POSITION OF OBSERVER. 267 



noticing" all the points to which your attention has been 

 called ; but these various changes of the collar should 

 be much less limited in extent. We have now further 

 items for you to observe. Examine the since carefully; 

 see if they continue quite to the edge of the shell, or do 

 they stop a little short thereof, the edge becoming as 

 clear, sharp and distinct as that of a razor. Look with 

 an eagle eye, too, into that shadow under the median, 

 line. Has it become almost self-luminous and trans- 

 parent? And has it drawn itself upwards and together r 

 forming a narrow but brilliant band (so to speak) ad- 

 jacent to the under side of the median line? Have all 

 appearances of diffraction lines left? Does the shell lay 

 well down and flat? All of these interrogatories must 

 receive your devoted attention. 



When things are just right, the valve laying flat in 

 the balsam, the striae will not quite extend to the edge 

 of the shell. The dark band under the median line 

 will be contracted as described, but in the place of the 

 former dark and muggy shadow, it will have become 

 lively and brilliant. Ditto, as to all the other shadows- 

 seen in the field. In fact, everything has improved, be 

 it a diatom or a patch of dirt. The edge of your shell 

 ought to be sharply distinguishable, not by a line of 

 varying thickness, for there is no line there, and there- 

 fore you ought not to see anything of the kind. If the 

 adjustment be a trifle out, it will sometimes occur that 

 the valve will seem to "rise up;" i.e., appear nearer 

 to the eye than it should. It is impossible to describe 

 on paper just what I mean by this. I have no trouble, 



