854 MICROSCOPIC FORMS OF ANIMAL LIFE 



Islands. Few .microscopic objects are more beautiful than an 

 assemblage of the most remarkable forms of the Barbadian Poly- 

 cystina (fig. 644), especially when seen brightly illuminated upon 

 a black ground ; since (for the reason formerly explained) their 

 solid forms then become much more apparent than they are when 

 these objects are examined by light transmitted through them. And 

 when they are mounted in Canada balsam the black-ground illu- 

 mination 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 objects, 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. No class of object 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 beautiful effect all 

 their delicate sculpture. 1 



1 For a fuller description of the fossil forms of this group see Professor Ehrenberg'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 sandstone it? described by Mr. Fur- 

 long in the Quart. Journ. of Microsc. Sci. n.s. vol. i. 1861, p. 64. 



