SPECIES IDENTITY OF NUCLEUS-PLASMA NORM 473 



7. The difficulty of cutting straight edges in the thicker wax 

 plates necessitated to correspond to the magnification in the 

 two micra sections. An electrically heated knife was found 

 essential. The cut out wax layer was always checked by super- 

 imposing on the tracing and corrected. In several cases, a plate 

 of half thickness was used, and each layer reduplicated. 



8. Spherical aberration of microscopic lenses. It was noted 

 with the highest powers that the readings from the object microm- 

 eter were appreciably larger toward the periphery of the field. 

 In a cell of ten layers, this would have a decided influence on the 

 weight of the much larger plasma. The higher coefficient figure 

 which resulted on Cell 4, table 2, was thought to be due largely 

 to this, and at least the figure fell without altering consciously 

 any other factor in the repetition. The difficulty in general was 

 obviated by making as many cells as possible from the same 

 plate and at the same magnification. 



9. Inequalities in the thickness of the wax plates. This 

 refers to localized inequalities which might occur judging from 

 the appearance of discarded plates. The allowed inequalities 

 within one-tenth of a millimeter probably average up. 



10. The limitations in estimating the magnification. The 

 limits of certainty seemed to be within twenty or thirty; beyond 

 that it became an approximation. 



3. In the application of the prismoid formulas 



1. Incomplete end sections in the series of either the cell or 

 nucleus. Such layers of only a fraction of the full depth could 

 only be computed as cones and added to the results obtained by 

 Simpson's rule. 



2. Each cell is divided b}^ the knife into a certain number of 

 layers, each constituting a frustrum, whose bounding parallel 

 planes differ more or less in area according to the slope. The 

 tracing off of these layers from the microscope must in the main 

 follow the outline of the larger area, call it the base of the frustrum. 

 Theoretically, because on one side, for one-half of the spheroidal 

 body, the slope of the edges of the aggregated layers is the reverse 



