POSITION OF OBSERVER. 269 



find sediment congregated together in little colonies, the 



O O O 



coarser particles throwing an individual shadow over 

 the others. Observe these appearances closely. Now 

 place the collar intentionally out of adjustment (prefer- 

 ably opening the systems), and thus look over your 

 mount again. See, how the shadows have become mixed 

 up! What a labyrinth there is to be sure! A perfect 

 plexus of indeterminable, indefinable shadows proceed- 

 ing from nowhere and ending nowhere. The thickened 

 edges of the diatoms seem to cast a shadow on their 

 own account. Notice, too, how that some of these 

 large shells seem to have become badly distorted. Find 

 those little colonies again, and note that the individual 

 particles thereof have lost their brilliancy have become 

 muggy and indefinite. Here, too, are all sorts of shad- 

 ows, and more or less confusion generally. Return,, 

 now the collar to its proper place and review your 

 mount. Now, see how things have improved in general 

 brilliancy ; see how the shadows have become harnessed 

 properly into the traces. You can trace them now, and 

 get an idea of what they mean, where they came from,, 

 and where they end. Look at the little colonies once 

 more. Now they are all alive with brilliancy and sparkle 

 like very diamonds. 



When the student feels that he can follow me thus 

 far, let him study the tone of his objective. This will 

 depend on the object-glass employed. All superfine 

 objectives should be under corrected, i. e., there should 

 be a preponderance of the blue; but hardly any two 

 good objectives are exactly alike, some being under- 



