POSITION OF OBSERVER. 271 



thing in this way that is presumed to be characteristic, 

 then put the object-glass out of adjustment and focus- 

 sing anew make the comparison. If the student has 

 hit the proper effect, he will notice that with the glass 

 out of adjustment, said effect has vanished. Should (as 

 will most probably be the case with his first efforts) he 

 fail to mark any characteristic difference in color or 

 tint, replace the collar adjustment and repeat the ex- 

 periment; selecting, as near as he can judge, some 

 "probability," and again throwing the glass out of 

 proper adjustment examine again. The process must 

 be repeated time after time until the end shall be 

 gained. When by dint of practice the learner begins 

 to feel that he " sees the point" let him in a similar 

 manner examine all of the other diatoms on the mount 

 until he shall have become perfectly familiar with the 

 tone of his objective. 



Lesson Seventh. Having mastered tolerably well the 

 previous instructions, let the learner now turn his atten- 

 tion to the tone of his field. This is a finer "point" 

 than that of the preceding lesson, and demands in turn 

 more of the eye-training. The problem is in all 

 respects similar to that concerning the tone of the 

 objective, differing in this one element only. The 

 student must now endeavor to recognize a particular 

 tone to the whole field when the object glass is in per- 

 fect adjustment. The instructions are quite of the 

 character as those of the last lesson, an 1 need not be 

 repeated. The two lessons might have been reduced to 

 one, were it not that a little practice on the former 



