POSITION OF OBSERVER. 287 



light, that is, it will not appear to be equally illumi- 

 nated as was the case before. Now look to the adjust- 

 ment of the objective, and by slight manipulations of 

 the mirror display the striee, which will be right 

 straight along work. Seize the base of the condenser, 

 stand firmly, supporting the left forearm on the edges 

 of the draw and table; move the condenser just a bit 

 to and from the edge of the table, noticing the effect 

 in the field, getting it thus in the best position possible. 

 Look to your mirror again and see if that " line" is 

 still there; this cannot be dispensed with. Now if 

 the directions shal I be strictly carried out, the display 

 will be much finer than possibly could have been ob- 

 tained with the former illumination, the diatom will 

 seem to swim in a lake of fire, or as my pupils some- 

 times have said, " chain-lightning." 



Do not be satisfied with any apparent success, but 

 repeat the method of illumination, time and time 

 again, comparing notes. The student will not prob- 

 ably meet with perfect success the first evening, 

 although it may be that he will be quite satisfied with 

 his efforts. If it so be that the condenser stands at 

 quite an angle, more so than you think ought to be the 

 case, never mind; if you have the right effects in the 

 iield, you can study the rationale at your leisure. The 

 next thing in order is to study all the appearances 

 which have been pointed out in the preceding lessons. 



Now shifting the object-carrier, hunt up a valve next 

 the very smallest on the mount, one that lies flat. If 

 jou are using a one-tenth your inch ocular will do, but 



