280 SURFACE-MASKING S OF DIATOMACEjE. 



that an uncertainty still remains. These valves are commonly 

 spoken of as marked by strice, longitudinal, transverse, or oblique, 

 as the case may be ; but this term does not express the real nature 

 of the markings (the apparent lines being resolvable by Objectives 

 of sufficient magnifying power and angular aperture into ro%vs of 

 dots), and should only be used for the sake of concisely indicating 

 the degree of their approximation. If we examine Pleurosigma 

 angulatum,, one of the easier tests, with an objective of l-4th inch 

 focus (having an angular aperture of 90° and a magnifying power 

 of 500 diameters), we shall see very much what is represented in 

 Fig. 120, E ; 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. When, however, 

 the valve is examined with an objective of 1-1 2th inch focus, having 

 an angular aperture of 170° and a magnifying power of 1200 

 diameters, the appearance of its surface is that represented in Fig. 

 90, namely, a hexagonal areolation somewhat resembling that of 

 Triceratium (Fig. 119), in which the areola? can be made to appear 

 light, and the dividing network dark, or vice versa, according to 

 the adjustment of the focus. Now the question is, whether the 

 Areola? are here depressions or elevations ; and on this point a great 

 deal more has been said and written than its essential triviality 

 would seem to justify. The fact is, however, that although to the 

 Physiologist who studies the vital actions of the Diatoms it is a 

 matter of comparatively little importance whether the surfaces of 

 their valves are beset with rows of tubercles or are marked with 

 rows of punctations, it is of essential importance to the Microscopist 

 that he should certainly know how to interpret any such appear- 

 ances ; and the difficulty here resulting from the extreme minute- 

 ness of the objects, and the peculiar optical effect produced by them 

 (in virtue of their high refracting power) upon the light which 

 passes through them, is such as very rightly stimulates him to 

 devise every attainable means for its solution. Analogy would 

 obviously favour the idea that the hexagonal areolation of Pleuro- 

 sigma is of the same kind as that of Triceratium, and that the 

 Areola? are depressions in the former, as they certainly are in the 

 latter ; and it has been affirmed that such a continuous gradation 

 may be traced from the coarser to the finer kinds of areolation, as 

 establishes the unity of their nature throughout.* There is now, 

 however, a general agreement among those British Microscopists 



honeycomb structure is completed in many instances, as in Triceratium 

 and Cosciywdiscus, by the closing-in of its cells or depressed areolae with 

 siliceous facets on their outer as well as on their inner side. The Author 

 has not been able to satisfy himself, however, that such is the case ; and 

 he prefers to leave the question to be resolved by such observers as spe- 

 cially occupy themselves with this group. 



* See Dr. J. W. Griffith in the Introduction to the " Micrographic Dic- 

 tionary," 2nd Ed., p. xxxiii, and in Articles 'Angular Aperture' and 

 • Diatomacete.' 



