264 HOW TO SEE WITH THE MICROSCOPE. 



yourself instantly, repeat the programme until you can. 

 Now, reader, one-half hour conscientiously spent at this 

 exercise will assist you more than a half year of pro- 

 miscuous practice. Let me advise you to hang well to 

 it. You must learn to judge instantly as to the appear- 

 ances to which your attention has been called, nor can 

 the least " guess-work" be allowed. 



Well, now, premising that you have had sufficient of 

 this practice, so that you are able to assert, without fear 

 of contradiction, that after the turn of the collar adjust- 

 ment, there was, as we will assume, evident!} 7 more dis- 

 tortion noticeable at the ends of the valve, nor were 

 the edges as sharply defined, (it may be that one edge 

 cannot now be seen without specially focussing therefor. N 

 The shadow, too, under the median line is not only en- 

 tirely over the entire lower half of the shell, but it has 

 become deeper and more offensive than before. The 

 fact, then, becomes apparent that you turned the collar 

 in the wrong direction. Therefore, place the adjust- 

 ment in its original position ; look your shell well over, 

 thus to fortify yourself.. It will be well, now, to repeat 

 the experiment, selecting various valves of the rhom- 

 boicles, and, having become accustomed to the change 

 in appearances, then you may try many of the other 

 diatoms to be found on the mount. No matter what 

 particular one may be selected, you will in due time be 

 able to note the characteristic effect of this change in 

 the adjustment. 



Lesson Second. Having replaced the collar adjust- 

 ment to its first position, and selecting a tolerably small 



