1899] MICROSCOPICAL JOURNAL. 3 



edge, which proved the image to be unquestionably a true 

 one. In the case now before us, however, its Very min- 

 uteness prevents this test being applied, for it would be 

 quite impossible to see at a fractured edge such a struc- 

 ture, even though it were several degrees coarser. 



In microscopical images all fractures and edges are 

 bounded by an umbra, coma, or undefined margin. This 

 umbra tends to blot out all finer structures in its imme- 

 diate vicinity. Thus the umbra of a primary blots out 

 (either partially or wholly) the image of the secondary ; 

 much more, therefore, would it blot out a tertiary. For 

 example, the postage stamp fracture in a Triceratium 

 favus is not so easily seen as it might be on account of 

 the umbra of the large primary structure. On the other 

 hand there is no similar difficulty found with the postage 

 stamp fracture of a P. angulatum or a N. rhomboides, 

 for there is no coarser primary present. 



It is obvious, therefore, that we must resort to some 

 test of truth other than that of a fractured edge. Let 

 us, therefore, try counting the number of spots in each 

 aperture, for if they are ghosts they will be replicas of 

 some pattern — e. g. six spots surrounding a central one. 

 This, though a difficult operation, has been done, and 

 both a five and a four have been seen, and sometimes the 

 central one has been found wanting; further, these ob- 

 servations have been confirmed by an independent obser- 

 ver ; therefore we may conclude that' the image is not 

 that of a ghost, but an indication of an actual structure. 



If further evidence be required, it may be remarked 

 that six spots with a central dot is the interference image 

 or ghost of a quincunx pattern. Now, the images in 

 question cannot possibly be interference images of the 

 quincunx pattern of the primary polygonal structure of 

 the valve, because the position of the intercostal ghosts 

 would be in the intercostal areas ; again, as a dernier 

 resort, it may be said that it is an interference ghost o^ 



