POSITION OF OBSERVER. 279 



for the everyday work of the microscopist. The ob- 

 server who depends thereon leans on a treacherous staff. 

 Could we command sunlight, this objection would cer- 

 tainly have less force ; but even in that case we would 

 be compelled to fall back on artificial illumination in 

 the evening, and eight-tenths of all microscope work is 

 done after sundown. Hence it is of profound import- 

 ance that the microscopist select such illumination as he 

 can at all times control, and to learn to make the best 

 possible use of it. Again, all that we have said as to 

 sunlight applies with equal force to objectives that re- 

 quire to be worked with its aid. Let the reader note 

 well this fact. It has been our purpose here, as it is 

 every day with our pupils, to teach the use of the sim- 

 plest methods, both as regards illumination and other 

 items of management. 



Returning, now, to our diatom slides again, we remark 

 that the objective failing, with the stipulated illumina- 

 tion of showing the Leipsig, might be forced to do so 

 tolerably well by the assistance of monochromatic sun- 

 light. Admitting this, we still assert that such a glass 

 is unfit for the purposes of the working microscopist 

 beyond those it will respond to under the conditions 

 which we have presented. It further obtains that the 

 reader who, with such a glass, has followed us through 

 the Cherryfield and the Monmouth, can proceed no far- 

 ther until we are ready to change the illumination. 

 Presuming, then, that the objective is quite competent, 

 and that the observer has thus far followed us in a sat- 

 isfactory manner, we attempt some further hints and 

 suggestions, 



