294 HOW TO SEE WITH THE MICROSCOPE. 



tion to this essential item, knowing, too, that it is es- 

 sential "cumgrano salts." With our private pupils 

 we have often tried amusing- experiments, simply to 

 become assured of their proficiency in this matter, thus : 

 We sometimes place a drop of mucilage on the interior 

 surface of the field lens of the eye-piece, replacing the 

 same and watch for results. If the pupil is well trained 

 there is no deceiving him. He will assert roundly that 

 something is the matter. If I say to him, mildly, " why, 

 that's pretty well, isn't it?" He replies at once, in tones 

 that there is no dodging, " No, sir; there is something 

 wrong, sure." And then, again, I have had the pupil 

 pay me back in my own coin placing a similar drop of 

 mucilage in the interior surface of my own oculars. 



We have already said that no two lenses work ex- 

 actly alike. It remains, therefore, for the pupil to moke 

 a specialty of his own object glasses, first assuring him- 

 self, preferably by the advice ot some expert friend, that 

 he shall not waste time over an inferior glass. While 

 he thus becomes more and more familiar with his own 

 objectives, he will also acquire a general knowledge of 

 all, and will in due time be able to take a strange object 

 glass and work it nearly or quite up to its maximum. 



Unless the observer be provided with a really super- 

 fine objective, he can hardly make much headway in 

 the examination of the smaller shells of the Saxonica 

 from the Isle of Shoals. Should he have much trouble 

 in getting the striae on the very largest frustules, these 

 appearing quite obscure and of a generally uninviting 

 aspect, the edges badly defined, and more or less distor- 



