592 MICROSCOPIC FORMS OF VEGETABLE LIFE THALLOPHYTES 



. I. titurtii, magnified 2,000 times, is shown in fig. 6 in. the same 

 plate. 



The ' beaded ' appearance of diatom-Valves is so universal in all 

 those which have been examined, that it must be regarded as 

 common to all diatoms, although this is not yet absolutely proved. 

 But, while it is admitted that the beading of the valves may be 

 common to all diatoms, it cannot be regarded as proved that the 

 siliceous envelope is composed of globular particles of silex arranged 



FIG. 442. Triccratiumfavus: A, side view ; B, front view. 



in regular rows ; while the variety in the size and arrangement of 

 these particles shows that they are correlated with the vital pro- 

 cesses of the organisms, and afford characters for the discrimination 

 of the species. The nature of these granules, their size, and the 

 mode in which they are arranged have from the earlier days of micro- 

 scopy rendered diatoms of special value as * test- objects.' This 

 appearance has led to the use, in speaking of diatoms, of the incorrect 

 terms ' transverse,' ' longitudinal,' or ' oblique stri^,' these being in 

 truth simply the intervals which separate 

 the boundaries of the ' beads/ apertures, or 

 their equivalents, whatever they may ulti- 

 mately prove to be ; and this is clearly seen 

 when they are observed with objectives of 

 sufficient numerical aperture and propor- 

 tional power. Pleurosigma angulatum is 

 one of the most commonly employed test 

 objects, and at the same time one of the most 

 reliable, its remarkable constancy rendering 

 it especially valuable for this purpose; 

 while, on the contrary, Amphipleura pdlucida 

 is extremely variable, and is, as it were, the 

 torment of microscope-makers and rival 

 diatoin-resolvers, who do not take into account the variability of 

 this type, forgetting, in fact, that one A.pellucida maybe extremely 

 fine, and another, being in truth a varietal form, may be nearly as 

 coarse as Navicula rhomboides. The new apochromatic objectives, 

 and the compensating eye-pieces, both for the eye and for projection, 

 constructed by Zeiss, of Jena, have brought about such progress in 

 micrography that the image of P. anyulatum appears to some minds 



PIG. 443. Areolations in 

 Isthmia >iervosa. 



