282 



MICROSCOPIC FORMS OF VEGETABLE LIFE. 



light or of stops witli a central diaphragm, and of objectives of 

 larger and larger angular aperture ; until we come to those species 

 which present the greatest difficulty, and the nature of whose 

 markings seem most obscure. The more perfectly these mark- 

 ings can be defined, however, in any case, the more decidedly 

 are they found to correspond with what has been already seen. 

 Thus, if we examine Pleurosigma angulatum, one of the easier 

 tests ( 102), with an objective of l-4th inch focus and 75 aper- 

 ture, we shall see very much what is represented in Fig. 80, A ; 

 namely, a double series of somewhat interrupted lines, crossing 

 each other at an angle of 60 degrees, so as to have between them 

 imperfectly defined lozenge-shaped spaces. 1 When, however, 



A FIG. 80. 





* + * 



+ * 



Pleurosigma angulatum: A, entire 

 frustule, as seen under a power of 500 

 diam. ; B, hexagonal areolation, as seen 

 under a power of 1300 diam.; c, the 

 same, as seen under a power of 15,000 

 diam. 



the valve is examined with an objective of l-12th inch focus, 

 having an angular aperture of 130, and is illuminated by oblique 

 rays, its hexagonal areolation becomes very distinct, as shown at 

 B. And if a photographic representation obtained by such a 

 power be itself enlarged by photography, as has been accom- 



1 This representation is taken from a Photograph by Mr. Delves, which is imitated as 

 closely as Wood-engraving can imitate on a scale of such minuteness, but with a reversal 

 of the lights and shades. 



