POSITION OF OBSERVER. 255 



line shells of navicular rhomboides, and nicely assorted 

 iis to size and "difficulty." The larger, or even the 

 medium sized shells are easily displayed (we refer to 

 the transverse striae) by such a glass as the Spencer dry 

 one-fourth of 115, when nicely adjusted, while the 

 smaller frustules are more "difficult" than the No. 18 

 of the Moller plate. These mounts are therfore valu- 

 .able on account of their range, and the reader has 

 already our reasons governing the selection thereof. 

 When these slides are thoroughly mastered, the student 

 will be ready to attack the Saxonica from Leipsig. 

 These vary also, a medium shell being about as obsti- 

 nate as the No. 18 of the Moller plate, and will defeat 

 the Spencer one-fourth. The Saxonica from the Isle 

 of Shoals is still more bothersome. These frustules 

 are very smaii i nd very thin, and at the start the 

 student should select the very largest and most vigorous 

 shell he can find on the mount, proceeding carefully as 

 he may acquire skill, to the smaller ones, and before he 

 can establish any claim to have mastered this slide he 

 must be able to show the very smallest and thinnest 

 shell acceptably. In point of ''difficulty" these smaller 

 diatoms rival the No. 19 of the Moller plate, while the 

 smallest and the very thinnest are a fair match for the 

 No. 20. Of the amphipleuras, that from the " Bridge 

 of Allan," Scotland, is the easier of the two, and 

 should be studied first. This slide in common with the 

 two last mentioned calls for object-glasses of high 

 apertures. To attack them with any of the low angled 

 glasses is but a waste of time and patience. The mount 



