278 



THE MICROSCOPE AND ITS REVELATIONS. 



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

 Fig. 165, 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 higher power, having an angular aperture 



of 120 or more, and a mag- 

 nifying power of 1,200 diam 

 eters, an appearance like that 

 represented in Fig. 166, A) 

 may be obtained, namely, a 

 h e x a g onal areolation, in 

 which the areolae can be made 

 to appear light, and the di- 

 viding network dark, or vice 

 versa, according to the ad- 

 justment of the focus (Fig. 

 115). That the areolae are 

 here elevations, and not (like 

 those of Triceratum) depres- 

 sions, is indicated by the 

 comparative results of the ex- 

 amination of fractured valves. 

 For in Triceratium the frac- 

 tures pass through the ap- 

 parent depressions, and co- 

 incide with various optical 

 indications in establishing 

 their reality. Fractured 

 valves of P. angulatum and 

 allied species show that the 



S 



Outline of Pleurosigma quadratum, as seen under a Weakest parts are between the 

 power ot 400 diameters: at A, B, D, are shown the direc- bead-rOWSI and Single beads 

 tions of the lines seen under a power of 1,300, the illumin- ? , 



ating rays falling obliquely (in each case) in a direction at may Olten D6 Seen 

 right angles to the lines; at E are shown two sets of + n oViorrk nno-nlnv 

 lines, as tee 

 of the mid 



markings when illuminated with an Achromatic Conden- D7 .;._ 

 ser of large angular aperture, the spherules being within r leUTOSigma, 



rigt anges to e nes; a E are sown wo ses o + n oViorrk nno-nlnv 



lines, as teen when the oblique rays fall in the direction tin g & n ^ r P angUlai 



of the midrib; and at c is shown the appearance of the Further, Wlien Specimens, of 



markings when illuminated with an Achromatic Conden- D7 .;._ j_^j ~u 



ser of large angular aperture, the spherules being within r leUTOSigma, mounted D6- 



the focus, and the portion left blank showing the obli- neath 2"laSS have had their 



teration of the markings by moisture. ' . & 



markings obscured by mois- 



ture, the obscurity is dissipated by the application of a gentle heat, in a 

 way that is readily explicable on the supposition that the markings are 

 elevations, but seems unintelligible on the idea of their being depres- 

 sions. 1 Notwithstanding these considerations, however, it must be freely 

 admitted that there is still considerable uncertainty respecting the real 

 structure of the Diatom-valve. For it cannot be positively asserted that 

 the focal adjustment which gives the image represented in Fig. 166, A, is 

 more correct than that which gives the equally distinct images B, c, D of 

 other parts of the same valve, of which the last departs in the most 

 marked manner from what is commonly regarded as the normal type. 

 And now that it has been shown that these images are not formed diop- 

 trically, but are resultants of the combination of numerous ' diffraction- 

 spectra' ( 157), it is impossible to entertain the same confidence as 

 before that they truly picture the surface marking they are supposed to 



1 See Mr. G. Hunt in :< Quart. Journ. of Microsc. Sci.," Vol. iii. (1855), p. 174. 



