WORK WITH THE HIGHER " POWERS." 243 



We intended to refer to this bye-and-bye, but it finds 

 an appropriate place rig-lit here. We may, however, 

 refer to it at some future time. I allude to that class 

 of work intimately connected with the use of object 

 glasses of wide apertures, and over exceedingly difficult 

 lined structures; for instance, the display of the striae 

 of amphipleura pellucida. But let the reader remember 

 that this class of work is not confined to the study of 

 diatoms. 



To illustrate what we have in hand, reference is made 

 to the following sketch : Let C-D and G-H be a sec- 

 tional view of some " difficult " diatom, such as amphr 

 pleura pellucida or the like, the short lines 1, 2, 3, 4 ? 

 etc., representing in section the elevations of the striae; 

 M and N being views " in plan" as seen conditionally 

 in the microscope. Let us first consider the effect of 

 illuminating C-D with direct central light, as indicated 

 by the line A-B. The effect will be as shown in plan 

 at N, to wit : There will appear but a series of exceed- 

 ingly fine lines; so fine (mark the words, not necessa- 

 rily close) that it will be impossible to see them with 

 any glass extant. Recourse must then be had to " ob- 

 lique illumination." Now let C-D, as duplicated at 

 G-H, be illuminated by the oblique beam E-F. The 

 effect of this is shown in section at 1', 2', 3', etc., and 

 in plan at K, where we have the view as displayed in 

 the microscope. Here we have two things successfully 

 accomplished; the striae which in the former case were 

 so " fine " as to be invisible have now become broad and 

 can easily be distinguished by the eye; and, secondly, 



