POSITION OF OBSERVER. 273 



Hand the expert a strange mount of the Monmouth 

 or Cherryfield rhomboides. It is possible for him with- 

 out looking at it, and by the sense of feeling alone to 

 state thus : " This slide cannot be used with the one- 

 tenth and glycerine contact. Use water and with the 

 collar nearly at closed. If the one-sixth be employed 

 use glycerine, and the objective will correct within 

 four or five divisions from the closed point." Either 

 statement being found on the actual test or trial to be 

 the fact. In truth, any such statement from the expert 

 founded on his sense of feeling alone would be worth 

 more than two hours spent over the tube by the novice. 

 Hence the importance of the learner's acquiring the 

 same tact (" knack " is the popular word), nor is there 

 anything so very difficult in its acquisition. 



Lesson Eighth. Returning to the slide of Monmouth 

 Rhomboides, and choosing a frustule somewhat larger 

 than a medium one, place it in the centre of the field, 

 adjusting the objective, and displaying the striae, em- 

 ploying an obliquity of say 70 from axis. Next rotate 

 the stage through an arc of 45 ; if necessary centre the 

 object again. It will now have assumed a position in- 

 termediate between the horizontal and the vertical. 

 Attempt, by slight changes in the position of the mirror, 

 to display simultaneously both sets of lines. The inch 

 ocular may now be substituted in place of the two inch. 

 To do this nicely, getting both sets with equal force, 

 requires indeed some little "knack." The collar ad- 

 justment being already correct, the manipulation will 

 entirely be confined to the illumination. Possibly it 



18 Microscopy. 



