ACTION OF ULTRAVIOLET LIGHT ON VIRUSES 



155 



The fact that tobacco mosaic virus consists of long, thin rods, 

 and so can be oriented, means that the powerful tool of polar- 

 ized ultraviolet absorption can be used. This was done by 

 Butenandt, Friedrich-Freksa, Hartwig, and Scheibe. Tobacco 

 mosaic virus is pressed into a fine capillary in a quartz tube and 

 thereby acquires the orientation of the capillary. Light then 

 passes through this capillary on to the slit of a spectrograph. 



1.2 - 



1.0 - 



^0.8 



S0 6- 



0.4- 



0,2 



2450 2500 



2600 ^^^ , .^.2700 

 Wavelength 



2800 



2900 



Fig. 6.6. Absorption spectrum of TMV at room and low temperatures, 

 taken by Suprynowicz. Some sharpening is apparent, but no great increase in 

 structure over what can be seen by careful examination of plates at room 

 temperature. 



At the exit end of the spectrograph is placed a sheet of crystal- 

 line quartz cut perpendicular to its optical axis and tilted at a 

 slight angle so that the light having one plane of polarization is 

 displaced to a different point than is light having a perpendicu- 

 lar plane of polarization. This occurs because the two classes of 

 light have different refractive indices in the quartz. The spectro- 

 graph thus gives two images, one polarized parallel to the TMV 

 in the capillary, and one perpendicular to it. The relative in- 

 tensities of these, as a function of wavelength, are then measured. 



