116 THE MICROSCOPE AND ITS REVELATIONS. 



The internal portion of each mass is composed of an aggregation of large 

 vesicle-like bodies, imbedded in a softer sarcodic substance. 1 



507. From the researches made (Juring the 'Challenger' expediti* n, 

 it appears that the Radiolaria are very widely diffused through the waters 

 of the ocean, some forms being more abundant in tropical and others in 

 temperature seas; and that they live not only at or near the surface, but 

 also at considerable depths. Their siliceous skeletons accumulate in some 

 localities (in which the calcareous remains of Foraminifera are wanting) 

 to such an extent as to form a 'Kadiolarian ooze;' and it is obvious that 

 the elevation of such a deposit into dryland would form a bed of siliceous 

 sandstone resembling the well-known Barbadoes rock, which is said to 

 attain a thickness of 1100 feet, or a similar rock of yet greater thickness 

 in the Nicobar Islands. Few Microscopic objects are more beautiful 

 than an assemblage of the most remarkable forms of the Barbadian Poly- 

 cystina (Fig. 345), especially when seen brightly illuminated upon a 

 black ground; since (for the reason formerly explained, 103) their solid 

 forms then become much more apparent than they are when these ob- 

 jects are examined by light transmitted through them. And when they 

 are mounted in Canada-balsam, the Black-ground illumination, either by 

 the Webster-condenser ( 100), the Spot-lens (104), or the Paraboloid 

 ( 105), is much to be preferred for the purpose of display, although 

 minute details of structure can be better made out when they are viewed 

 as transparent objects with higher powers. Many of the more solid forms, 

 when exposed to a high temperature on a slip of platinum foil, undergo 

 a change in aspect which renders them peculiarly beautiful as opaque ob- 

 jects; their glassy transparence giving place to an enamel-like opacity. 

 They may then be mounted on a black ground, and illuminated either 

 with a Side-condenser, or with the Parabolic Speculum ( 114). No 

 class of objects is more suitable than these to the Binocular Microscope; 

 its stereoscopic projection causing them to be presented to the mind's eye 

 in complete relief, so as to bring-out with the most marvellous and beauti- 

 ful effect all their delicate sculpture. 2 



1 See Prof. Huxley (to whom we owe our first knowledge of these forms) in 

 "Ann. Nat. Hist.," Ser. 2, Vol. viii. (1851), p. 433; also Prof. Muller, of Berlin, in 

 " Quart. Journ. Microsc. Sci.," Vol. iv. (1856), p. 72, and in his Treatise " Ueber 

 die Thalassicollen, Polycystinen, und Acanthometren des Mittelmeeres;'' and the 

 magnificent work of Prof. Haeckel, "Die Radiolarien." Great additions to our 

 knowledge of this group may be expected from the collections made in the 

 ' Challenger ' expedition. 



2 For a fuller description of the Fossil forms of this group, see Prof. Ehren- 

 g's Memoirs in the " Monatsberichte " of the Berlin Academy for 1846, 1847, 



and 1850; also his ' Microgeologie,' 1854; and "Ann. of Nat. Hist.," Vol. xx. 



(1847). The best method of separating the Polycystina from the Barbadoes sand- 



stone is described by Mr. Furlong in the " Quart. Journ. of Microsc, Sci.," N. S., 



Vol. i. (1861), p. 64. 



