282 HOW TO SEE WITH THE MICROSCOPE. 



time to time all the larger frustules to be found on the 

 mount. 



Before proceeding farther we have to say that the 

 foregoing ten lessons must be well studied and thor- 

 oughly understood. They have been arranged and pre- 

 sented so as to lead the student safely and progressively 

 through what has proved to many a labyrinth. We 

 have hinted at the difficulties which beset us and satu- 

 rate (so to speak) any attempt of ours to render our in- 

 structions perfectly intelligible to all. It follows, then,, 

 perforce, that defects and shortcomings in the way of 

 elucidation ought to be compensated for on the part of 

 the pupil by his vigorous determination to master the 

 situation. Again, in the exercise thus far given the in- 

 tention has been steadily held in mind ; first, to present 

 as plain a chart for the guidance of the learner as was 

 within our power; and, secondly, to keep well in hand 

 the necessary education of his eyes ; and in the latter 

 acquisition the element of time is quite as much a factor 

 as that of diligence and determination. We strongly 

 advise, then, the learner to confine himself to the sug- 

 gestions already given, until he shall feel himself thor- 

 oughly and practically familiar with them. 



Lesson .Eleventh. We will now study the Mon mouth 

 or the Cherryfieid (either will answer our purpose) once 

 more. Place the same on the stage, illuminate at an 

 angle of, say seventy degrees from axis. Bring one of 

 the largest shells to the centre of the field ; adjust the 

 glass and focus, getting the very nicest display of the 

 transverse striae obtainable. Now gradually let down 



