BACTERIAL PHOTOSYNTHESIS 193 



coincides with the surface. The average diameter observed in tlie 

 thin sections of the chromatophorcs is 300 A, which agrees very well 

 with the value (320 A) calculated by Stokes' relation from the sedi- 

 mentation constant and density. We employ the latter as the diam- 

 eter of the unfixed chromatophore. This agreement between the 

 different measurements suggests that shrinkage has been minimized 

 in the specimens used for electron microscopy. 



The objective lens of the electron microscope has considerable 

 depth of focus, and points which are separated by considerable dis- 

 tances in the direction of the axis of the lens can be superimposed in 

 the final image. Since the chromatophorcs tend to pack very closely 

 in the cell, image-overlap due to the depth of focus will obscure 

 the chromatophorcs and produce optical artifacts if the sections 

 contain more than one laver of chromatophorcs. This effect also 

 makes it difficult to assign a value to the thickness of the cortex of 

 the chromatophore. There are at least four intrinsic factors which 

 influence the apparent thickness of the cortex in a section: the actual 

 thickness of the cortex, the efficiency with which the electrons are 

 scattered by this region, the thickness of the section, and the diam- 

 eter of the chromatophore. The apparent thickness of the cortex in 

 the thinnest sections is about 70 A, but the actual thickness of this 

 region must be less than this value. The magnitude of the deviation 

 becomes clear ( Fig. 8 ) when we compare the effect on the image of 

 varying the thickness of sections through the center of a sphere 300 A 

 in diameter with a completely opaque cortex. An apparent cortical 

 thickness of 70 A can be obtained for actual values ranging from 25 

 to 60 A as the section thickness decreases from 200 to 100 A. Since 

 the cortex of the chromatophore is only moderatelv opaque to 50- 

 kilovolt electrons and the thickness of the sections appears to be 

 less than 200 A, a cortical thickness of about 60 A seems reasonable. 



Since the intrinsic electron density of most cellular constituents 

 is rather low, procedures which enhance electron densitv are nor- 

 mally employed for electron microscopy. Practice has established 

 osmium tetroxide, particularlv in the buftered solution advocated by 

 Palade (1952), as the standard or reference fixative. A classical 

 aphorism, namely, that osmium tetroxide fixes and blackens fats, has 

 been responsible for a tendency to assume that the patterns of elec- 

 tron density produced bv this reagent reflect the deposition of the 

 metal or its oxides in the lipid constituents of cellular structures. 

 Nevertheless, the data have often suggested that protein rather than 



