342 



R. DULBECCO 



[vol. 59 



pigment that absorbs the reactivating hght (see Loofbourow, 1948, for dis- 

 cussion of this relation); we tried, therefore, to obtain on this basis some infor- 

 mation about the photosensitive pigment. The action spectrum is not detailed 

 enough to give a specific indication; it shows, however, that the pigment is not 

 contained in the unmodified phage, since the absorption spectrum of purified 



320 340 360 380 



Wave Len^i-h J mu 



440 



Figure 7 . Absorption spectra of purified phage T2. The optical density is plotted against 

 the wave length. The spectra were obtained with a Beckman quartz spectrophotometer. 

 The phage was suspended in phosphate buffer (m/15, pH 7) plus MgS04 10"' u. A. Active 

 phage, concentration 5.5 X 10^" infecting units per ml. B. Upper curve : absorption spectrum 

 of active phage for wave lengths longer than 320 m^t, concentration 2.3 X 10" infecting units 

 per ml; lower curve: absorption spectrum of the same phage after 2 hours' irradiation with 

 the germicidal lamp at a distance of 12 inches. 



phage has no band comparable to the action spectrum of PHTR. This is shown 

 by figure lA, which reproduces the absorption spectrum of purified phage T2. 

 For wave lengths longer than 320 mju the absorption closely follows Rayleigh's 

 law of scattering and is, therefore, due to scattering of the light. This is shown 

 more convincingly by plotting the logarithm of the optical density at different 

 wave lengths against the logarithm of the wave length; according to Rayleigh's 



241 



