POSITION OF OBSERVER. 277 



may be advantageous to the student if we repeat ejacu- 

 lations such as are heard from scholars, thus : " I can't 

 make this shell lay down." " I have a tolerably good 

 show, but the valve is restless.'"' " You seem to anchor 

 your diatoms; mine are all out at sea." "The valve 

 will float in spite of me." "I have it; the shell lays 

 as quiet as a summer morn." "This valve seems to 

 come right up to the eye-piece," (referring generally to 

 the one-half or one-fourth inch.) Such and similar ex- 

 pressions we have heard over and over again. The 

 student can select the one that will be of the most 

 service. 



The tone of the objective should be studied with the 

 Leipsig, just as has before been noticed when discussing 

 the Monmouth and the Cherryfield. The observer may, 

 at the commencement, entertain the notion that in re- 

 gard to tone, the objective behaves very differently when 

 over the more difficult mount. If, however, the pre- 

 ceding lessons have become perfectly familiar there will 

 be but little trouble in recognizing the same peculiar 

 and characteristic tone not only pervading the valve, 

 but the entire field of the instrument. 



Suppose, now, that the manipulator, having followed 

 the author with satisfactory success through the Mon- 

 mouth and the Cherryfield, should fail thus to do when 

 the Leipsig is taken in hand, thus : There appears a cer- 

 tain indistinctness accompanying .the images of the latter, 

 besides, the valves seem considerably distorted, and the 

 band of light behaves badly ; so much so that the direc- 

 tions cannot be followed with any degree of certainty, 



