266 HOW TO SEE WITH THE MICROSCOPE. 



sition, as per lesson second, focus your new shell and 

 examine it critically, as has already been directed ; next,, 

 repeat the turn of the collar, in the same direction, but 

 varying" the extent of the movement, focussing 1 again 

 as before. Bye-and-bye, when the collar shall have been 

 turned to approximately the correct place, the strifie will 

 of course be again visible, and it is more than probable 

 will be even better seen than before. If this shall be 

 the case, try, by slight manipulations of the mirror 

 alone, to obtain the very best display of the lines. And 

 we now approach close work. If the eyes are tired, or 

 the general condition of health be below par, it will be 

 better, if any further work be determined upon, to go 

 back and take a review of what has already been accom- 

 plished; but on the other hand, if conditions are gen- 

 erally favorable, proceed thus : 



Lesson Fourth. With your frustule as nicely exhi- 

 bited as possible, the shell selected being as small as you 

 can successfully attack to show well the stride, examine 

 it well closely; see if the upper and under edge are 

 both equally well in focus. It may be that one edge is. 

 much better denned than the other, or it may be that 

 one edge cannot be seen at all. This indicates that the 

 valve lies sidewise in the balsam, in which event discard 

 the shell and hunt up another that lies exactly flat, and 

 of the same size. 



Now work over this diatom, generally as you have 

 been instructed in the foregoing lessons, accepting the 

 position you now have of the adjustment as the base of 

 farther trials. Change the collar to the right or left* 



