172 



THE MICROSCOPE AND ITS REVELATIONS. 



direction transverse to its axis; while B shows a portion more highly 

 magnified under the last illumination. This Diatom, however, has been 

 successfully photographed by Dr. Woodward (Fig. 118, c), who says of 

 it: "A careful examination of specimens mounted dry, has satisfied me 

 that Hartnack's interpretation is erroneous. The fine striae are, I think, 

 rows of minute hemispherical beads; the appearance of hexagons is the 

 optical result of imperfect definition or of unsuitable illumination. For 

 photographing this object, I have selected a frustule of somewhat less 

 than the medium size. It measures 1290th of an inch in length. 

 Longitudinally the fine striae count at the rate of 72,000 to the inch. 

 These striae are resolved into beaded appearances, which count laterally 

 84,000 to the inch." 1 



162. As a test for those qualities of Objectives which best fit them 

 for the general purposes of Biological investigation, the Author remains 

 of the opinion (which he finds to be shared by many able and experienced 



Valve of Surirella gemma, with portion (B) more highly magnified, showing two systems of 

 markings a and 6, as represented by Hartnack; while c is copied from a photograph taken &y Dr. 

 Woodward. 



Microscopists, and by Makers specially familiar with their requirements) 

 that nothing is better than the scale of the Lepidocyrtus cervicottis, 

 commonly known as the Podura (Fig. 419). It is a fact perfectly 

 familiar to such Makers, that an Objective may serve, in virtue of its 

 wide Angular aperture, to resolve Diatom-tests of considerable difficulty, 

 and may yet fail utterly on the Podura-scale, in consequence of its 

 inferior defining power; and such an Objective can be of very little ser- 

 vice to the Biological investigator. On the other hand, although the 

 exact structure of the Podura-scale is still (like that of the Diatom-valve) 

 a matter of discussion, yet all are agreed as to the appearances it presents 

 under Objectives that combine in the fullest degree the attributes already 

 specified as best qualifying them for Scientific work; so that any glass 

 which shows these appearances satisfactorily, may be safely accounted 

 suitable for that purpose. The surface of this scale, when viewed under 



1 ''Monthly Microsc. Journ.," Vol. vi. (1671), p. 100. 



