268 HOW TO SEE WITH THE MICROSCOPE. 



however, in getting pupils to appreciate this phenome- 

 non. This effect is the more palpable under a high oc- 

 ular, say the one-fourth inch. Let the student try the 

 highest one he has, and he will notice that a slight chancre 



<^ DO 



of the collar will cause the shell to " lay down " prop- 

 erly. But he must nevertheless be able to make this 

 correction from noticing the condition when employing 

 the two-inch eye-piece, And here it may be observed 

 that with different objectives some leeway must be 

 allowed. It is hardly probable that the student can 

 take his particular object glass and follow me to the 

 very letter; or is it even certain that the author could, 

 with said glass, demonstrate to an expert what has been 

 written in these instructions ; yet after allowing due and 

 proper margin, the hints presented must prove of value, 

 and if we could have been told as much eight years ago 

 it would have saved us hours and hours of the toughest 

 work. And right here a thought presents itself, for- 

 eign, perhaps, to these lessons, but let it go on record, 

 namely : If it be so that we can assist the student, we 

 have in very truth no mean reward for time spent in 

 the past in the study of object glasses. 



Lesson Fifth. Having noted the division correspond- 

 ing to the exact adjustment of your objective, so that 

 you shall be able to place the collar at the right spot at 

 -once, without loss of time, leave the rhomboidcs and 

 examine your mount generally. Notice the play of light 

 and shade over the several diatoms. Hunt up those por- 

 tions of the slide containing a mass of the larger forms 

 huddled together; in other places you will probably 



