have necessi- 

 tated a correction to be applied to this spectrum at wavelengths below 



