280 HOW TO SEE WITH THE MICROSCOPE. 



Lesson Tenth. Having succeeded in displaying the 

 striae of the Leipsig, selecting one of the larger and 

 most vigorous valves, it will be instructive to notice 

 once more, and particularly the edges of the shell, if 

 the frustule is flat and the objective equal to the work. 

 There ought not, as has been before intimated, be the 

 slightest indication of any line bounding the shell, such 

 as would be seen in a drawing on paper; but in place 

 thereof (so to speak) ; on looking closely, there may be 

 discovered, when things are just right, an exceedingly 

 fine band of peculiar light, the exact tint of which will 

 vary with the particular objective employed; and, as a 

 rule, the most brilliant tone will be noticed on the upper 

 half of the shell. If necessary, manipulate the mirror a 

 bit, and endeavor to "gather up" this band so as to 

 restrict it to the narrow space bounded by the very 

 edges and the termination of the striae. This band or 



O 



tint cannot be well seen on the Cherryfield or the Mon- 

 mouth, unless, indeed, on the very smallest shells, which 

 for the present have been held in reserve. Should there 

 be any tendency of the "lines" to "rise up," try a 

 deeper eye-piece, and, shifting the adjustment a trifle, 

 seek to remove the trouble, returning to the inch ocular 

 as quickly as practicable. 



This pale boundary tint may, with different glasses, 

 assume quite different tints. With one it may appear 

 as a pale blue; another will show it of a delicate fawn 

 color, or even a pale pink. Objectives having the most 

 perfect corrections (I do not mean the most achromatic) 

 will give this band in what I call an " apple green," 



