276 HOW TO SEE WITH THE MICROSCOPE. 



cent to the median line, while the entire lower half of 

 the shell is of a lower tone than that of the upper. 



Should the tryo have got along- thus far tolerably 

 well, it is more than probable that he will have been 

 rewarded by at least a glimpse of the transverse striae. 

 He may now, by minute changes of the mirror, and by 

 raising or lowering the wick of the lamp, endeavor to 

 show the markings as strongly (that's a bad word) as 

 neatly as possible. Having thus succeeded, look sharply 

 and see if the lines extend quite to the edges of the 

 frustule ; it may be that some of them will be found to 

 project ; in either case something is wrong. Look first 

 to the illumination, and, observing the shadow line 

 under the median line, endeavor to contract and render 

 this as narrow as possible. If this does not prevent the 

 striae from showing to the extreme edges of this valve, 

 then recourse must be had to slight alterations in the 

 position of the collar adjustment. It may be advisable 

 to try a stronger ocular, say the " D " solid, if one is at 

 hand. If under this the lines seem to " rise," try and 

 correct this, as before instructed. 



This apparent "rising" will perhaps strike the reader 

 as something novel. Certain it is that we have never 

 seen anything of the kind in print. The effect was, 

 however, noticed long ago by the author, and it soon 

 became one of his " points" in the tuition of his pupils, 

 the latter, at the onset, seldom ever get his meaning. 

 Nevertheless, in a very little time they " see it " plain 

 enough. We have not command of a word that will 

 precisely and accurately express what we desire. It 



