Electron Microscopy of the Lymphocyte 55 



cised. The thinness of the sections used is of critical importance to the 

 observer. A section of average thickness passing through the equator of a 

 rounded-up cell contains considerably less than 1 per cent of the total 

 volume of the cell. It is obvious that structures visualized in micrographs of 

 this section are surely present in the cell, but the absence of any known 

 or anticipated structure is not an indication that the cell does not possess it. 

 The thin sections of electron microscopy also impose severe limitations on 

 measurements of size, judgment of contours, and estimates of population 

 densities. A discussion of these limitations, with special reference to blood 

 cells, has been presented elsewhere (Reference 11, pp. 2-7). The proper 

 interpretation of very high resolution electron micrographs may be attended 

 by similar difficulty but for a distinctly different reason. In micrographs now 

 routinely obtainable, resolution far surpasses section thinness, so that great 

 difficulty may be encountered with proper judgment of size, shape, number, 

 density, etc., when very small structures are being interpreted. This is be- 

 cause the optical situation in an electron microscope is such that the entire 

 thickness of a section is equally clearly in focus. Depth of field exceeds 

 section thickness in electron microscopy, whereas the reverse is true in light 

 microscopy. The bearing of this phenomenon on interpretative problems, 

 including a comparison with the optical situation in light microscopy, has 

 been discussed in a previous paper (Reference 14, pp. 242-246) . Both thin 

 sections and the unique optics of the electron miscroscope are responsible 

 for characteristics in electron micrographs that are unfamiliar to light micro- 

 scopists. Their fundamental nature is such that they point up the need for 

 an essentially statistical approach to the analysis of protoplasmic ultra- 

 structure. Final judgments may be considered safe only when based on a 

 large number of electron micrographs. These, in turn, must be interpreted 

 with due regard for their inherent limitations as well as for the wealth 

 of new detail they present. 



Special interpretative care should also be taken when comparing electron 

 micrographs with light micrographs of dried smear preparations. In the 

 latter all the contents of the cell remaining after drying and fixation are 

 piled one on top of the other on the slide. The resultant patterns may be 

 different from those of the thin sections of electron microscopy, where onlv 

 a thin lamina of the cell is present. Furthermore, the patterns visualized by 

 the chromatic dyes of light microscopy differ from those visualized in elec- 

 tron micrographs, since the latter are largely an expression of the affinity of 

 Os0 4 for ethylenic double bonds. 15 



The morphologic patterns familiar to the light microscopist cannot be 

 expected to repeat themselves exactly in electron microscopy. But the wealth 

 of new information afforded by electron micrographs more than counter- 



