POSITION OF OBSERVER. 295 



tion prevailing, he may be pretty sure that his glass is 

 at fault. His time will therefore be much better spent 

 in the examination of such objects as come within its 

 powers. This point is well worth close attention, thus, 

 perhaps, saving a double outlay of time and patience; 

 and there has been a vast amount of valuable time 

 wasted in fruitless attempts to resolve severe tests, 

 for which purpose the objective was totally unfit and 

 incapable. 



The illumination we have just described is quite suffi- 

 cient for the display of the transverse striae of either 

 slide of Amphipleura. As we have before stated, the 

 " Bridge of Allan" is the easier of the two and should 

 be studied first, and the student should give preference 

 to the cleanest shells on the mount. If the smaller 

 scales of the Isle of Shoals are well seen, and the 

 " Bridge of Allan " resist, the fault will certainly be in 

 the adjustment of the objective over the latter mount; 

 possibly it may occur that the observer has selected an 

 exceptionally difficult shell of the Pellucida. The bet- 

 ter way will be to look over the slide in detail, of course, 

 as before advised, giving preference to the cleaner and 

 most inviting frustule. 



If so be that the student has the Moller probbe plate, 

 the illumination given is also quite sufficient to display 

 the Nos. 18, 19, and 20. After he shall have become 

 well advanced, there are some particular advantages 

 connected with the study of this plate. He ought to 

 see the Nos. 18 and 19 with but little trouble, and in a 

 little time with the radial arm at 45 from axis. On this 



