POSITION OF OBSERVER. 283 



on the angle of the illumination little by little, adjust- 

 ing the mirror so as not to lose the show of the lines. 

 Now if the student has studied well what has thus far 

 been taught, he ought to be astounded at two conditions 

 which will flash on his intelligence. In the first place, 

 he will find that he can decrease the angle of the illumina- 

 tion down to, say 35, all the while holding the stria? 

 well in hand. The probability is that he has now too- 

 much light, and it will be well to try the effect of turn- 

 ing down the wick of the little lamp a little. Having 

 tound the very lowest angle of the illumination that 

 will command a full show of the striae, return the mir- 

 ror just a trifle just enough to reinforce the display of 

 the lines. Compare the effect with that of the previous- 

 lessons. Notice that your strise shell and all have be- 

 come more transparent. There is not even a suspicion 

 of oblique illumination. Your diatom (and all the 

 others decently in focus in the field) swim in a sea of 

 fire. That's a cold word "lightning" expresses the 

 idea better. Observe how flat the shell is! Not one- 

 particle of distortion. Observe, too, that shadow we- 

 have before called attention to, under the median line. 

 See how transparent it has become. Did you say that 

 you cannot see it at all? Yes, it's there ; look sharp 

 for it I Now look down between the striae. See how 

 nicely, how exquisitely they are cut apart. The spaces 

 between them are as visible as the striae themselves. 

 Notice, too, how that the most infinitesimal flickering^ 

 of the lamp flame plays with consummate grace in and 

 among the lines. Look well to the edges of the valve. 



