304 



HOW TO SEE WITH THE MICROSCOPE. 



which we have had occasion to allude to in previous 

 pages, when treating of certain diatom tests. Now 

 this same effect is easily seen in the podura when under 

 the objective, and we have never been entirely able to 

 get rid of it, especially when a glass of moderate angle 

 is employed. Now the reader knows from personal 

 experience just as much of this borrowed cut as does 



the author. We will, 

 however, hazard the 

 assertion that the il- 

 lumination shown was 

 about 20 from axis, 

 the glass having aper- 

 ture as above stated, 

 of perhaps 120 or 130. 

 Let us imagine then 

 that we thus have such 

 a display under the mi- 

 croscope; and again r 

 that we remove the 

 objective and replace 

 with another of high 



balsam angle. In the cut on this page we have endeav- 

 ored to depict the changes. 



First it will be seen that with the radial bar in pre- 

 cisely the same position we have the effect of greater 

 obliquity; the shading is more decided and (over the 

 tube) the lights are more intense and brilliant. Now 

 examine this cut with the paper tube ; it will be ob- 

 served that the exclamation points do not " rise " so 



